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Numerical and comparative analyses of the modern systems of classification of the flowering plants.



II. Introduction

Often we are given no idea what evidence is used in erecting the classification. - M. T. Ghiselin (1984)

Plant taxonomists can refer to four major and modern classification systems: Cronquist (1988), Dahlgren et al. (1985; see also G. Dahlgren, 1989a, 1989b), Takhtajan (1987), and Thorne (1992b). These systems are important because they serve as a basis for phylogenetic phy·lo·ge·net·ic
adj.
1. Of or relating to phylogeny or phylogenetics.

2. Relating to or based on evolutionary development or history.
 studies, whether the latter use morphological or molecular data. For example, Chase et al. (1993) wrote that their analysis stems from the works of Cronquist (1981) and of Dahlgren et al. (1985). Also, in the discussion of their results, they compare intuitively their cladogram, obtained from rbcL data, to Cronquist's, Dahlgren's, Takhtajan's, and Thorne's classifications. Since studies in molecular systematics systematics: see classification.  increase in number and because they use the modern classifications as a starting point Noun 1. starting point - earliest limiting point
terminus a quo

commencement, get-go, offset, outset, showtime, starting time, beginning, start, kickoff, first - the time at which something is supposed to begin; "they got an early start"; "she knew from the
, it is becoming important to understand the structure of these different classifications and to define their underlying principles and concepts. Yet modern classifications are little studied; we do not know, among other things, if they differ. A more profound knowledge of these recent classifications matters not only because we use them as a basis for our taxonomic tax·o·nom·ic   also tax·o·nom·i·cal
adj.
Of or relating to taxonomy: a taxonomic designation.



tax
 analyses but also because we are comparing them to our results, judging qualitatively whether they are congruent con·gru·ent  
adj.
1. Corresponding; congruous.

2. Mathematics
a. Coinciding exactly when superimposed: congruent triangles.

b.
 or not. Indeed, such comparisons are precarious, for it is difficult to compare what is known to what is hardly known.

Apart from a few studies that used the size distribution of taxa (Holman, 1985, 1992; Cronk Verb 1. cronk - utter a hoarse sound, like a raven
croak

let loose, let out, utter, emit - express audibly; utter sounds (not necessarily words); "She let out a big heavy sigh"; "He uttered strange sounds that nobody could understand"

2.
, 1989, 1990), historical studies of classification schemes have remained qualitative (Lu, 1981; Barabe & Brouillet, 1982; Barabe, 1984, 1993; Stevens, 1984b, 1986, 1994). Sivarajan (1991) mentioned that knowing the history of classification systems is the only way to foster in taxonomists a true understanding of the modern systems. For Stevens (1994: 269), "systematics can free itself of its undue reverence for tradition if systematists come to understand more about the development of that tradition in its historical context."

Nonetheless, the history of plant systematics The history of plant systematics—the biological classification of plants—stretches from the work of ancient Greek to modern evolutionary biologists. As a field of science, plant systematics came into being only slowly, early plant lore usually being treated as part of  awaits further development. Apart from works on Linnaeus or Adanson, few studies have been done on other plant taxonomists (Turrill, 1963; Kiger, 1971; Barabe & Brouillet, 1982; Mabberley, 1985; Williams, 1988; Barabe & Vieth, 1990; Stevens & Cullen, 1990; Endress, 1993; Stevens, 1984a, 1984b, 1991, 1994; Cain, 1994; Cuerrier et al., 1996). Among these, few have insisted on recent aspects of taxonomy.

Benson (1957), Lu (1981), and Woodland (1991) have graphically represented a theoretical genealogy genealogy (jē'nēŏl`əjē, –ăl`–, jĕ–), the study of family lineage. Genealogies have existed since ancient times.  of classification systems. Figure 1 shows that, for these authors, all modern systems originate directly or indirectly from the Bessey system A system of plant taxonomy, the Bessey system was published in
Charles E. Bessey (1915). "The phylogenetic taxonomy of flowering plants". Annals of the Missouri Botanical Garden 2: 109-164.
; one should recognize, though, that Hallier's system is not the German counterpart of Bessey's system (see Cuerrier, in press), contrary to what Core (1955), Benson (1957), Stace (1989), and Woodland (1991) have written. Cronquist (1965: 285) stipulated: "Nearly all modern systems of angiosperms fall into the de Candolle-Bentham and Hooker-Bessey tradition." Cuerrier et al. (1992) have otherwise shown that if Engler's classification topology differs from Bessey's, their ordinal (mathematics) ordinal - An isomorphism class of well-ordered sets.  circumscriptions (i.e., family affinities) stay statistically similar.

Therefore, in this article, we wish to extend our knowledge of the modern and past systems of classification of the Angiosperms by comparing classifications of Cronquist (1988), Dahlgren et al. (1985), G. Dahlgren (1989a), Takhtaj an (1987), and Thorne (1992b) with one another, using statistical tools. A comparative study of the principles underlying these systems will parallel the numerical comparisons and they will serve to interpret the statistical results. Stevens (1986, 1994) mentioned that modern classifications were derived from those of the past. Hence, we will also compare the modern classifications to those published at the beginning of this century by Bessey (1915), Engler (1909), and Hallier (1912). Takhtajan's (1987) classification will be compared to Gobi's (1916) since Takhtajan himself asserted that he was influenced by Gobi. Our study will answer specifically the following questions:

* Are modern classifications statistically similar?

* Are they significantly different from classifications published at the end of the 19th and beginning of the 20th centuries?

* Are they more similar to Bessey's classification than to other past classifications as underlined by many authors (see above)?

* What are the relationships among the seven classifications studied in this article? Are these relationships the same as the ones depicted by Benson, Lu, and Woodland?

Answering these questions will help to verify the thesis defended, among others, by Stevens (1994), who stipulated that modern classifications have changed little over the last two centuries. Is this absence of changes visible solely in classification topologies or is it equally visible in the classificatory approach and in the principles used by taxonomists?

III. Material and Methods

For numerical analyses, only the most recent classification published by modern authors has been used: Cronquist (1988), Dahlgren et al. (1985; for Dicotyledons Dicotyledons

A large group of flowering plants (angiosperms) that for many years has been considered one of the two main categories of plants, the other being monocotyledons.
, G. Dahlgren, 1989a), Takhtajan (1987), and Thorne (1992b). [Takhtajan's (1997) new book, which includes a classification scheme, was only available after all comparisons were completed and the article written.] As for the past classifications, Bessey's (1915), Engler's (1909), Gobi's (1916), and Hallier's (1912) were chosen. Analyses have been done independently for Monocotyledons Monocotyledons

This group of flowering plants (angiosperms), with one seed leaf, was previously thought to be one of the two major categories of flowering plants (the other group is dicotyledons).
 and Dicotyledons, because these two group circumscriptions are identical among all authors. Additionally this was done to avoid an overestimation o·ver·es·ti·mate  
tr.v. o·ver·es·ti·mat·ed, o·ver·es·ti·mat·ing, o·ver·es·ti·mates
1. To estimate too highly.

2. To esteem too greatly.
 of similarity among classifications caused by identical partitions at the most inclusive rank (Lapointe & Legendre, 1990). Moreover, monocotyledon classifications are usually formed independently of those of Dicotyledons.

Two types of statistical analyses were first performed: Mantel and consensus tests. The former is a local test and the latter is a global one. That is, the Mantel test The Mantel test is a statistical test of the correlation between two matrices. The matrices must be of the same rank, and in the usual applications, they are matrices of interrelations between the same vectors of objects.  compares terminal taxa only, while the consensus test takes into account taxonomic ranks and classification topologies (or structures) as well. All three formal properties of nested classification are thus analyzed (Lapointe & Legendre, 1990). One must realize, though, that these tests do not evaluate another component of evolutionary classifications: the linear sequence of taxa (from ancestor to descendant). This information is more or less clearly marked in classifications and thus remains elusive. For example, for Thorne, each taxon taxon (pl. taxa), in biology, a term used to denote any group or rank in the classification of organisms, e.g., class, order, family.  in his classification is analogous to a ladder at the bottom of which lie primitive groups and at the top, more advanced groups. It is impossible to statistically analyze this information since it is not always available and, moreover, because Linnean classification expresses it only with great difficulty (Cuerrier et al., 1992, 1996; Rieppel, 1991). Even though authors establish ancestor-descendant sequences, they acknowledge at the same time that taxa are not direct ancestors but solely the nearest groups.

For both Mantel and consensus tests, matrix dimensions have to be identical for objects to be comparable. This constraint requires that only families recognized by authors in their respective classifications must be used for comparative purpose (Cuerrier et al., 1992), even if families of one author are considered as subfamilies by another, which happens frequently. In the latter case, we have ignored the information. This constraint is a lesser evil because it has more to do with similarities than with differences at distinctive ranks; and similarity among classifications is already strong (Table I). Between modern and past classifications an important disparity in family number can be seen: Modern classifications have more families. Therefore, comparisons involving these two groups of classifications have been performed with those subfamilies (mostly those of past classifications) taken into account when recognized at the family level by the other author (mostly modern).

When simultaneous statistical tests are carried out, p-values (i.e., level of significance) need readjustment (Wright, 1992). To do this we have chosen the Bonferroni correction In statistics, the Bonferroni correction states that if an experimenter is testing n independent hypotheses on a set of data, then the statistical significance level that should be used for each hypothesis separately is 1/n , which divides the level of significance by the number of tests performed. This correction limits the probability of a type I error (erroneous rejection of the null hypothesis null hypothesis,
n theoretical assumption that a given therapy will have results not statistically different from another treatment.

null hypothesis,
n
) and renders more rigorous the series of tests used.
Table I

Results of the consensus (NISI) and Mantel (r) tests between modern
classifications. Mantel tests were performed at the ordinal level,
and only between the monocotyledon classifications, due to the
algorithm limits. For [Alpha] = 0.01, the associated probability of
the Normalized Intermediate Similarity Index, P(NISI), and of the
standardized Mantel statistic, P(r), is p = 0.00167, after
correcting the level with the Bonferroni method (level/number of
pair-wise comparisons). The number of permutations has been
established at 4000 for the Monocotyledons and at 10000 for the
Dicotyledons.

Classifications               NISI    P(NISI)       r      P(r)

Cronquist/Dahlgren

Monocotyledons (106 OTUs)   0.85021   0.00025   0.59443   0.00049
Dicotyledons (306 OTUs)     0.74547   0.00009       ?        ?

Cronquist/Takhtajan

Monocotyledons (107 OTUs)   0.73486   0.00025   0.36922   0.00049
Dicotyledons (313 OTUs)     0.88514   0.00009       ?        ?

Cronquist/Thorne

Monocotyledons (91 OTUs)    0.78652   0.00025   0.57833   0.00049
Dicotyledons (290 OTUs)     0.74141   0.00009       ?        ?

Dahlgren/Takhtajan

Monocotyledons (111 OTUs)   0.80338   0.00025   0.55035   0.00049
Dicotyledons (361 OTUs)     0.78661   0.00009       ?        ?

Dahlgren/Thorne

Monocotyledons (103 OTUs)   0.91234   0.00025   0.89108   0.00049
Dicotyledons (318 OTUs)     0.87091   0.00009       ?        ?

Takhtajan/Thorne

Monocotyledons (103 OTUs)   0.64725   0.00025   0.26337   0.00049
Dicotyledons (331 OTUs)     0.75969   0.00009       ?        ?


Figure 2 pictures the creation of the matrices used by both tests and shows the steps leading to meta-analyses (clustering analyses and ordinations of the seven classifications, i.e., a classification of classifications).

A. CONSENSUS TEST

Based on the calculation of a consensus index, this test is meant to measure the similarity between two classifications. As was pointed out previously, Lapointe and Legendre's (1990) algorithm was used because it takes advantage of the three properties of dendrogram A dendrogram is a tree diagram frequently used to illustrate the arrangement of the clusters produced by a clustering algorithm (see cluster analysis). Dendrograms are often used in computational biology to illustrate the clustering of genes.  (taxonomic ranks, family position, and topology). Their algorithm proceeds through a double permutation One possible combination of items out of a larger set of items. For example, with the set of numbers 1, 2 and 3, there are six possible permutations: 12, 21, 13, 31, 23 and 32.

(mathematics) permutation - 1.
 of the ultrametric matrices formed from the above-mentioned classifications. Cuerrier et al. (1992) applied this consensus test in their comparison of Bessey's and Engler's classifications. The null hypothesis works as follows: The two classifications compared are no more similar than classifications randomly generated. If the null hypothesis is rejected, then the two classifications compared are considered to be statistically similar. Furthermore, the consensus indices obtained (which are normalized intermediate similarity indices, or NISI) can serve to form an input matrix that can be used for clustering analyses and ordinations. Contrary to matrix correlations (r) obtained from Mantel tests, NISI are not affected by the number of objects or matrix dimensions (F.-J. Lapointe, pers. comm.).

B. MANTEL TEST

This permutation test was first conceived by Mantel (1967) to statistically compare two matrices of the same dimension and produce a matrix correlation. Cuerrier et al. (1990, 1992) give a detailed description of the Mantel test as it is applied in this paper. The null hypothesis can be stated as follows: Matrix A does not statistically resemble matrix B more than does a matrix randomly generated. So the null hypothesis is accepted if the content of taxa differs between the classifications being compared, whereas it is rejected if contents are similar. Results of Mantel tests among dicotyledon classifications of modern authors have not been retained since the operational taxonomic units (OTUs; i.e., number of families) exceeded the algorithm limit. However, results of Mantel tests between matrices at the rank of subclass In programming, to add custom processing to an existing function or subroutine by hooking into the routine at a predefined point and adding additional lines of code.

subclass - derived class
 and superorder su·per·or·der  
n.
A taxonomic category of related organisms ranking below a class or subclass and above an order.



superorder  
 are discussed; they were all significant, and could possibly be correctly interpreted.

Matrix correlations of the Mantel tests have further served to produce input matrices for clustering analyses and ordinations of the seven classifications. However, matrix correlations are affected by matrix dimensions (number of objects; i.e., OTUs). Podani (pers. comm.) mentioned that interpretations of these analyses using matrix correlations based on different OTUs must be made with extreme precaution (see below).

C. CLUSTERING ANALYSES

As stated above, the results of both the Mantel and consensus tests can serve to generate classifications (dendrograms and ordinations) of the classifications compared, so that these can be called metataxonomic analyses or recta-analyses, in which OTUs themselves happen to be the seven classifications.

Two different clustering strategies were used: single linkage and complete linkage complete linkage Genetics An inheritance pattern for 2 gene loci on the same chromosome, in which the observed crossover frequency between the loci is zero. See Chromosome, Crossing over, Gene, Inheritance, Linkage, Locus, Nonlinkage, Partial linkage. . When dendrograms from both types of analysis had identical topologies, only the one produced by single linkage was reproduced in this paper. One should know that single linkage merges objects and groups together while contracting the space of reference; thus a chaining of groups with only one object can follow (Sneath & Sokal, 1973; Abbott et al., 1985). On the contrary, the dilation dilation /di·la·tion/ (di-la´shun)
1. the act of dilating or stretching.

2. dilatation.


di·la·tion
n.
1.
 of space occurring with complete linkage favors the distance between clusters and creates, in general, a smaller number of clusters. Thus, complete linkage can reduce the similaxity between objects or between clusters which can lead to artificial clusters (Abbott et al., 1985). Program NCLAS of the SYN-TAX IV software (Podani, 1990) was used to perform all clustering analyses. Once dendrograms of monocotyledon and of dicotyledon classifications were independently obtained, repeated analyses were done using an input matrix of combined results (i.e., NISI of Monocotyledons + NISI of Dicotyledons/2). This procedure is justified because the index used is normalized.

D. PRINCIPAL COMPONENT AND COORDINATE ANALYSES

Again, using the consensus test results (NISI) as an input matrix, principal coordinate analyses (PRINCOOR) were carried out. Since the Mantel test results in matrix correlations (r), and not in similarity indices, principal component analyses (PRINCOMP, option "PCA using correlation") were chosen instead. As was done with the clustering analyses, consensus test results obtained by comparing monocotyledon classifications with one another and dicotyledon classifications with one another were combined. The new input matrix served to produce an ordination of angiosperm angiosperm (ăn`jēəspûrm'), term denoting seed plants in which the ovules, or young seeds, are enclosed within the ovary (that part of the pistil specialized for seed production), in contrast to the gymnosperms, in which the seeds  (Monocotyledons and Dicotyledons) classifications.

IV. Results

A. COMPARISONS OF THE MODERN CLASSIFICATIONS

Results taken from the multiple consensus and Mantel tests between modern classifications of Monocotyledons and of Dicotyledons are compiled in Table I.

Results from both tests are in agreement: The null hypothesis is always rejected. Indeed, monocotyledon classifications of Cronquist, Dahlgren et al., Takhtajan, and Thorne are statistically similar, as much for their overall structure as for their content of orders. Modem authors share similar monocotyledon and dicotyledon classifications at the family level and above. Minor differences seen among modern classifications are not important enough for statistical results to be affected.

B. COMPARISONS OF THE MODERN AND PAST CLASSIFICATIONS

Table II summarizes the results of the consensus and Mantel tests between modern and past classifications. In general, the classificatory structure does not differ significantly among authors. However, the null hypothesis is accepted for some comparisons: between the classifications of Cronquist and Bessey (Dicotyledons), between those of Dahlgren and Engler (Dicotyledons), and between those of Takhtajan and Bessey (Monocotyledons). These classifications differ statistically. Even though the null hypothesis is also accepted when Cronquist's and Bessey's classifications (Monocotyledons) are compared - or Cronquist's and Engler's (Dicotyledons), or Takhtajan's and Bessey's, or Takhtajan's and Engler's (Dicotyledons), or Thorne's and Engler's (Dicotyledons) - the probabilities obtained are near the level of significance. Indeed, without the Bonferroni correction, results would have emphasized the similarity among classifications. This correction is used as a conservative measure that accentuates the rigor rigor /rig·or/ (rig´er) [L.] chill; rigidity.

rigor mor´tis  the stiffening of a dead body accompanying depletion of adenosine triphosphate in the muscle fibers.
 when simultaneous tests have been repeated. The dicotyledon and monocotyledon classifications produced by Hallier are both similar to modern classifications. The monocotyledon classification of Engler is also similar to those of the modern authors. Therefore, differences are observed between modern and past classifications only when Bessey's (Monocotyledons and Dicotyledons) or Engler's (Dicotyledons) classifications are involved.

Comparisons between modern authors and the three main authors, who published their systems at the end of the 19th and beginning of the 20th centuries, do not statistically reveal significant differences in the content of orders (Table II). Thus, the ordinal circumscription cir·cum·scrip·tion  
n.
1. The act of circumscribing or the state of being circumscribed.

2. Something, such as a limit or restriction, that circumscribes.

3. A circumscribed space or area.

4.
 did not change significantly over the last century.

In addition to the three past authors, Gobi's (1916) classification was compared to Takhtajan's (Table III), since the latter recognized the influence of Gobi's system as well as those of Bessey and more especially Hallier. Consensus tests show that Takhtajan's classification is more similar to Gobi's for the Monocotyledons (r = 0.67003) and to Hallier's for the Dicotyledons (r = 0.66705). Likewise, results of the Mantel test indicate that Takhtajan's content of orders resembles more that of Gobi and less those of Hallier and Engler (Monocotyledons). However, differences among these results are shallow, especially the ones stemming from the Mantel test. One must realize that 1) the dicotyledon classification of Engler differs significantly from that of Takhtajan (p [greater than] 0.0025) and 2) more importantly, Bessey's classification differs statistically from Takhtajan's (consensus test; Table III). Also, results (i.e., correlations) of the Mantel test between Bessey and Takhtajan are the weakest (Table III).

Tables IV and V give the results of the Mantel tests at the superorder and subclass ranks. These comparisons examine the familial content of both taxonomic ranks and assess their similarity among authors who use them in their classification. Results originating from the dicotyledon classifications of the modern authors have been transcribed even though they remain uncertain, this uncertainty being caused by the constraint imposed by algorithms upon the number of objects that can be treated at once. The superorder content among taxonomists is statistically similar (Table IV).

At the subclass rank, dissimilarity is found between the classifications of Bessey and of Takhtajan. Contrary to this dissimilarity and the one shown by their topology (consensus test; Table III), the content of orders is similar between these authors (Mantel test; Table III). The [TABULAR DATA FOR TABLE II OMITTED] [TABULAR DATA FOR TABLE III OMITTED] asymmetry observed at the subclass rank (Liliidae of Takhtajan vs. Alternifoliae-Cotyloideae of Bessey) then vanishes at the ordinal rank. The content of subclasses also differs between Engler and Bessey or Hallier, the difference being so high that correlations are negative (Table V). No other comparisons of the content of subclasses recorded in Table V display any significant differences; therefore, the content of subclasses is statistically similar.
Table IV

Results of the Mantel tests between classifications using
super-ordinal level. P(r) = associated probability of standardized
Mantel statistic (r). The level of significance has been adjusted
through the Bonferroni method (0.01/number of pair-wise
comparisons); it is then equal to 0.00333 (Monocots) and 0.00167
(Dicots). The number of permutations has been established at 4000
for the Monocotyledons and at 10000 for the Dicotyledons. Results
shown in italics must be carefully interpreted since matrices used
for the test were near the algorithm limits.

Matrices                                         r             P(r)

Bessey/Dahlgren
Dicotyledons (234 OTUs)                       0.15225        0.00009
Bessey/Takhtajan
Dicotyledons (231 OTUs)                       0.15872        0.00009
Bessey/Thorne
Dicotyledons (222 OTUs)                       0.16315        0.00009
Dahlgren/Takhtajan
Monocotyledons (111 OTUs)                     0.89550        0.00049
Dicotyledons (361 OTUs)                       0.04308        0.00009
Dahlgren/Thorne
Monocotyledons (103 OTUs)                     0.86566        0.00049
Dicotyledons (318 OTUs)                       0.03460        0.00009
Takhtajan/Thorne
Monocotyledons (103 OTUs)                     0.79567        0.00049
Dicotyledons (331 OTUs)                       0.03596        0.00009
Table V

Results of the Mantel tests between classifications at the subclass
level. P(r) = associated probability of the standardized Mantel
statistic (r). The level of significance has been adjusted through
the Bonferroni method (0.01/number of pair-wise comparisons); it is
then equal to 0.00333 (Monocots) and 0.00010 (Dicots). The number of
permutations has been established at 4000 for the Monocotyledons and
at 10,000 for the Dicotyledons. The result shown in italics must be
carefully interpreted since matrices used for the test were near the
algorithm limits.

Matrices                                         r             P(r)

Bessey/Cronquist
Monocotyledons (51 OTUs)                      0.24186        0.00049
Dicotyledons (233 OTUs)                       0.10838        0.00009
Bessey/Engler
Dicotyledons (234 OTUs)                      -0.02089        0.15321
Bessey/Hallier
Dicotyledons (256 OTUs)                       0.02534        0.00009
Bessey/Takhtajan
Monocotyledons (53 OTUs)                     -0.05682        0.09323
Dicotyledons (231 OTUs)                       0.14034        0.00009
Cronquist/Engler
Dicotyledons (229 OTUs)                       0.23302        0.00009
Cronquist/Hallier
Dicotyledons (175 OTUs)                       0.17941        0.00009
Cronquist/Takhtajan
Monocotyledons (107 OTUs)                     0.52233        0.00049
Dicotyledons (313 OTUs)                       0.12216        0.00009
Engler/Hallier
Dicotyledons (240 OTUs)                      -0.04124        0.00019
Engler/Takhtajan
Dicotyledons (230 OTUs)                       0.16894        0.00009
Hallier/Takhtajan
Dicotyledons (175 OTUs)                       0.20300        0.00009


C. META-ANALYSES

Results of the above statistical tests were used to produce input matrices which in turn were helpful to create a classification of classifications. Input matrices can be obtained from the primary author.

1. Monocotyledons

The dendrogram and ordination [ILLUSTRATION FOR FIGURES 3 & 4 OMITTED] obtained from the results of the consensus tests involving monocotyledon classifications have produced similar relationships among the seven classifications. The dendrogram [ILLUSTRATION FOR FIGURE 3 OMITTED] reveals two groups of classifications: past (Bessey, Engler, and Hallier) and modern (Cronquist, Dahlgren, Thorne, and Takhtajan). Engler's and Hallier's classifications form a group to which Bessey's classification is grafted. The first two axes of the ordination show a structure similar to the hierarchical clustering. Modem and past classifications are separated into two groups along axis 1 and Engler-Hallier and Dahlgren-Thorne are combined in two groups whereas Bessey and Takhtajan are isolated along axis 2. Classifications of Dahlgren and Thorne always constitute a cluster in all dendrograms; this cluster is also found in ordinations.

Results of the Mantel tests using monocotyledon classifications generate two different dendrograms depending upon the method [ILLUSTRATION FOR FIGURES 5 & 6 OMITTED]. The complete linkage maximizes the grouping of Bessey's and Hallier's classifications and minimizes the similarity between Takhtajan's and the other modern classifications [ILLUSTRATION FOR FIGURE 5 OMITTED]. Nonetheless, on these dendrograms, the same cluster appears composed of the classifications of Engler, Cronquist, Dahlgren, and Thorne. The groupings in modern and past classifications are absent; both the single and complete linkages join together the classifications of Engler and Cronquist, due to the high correlation (r = 0.61962). This cluster then joins that of Dahlgren and Thorne. Complete linkage [ILLUSTRATION FOR FIGURE 5 OMITTED] groups the classifications of Bessey and of Hallier and links the resulting group to the cluster comprising Cronquist, Engler, Dahlgren, and Thorne; whereas the single linkage [ILLUSTRATION FOR FIGURE 6 OMITTED] adds successively to the same cluster the classifications of Takhtajan, of Hallier, and finally of Bessey.

The ordination obtained from the principal component analysis [ILLUSTRATION FOR FIGURE 7 OMITTED] illustrates a cluster comprising the classifications of Engler, Cronquist, Thorne, and Dahlgren; whereas Bessey's, Hallier's, and especially Takhtajan's classifications are isolated, as in the clustering analyses. Whatever the axes used, Engler and Cronquist as well as Thorne and Dahlgren are always linked together, and Bessey, Hallier, and Takhtajan undergo slight changes of position, imitating in that regard the results of clustering analyses.

2. Dicotyledons

The dendrogram produced by single linkage using the results of the consensus tests pictures the relationships among all seven dicotyledon classifications [ILLUSTRATION FOR FIGURE 8 OMITTED]. Dahlgren and Thorne again are combined in a cluster to which is joined those of Cronquist and Takhtajan. Thus, modern classifications are clustered together as in Figures 3 and 4 (produced from the monocotyledon classifications). Thereafter, Hallier, Engler, and Bessey are successively linked to the "modern" cluster. The same clusters resulted from the ordination [ILLUSTRATION FOR FIGURE 9 OMITTED]: Modern classifications form a group, whereas Hallier's joins first, followed by Engler's and Bessey's classifications.

3. Angiosperms

Results of the consensus tests done separately on the Monocotyledons and the Dicotyledons can be combined for further clustering and ordination analyses. The single linkage dendrogram [ILLUSTRATION FOR FIGURE 10 OMITTED] shows a topology different from that of the complete linkage dendrogram [ILLUSTRATION FOR FIGURE 11 OMITTED]. These two dendrograms present, however, some similarity in the grouping of the modern classifications. Not surprisingly, this grouping was also obtained when the Monocotyledons and the Dicotyledons were analyzed separately [ILLUSTRATION FOR FIGURES 3 & 8 OMITTED]. In Figure 11 the complete linkage joins the three past classifications as in Figure 3. As for the single linkage dendrogram, the classification of Bessey is isolated whereas those of Hallier and Engler constitute a cluster which then joins the modern classification cluster [ILLUSTRATION FOR FIGURE 10 OMITTED]. The ordination originating from the same input matrix separates the modern from the past classifications [ILLUSTRATION FOR FIGURE 12 OMITTED]. Again, Bessey is isolated, whereas Hallier and Engler form one group and Thorne and Dahlgren form another. The Cronquist-Takhtajan cluster, shown on the dendrograms of Figures 10 and 11, stands out again when axes 1 and 3 (not shown) are used in the same analysis that generates Figure 12.

V. Discussion

Using the results of the consensus and Mantel tests, and of the meta-analyses, the four questions raised in the introduction can be tentatively answered.

A. ARE MODERN CLASSIFICATIONS STATISTICALLY SIMILAR?

The classificatory structure and content of orders (and of superorders or subclasses) do not differ statistically among the modern classifications. The classifications of Cronquist, Dahlgren, Takhtajan, and Thorne show more global resemblance than local differences. Indeed, Figures 3 and 4 (dealing with the monocotyledon classifications), Figure 8 (dealing with the dicotyledon classifications), and especially Figures 10, 11, and 12 (dealing with the angiosperm classifications) indicate that the four modern classifications join together in a cluster on the basis of the similarities obtained from the two types of tests used.

This similarity of classifications contradicts the authors of the systems. For instance, Takhtajan (1964:160; 1980) wrote: "In the systems of classification of the higher plants proposed during the last decades there reign extraordinary differences of opinion on the content and size of the higher taxa." Cronquist (1976: 2) echoed the notion: "The difficulties, and the great differences of opinion, come instead in how to organize the families of monocots and dicots into orders and superorders or subclasses." Thorne (1977) also emphasized the enormous difference between his system and those of the other modern authors. These opinions cannot be entirely accepted. Cronquist (1969b, 1974), however, stated that his system was similar to that of Takhtajan. Since 1957, these two taxonomists frequently consulted one another about the modifications they brought to their classifications (Cronquist, 1969b, 1974). Obviously, when an author proposes a new system, he underlines the differences and originality of his own system, whereas the same author neglects these differences and, instead, emphasizes the resemblances when he or she wants to show that all systems converge due to the advancement of knowledge in systematics (Stevens, pers. comm.). Many authors have also stipulated that modern classifications resemble one another (among others, Stevens, 1986). But does this resemblance reflect a shared taxonomic approach? Tables VI-VIII allow us to examine the numerical results in greater depth and formulate an answer to that question.

First, the same conceptual constraint plays a fundamental role in all four systems: the unique origin of the Angiosperms and of most taxa (Table VIII). This constraint is not accepted by all botanists This is a list of botanists who have articles, in alphabetical order by surname. See also the list of botanists by author abbreviation and . A
  • Erik Acharius
  • Julián Acuña Galé
  • Johann Friedrich Adam
  • Michel Adanson
  • Adam Afzelius
  • Carl Adolph Agardh
 (Meeuse, 1987, 1992; Krassilov, 1991; Hughes, 1994). Second, all authors agree in viewing one of the families of the Magnolianae as the most primitive taxa in the Angiosperms and the Nymphaeales as the probable ancestor of the Monocotyledons. These similarities vanish, however, in the choice of the probable ancestor of the Angiosperms and of the most primitive family of the Monocotyledons (Table VI). Thus, for Thorne (1992a) and Dahlgren et al. (1985), the Melanthiaceae and Trichopodaceae are the primitive families of the Monocotyledons, respectively, whereas for Cronquist (1988) and Takhtajan (1987), the Butomaceae are still viewed, in a more traditional fashion, as one of the most primitive members of the Monocotyledons. Therefore, there are both differences and resemblances among the four modern authors about ancestors and primitive taxa (Table VI). Figures 8 and 10-12 support the dichotomy Dahlgren+Thorne vs. Cronquist+Takhtajan, although, as noted before, all four classifications are very similar to one another. Thus, classifications can be similar and yet be in disagreement about the origin of some taxa. Among past authors, Hallier and Engler have produced similar monocotyledon classifications [ILLUSTRATION FOR FIGURES 3 & 4 OMITTED], even if they did propose different origins of that taxon. For example, Engler saw in the Pandanales, among others, a primitive order; Hallier held the idea that the Liliaceae were the most primitive family of the Monocotyledons, which were derived from a Pro-Berberideae very similar to the Lardizabaleae (Cuerrier, in press). Here one must keep in mind that the consensus test does not evaluate sequences of primitive and derived taxa (ancestors-descendants). This information does not constitute one of the formal properties of dendrograms; a Linnean scheme can hardly represent it. For an evolutionary interpretation of modern as well as past classifications, one must have recourse to the text or to those diagrams that explicitly illustrate the genealogy of plant taxa. For example, Takhtajan's (1987) phylogenetic diagrams (genealogical ge·ne·al·o·gy  
n. pl. ge·ne·al·o·gies
1. A record or table of the descent of a person, family, or group from an ancestor or ancestors; a family tree.

2. Direct descent from an ancestor; lineage or pedigree.
 trees) cannot be extirpated from his classification. Unless one adopts a convention which is not inherent to the Linnean scheme, the phylogenetic information will not be recorded in the classification. Indeed, modern classifications insufficiently render the angiosperm phylogeny. This explains, at least in part, why modern classifications resemble one another in spite of divergent viewpoints concerning the phylogeny of the flowering plants.

Results of statistical analyses seem to underline a common taxonomic approach by the four authors. Table VIII shows the conceptual concordance among systems. All authors adhere to adhere to
verb 1. follow, keep, maintain, respect, observe, be true, fulfil, obey, heed, keep to, abide by, be loyal, mind, be constant, be faithful

2.
 the classical primitive flower concept from which they formulate their evolutionary tendencies. They believe in the same tendencies (Table VIII), notably those of Bessey (dicta Opinions of a judge that do not embody the resolution or determination of the specific case before the court. Expressions in a court's opinion that go beyond the facts before the court and therefore are individual views of the author of the opinion and not binding in subsequent cases ). Cronquist (1957, 1983), for instance, recognized the paternity The state or condition of a father; the relationship of a father.

English and U.S. Common Law have recognized the importance of establishing the paternity of children.
 of those principles and evolutionary tendencies published by Bessey (1915) and consolidated by Thorne (1958, 1963). Some of these general tendencies, however, are not accepted by Dahlgren et al. (1985). Barabe (1984, 1993: tab. 2) has shown how few changes there were among the tendencies used by Bessey, Engler, Hutchinson, and Takhtajan.

Also, in rank determination, a similar approach can be observed (Table VIII) where authors take into consideration the taxonomic experience, the need for clarity (didactic di·dac·tic
adj.
Of or relating to medical teaching by lectures or textbooks as distinguished from clinical demonstration with patients.
 value), and the size of discontinuities (i.e., gaps). For them, ranks have a practical dimension, even though they refer to the size of discontinuities. Therefore, the rank determination is based on conflicting criteria: on one hand, a need to establish conventions, and on the other [TABULAR DATA FOR TABLE VI OMITTED] [TABULAR DATA FOR TABLE VII OMITTED] hand, a need to represent phylogenetic data. Stevens (1996) mentioned that such a problem also arose in the work of Bentham and Hooker. But, then, on which principle do authors establish the size of discontinuities? Such a principle still needs to be found or made explicit.

This subjectivity in regard to the determination of taxonomic ranks has produced classifications that differ in the number of ranks used (Table VII). Similar attitudes, then, do not guarantee similar results, and here, more than anywhere else, the idiosyncrasy idiosyncrasy /id·io·syn·cra·sy/ (-sing´krah-se)
1. a habit peculiar to an individual.

2. an abnormal susceptibility to an agent (e.g., a drug) peculiar to an individual.
 of the four modern authors is particularly strong. Cronquist (1988), Dahlgren et al. (1985), and G. Dahlgren (1989a) use two ranks higher than that of family, but one of them differs for these two authors [TABULAR DATA FOR TABLE VIII OMITTED] without, however, affecting the global similarity of their classifications. Thorne (1992b) and Takhtajan (1987) had recourse to three and four ranks, respectively. Again, this difference is less important than taxon circumscription itself, since the consensus tests have shown the global similarity among the four classifications. Thus, as it will become clear below, the modification of the rank of a taxon, whose circumscription remains similar, is but a minor fact.

The taxonomic approach of the modern authors also includes the use of intermediates for establishing the affinity of taxa. Equally important is the value of new characters that authors judge in regard to their own classifications. The similarity in these notions and in the ones discussed above explains, at least in part, the resemblance among modern classifications [ILLUSTRATION FOR FIGURES 10-12 OMITTED].

These authors diverge diverge - If a series of approximations to some value get progressively further from it then the series is said to diverge.

The reduction of some term under some evaluation strategy diverges if it does not reach a normal form after a finite number of reductions.
, however, in their choice of characters; this is particularly true of Dahlgren, who emphasizes the use of phytochemical phy·to·chem·i·cal
n.
A nonnutritive bioactive plant substance, such as a flavonoid or carotenoid, considered to have a beneficial effect on human health.
 and embryological characters. In Table VIII, the other notions showing dissimilarity among authors either have no consequence for classifications or are simply deceptive. For example, the concept of heterochrony, used mostly by Takhtajan (1943, 1954, 1972, 1976, 1983), applies more at the level of explanation than at taxon delimitations and the classificatory approach per se.

The similarity among modern classifications is imputable im·put·a·ble  
adj.
Possible to impute or ascribe; attributable: imputable oversights.



im·put
 not only to common principles and to a reciprocal influence but also, and more especially, to the use of past classifications as a starting point. Here, statistical tests performed between modern and past classifications were of particular interest.

B. ARE MODERN CLASSIFICATIONS SIGNIFICANTLY DIFFERENT FROM THOSE PUBLISHED AT THE END OF THE 19TH AND BEGINNING OF THE 20TH CENTURIES?

Modern classifications present a high resemblance to past classifications, to which they were compared (Table II). One must keep in mind, however, that classifications published at the end of the 19th and beginning of the 20th centuries (e.g., van Tieghem [1918] and Camel [1889]) did differ from those that we have chosen to compare (i.e., Bessey, Engler, Hallier). These classifications seem to have had a minor influence, at the most, on modern ones. These classification schemes never gained any importance among botanists as in the case of Bessey, Hallier, and especially Engler, and modern authors do not refer to them (except in the case of Takhtajan, who mentioned the influence of van Tieghem on his system [D. Stevenson, pers. comm.]). A sociohistorical study would be needed to understand why some classifications became popular on the international scene while others did not.

Even though the number of taxa differs among the different categories used by past (Bessey, Engler, Hallier) and modern authors (Cronquist, Dahlgren, Takhtajan, Thorne), the global similarity of classifications was not significantly affected. Results of the Mantel tests indicate that taxa have similar delimitations. Table VII clearly shows the increase in the number of families among modern authors in comparison to past ones. This modification is a minor one since newly recognized families by modern authors were taxa relegated to a lower rank by past taxonomists. During the 19th and 20th centuries, many taxa had their rank changed (especially from subfamily subfamily /sub·fam·i·ly/ (sub´fam-i-le) a taxonomic division between a family and a tribe.

sub·fam·i·ly
n.
A taxonomic category ranking between a family and a genus.
 or genus to family) without changes in their circumscription (Watson, 1964; see also Stevens, 1986, 1994; Thorne, 1992b: 244; Cuerrier et al., 1996). Watson (1964: 280) stated that"too many botanists seem to accept traditional systems uncritically and, unwittingly, to base their conclusions on nineteenth-century taxonomic philosophy." Watson's explanation is plausible, as the similarity among modern and past classifications tends to show (Table II).

Cronquist (1979) drew attention to the correspondence between the pre-Darwinian concept of natural classification and that of evolutionary classification. For him, an evolutionary approach In computer science, an evolutionary approach is an acquisition strategy that defines, develops, produces or acquires, and fields an initial hardware or software increment (or block) of operational capability.  to classification is compatible with that which prevailed before Darwin. This compatibility or correspondence appears to be connected to the concept of classification itself, a concept that does not allow an easy importation of phylogenetic data. Table VIII clearly shows among modern authors the practical and didactic goal of classifications. This constraint minimizes the impact of the Darwinian theory on the structure of classifications. For instance, this constraint can already be seen in the work of Bessey, Engler, and Baillon (Cuerrier et al., 1996); while Stevens (1984b) discussed it at some length. Therefore, a classification is concerned with two main features: its stability, which serves, among other criteria, as an argument between taxonomists to value their classifications (Barabe & Brouillet, 1982); and its memory device, which helps the human mind to grasp more easily the large amount of information secured in a classification.

In spite of the great similarity between modern and past classifications, some clustering analyses have underlined a temporal separation between these two groups of classifications [ILLUSTRATION FOR FIGURES 3, 4, 11 & 12 OMITTED]. This separation could be the consequence of the reciprocal influence among modern authors and of recent developments in different botanical disciplines, leading to an increasing number of available characters. In other respects, the clustering analysis, using the results of the Mantel tests on the monocotyledon classifications, shows that Engler shares more relationships with the modern authors than with the past ones [ILLUSTRATION FOR FIGURES 5-7 OMITTED]. This result will allow us to answer the third question raised in the introduction.

C. ARE MODERN CLASSIFICATIONS MORE SIMILAR TO BESSEY'S THAN TO OTHER PAST CLASSIFICATIONS AS UNDERLINED BY CORE, BENSON, STACE, AND WOODLAND?

Many taxonomists insist that Bessey's classification (1915) is the major influence on the modern systems [ILLUSTRATION FOR FIGURE 1 OMITTED]. Analyses conducted in this study indicate that modern classifications are, on the contrary, less similar to that of Bessey than to that of Engler or Hallier, in the structure as well as in the content of orders. It should be noted that the classifications of Cronquist and Bessey differ, even though the first author firmly recognized the influence of the latter: "We are all - or nearly all - Besseyans" (Cronquist, 1988: 162). He also admits that "large blocks, and groups of blocks, of the Engler system One of the prime systems of plant taxonomy, the Engler system was devised by Adolf Engler.

According to Engler, Syllabus der Pflanzenfamilien (1924) the main groups of plants are:
  • I. divisio Schizophyta
  • II.
 remain in all present systems, merely rearranged with respect to each other" (Cronquist, 1988: 162). This statement appears to be true: Orders circumscribed circumscribed /cir·cum·scribed/ (serk´um-skribd) bounded or limited; confined to a limited space.

cir·cum·scribed
adj.
Bounded by a line; limited or confined.
 by modern authors resemble less those of Bessey and Hallier than those of Engler (Table II; [ILLUSTRATION FOR FIGURES 5-7 OMITTED]), notwithstanding the supposed absurdities of the Engler system emphasized by Thorne (1973). Metataxonomic analyses show that Bessey's classification is either isolated or linked to the other past classifications; it is never directly connected to the modern classifications [ILLUSTRATION FOR FIGURES 3-12 OMITTED]. It seems that Bessey's position as an outlier outlier /out·li·er/ (out´li-er) an observation so distant from the central mass of the data that it noticeably influences results.

outlier

an extremely high or low value lying beyond the range of the bulk of the data.
 can be explained by his use of an analytic or dichotomic approach based on few characters. Cuerrier et al. (1992) underlined this difference in approach between Bessey and Engler, resulting in fewer taxa at the level of the most inclusive ranks in Bessey's classification. Indeed, in Table VII one can see that Bessey uses two taxa at the subclass rank, whereas Cronquist uses five or six. At the rank of superorder, Bessey's classification has five taxa whereas Thorne's, Dahlgren's, and Takhtajan's classifications have 19, 25, and 39 taxa, respectively. The large divisions in Bessey do not statistically differ, however, from those of the modern authors (Tables IV and V).

According to according to
prep.
1. As stated or indicated by; on the authority of: according to historians.

2. In keeping with: according to instructions.

3.
 Takhtajan (1980: 235), his own system derives from those of Hallier and Gobi. Nonetheless, the monocotyledon classifications of Takhtajan and Hallier are less similar to one another than that of Hallier is to the other modern authors (Table III). The difference in the number of ranks between Takhtajan (4) and Hallier (1) contributes to diminish the similarity between their monocotyledon classifications. Moreover, Hallier (1912) starts his monocotyledon classification with the Liliaceae, which originate from a plant similar to Berberis Berberis

genus in the plant family Berberidaceae; contains berberine, a pyridine alkaloid; causes cardiomyopathy and congestive heart failure. Called also barberries.
, whereas Takhtajan (1987), in accordance with Bessey, begins his with the Butomaceae, which he then derives from the Nymphaeales.

Results of the Mantel tests have also shown that the classifications of Takhtajan and Bessey differ at the level of subclasses. This difference appears to be due to the asymmetry caused by Takhtajan's subclass Liliidae Noun 1. subclass Liliidae - one of four subclasses or superorders of Monocotyledones; comprises 17 families including: Liliaceae; Alliaceae; Amaryllidaceae; Iridaceae; Orchidaceae; Trilliaceae
Liliidae
, since this subclass contains the majority of families whereas the three other subclasses are fewer in taxa. The classification of Bessey shows more symmetry (Cuerrier et al., 1992). One should also note that Takhtajan used four subclasses and Bessey only two (Table VII). These results again indicate that the classification of Bessey is unrelated to the modern ones, like that of Takhtajan.

D. WHAT ARE THE RELATIONSHIPS AMONG THE SEVEN CLASSIFICATIONS STUDIED IN THIS ARTICLE? ARE THESE RELATIONSHIPS THE SAME AS THE ONES DEPICTED BY BENSON, LU, AND WOODLAND?

The genealogy of the systems shown in Figure 1 has not been corroborated by metataxonomic analyses. As for the modern systems, Figure 1 reflects less the classification than the authors' opinions about ancestors and primitive taxa. In fact, Engler has influenced recent classifications more than Bessey did [ILLUSTRATION FOR FIGURES 5-7 OMITTED]. As for the primitive monocotyledon taxa, the ideas held by Dahlgren and Thorne refer to those of Hallier, not to those of Bessey or Engler (Table VI). Classifications of Dahlgren and Thorne show high similarities both at the level of the topology and the content of superior categories [ILLUSTRATION FOR FIGURES 3-12 OMITTED]. Indeed, Thorne mentions the influence that Dahlgren's studies on the Monocotyledons had on his own classification. These two authors collaborated on an article dealing with the Myrtales, but, nevertheless, differences of opinion were clearly stated in the article (Dahlgren & Thorne, 1984). Also, Thorne and Dahlgren used dahlgrenograms to represent taxon relationships, whereas Cronquist and Takhtajan chose minimum spanning trees, which allow direct ancestor-descendant links to be expressed (Table VIII). Therefore, a bird's-eye view (or a transversal section) for the first two authors and a side view (or a longitudinal section) for the latter two authors can be observed. This difference might be considered shallow if the "bubbles" of the dahlgrenogram are linked with one another. One should consider, however, dahlgrenograms as a bird's-eye view of Steiner trees (i.e., with no direct ancestor-descendant links). Throughout Takhtajan's numerous papers, trees have remained versions, more or less divergent, of a first genealogical tree a family lineage or genealogy drawn out under the form of a tree and its branches.

See also: Genealogical
 published in 1942 (104, [ILLUSTRATION FOR FIGURE 2 OMITTED]).

Some results [ILLUSTRATION FOR FIGURES 8-11 OMITTED] have grouped together the dicotyledon or angiosperm classifications of Cronquist and Takhtajan. This grouping probably reflects the Russian's influence upon the American botanist: Indeed, Cronquist (1969a) embraced the subclasses of Takhtajan's Magnoliatae. Moreover, Cronquist knew the Russian language and could read the works of Russian botanists.

Thus, a genealogical hierarchical tree representing the filiation fil·i·a·tion  
n.
1.
a. The condition or fact of being the child of a certain parent.

b. Law Judicial determination of paternity.

2. A line of descent; derivation.

3.
a.
 of the angiosperm systems of classification cannot be accepted as the ideal model: A network seems to be a more comprehensive model [ILLUSTRATION FOR FIGURE 13 OMITTED]. Indeed, the analyses have shown that all four modern systems are similar not only to one another but also to that of Bessey and, more especially, to those of Engler, Gobi, and Hallier. Moreover, this network of reciprocal influences follows a similar taxonomic approach by the authors; indeed, Table VIII indicates the concordance between notions or criteria used by the four modern authors. The fundamental principles at the basis of modern systems are those of Bessey. Nevertheless, they resemble those of Hallier (1912) and Engler (1909). Engler's influence is seen in ordinal delimitations (Table II). This influence can hardly be distinguished from those of Bessey and Hallier, since their contents of orders also resemble those of the modern authors (Cuerrier et al., 1992). In fact, the contents of orders are similar in all seven classifications. But it is more than reasonable to assume that the importance of Engler's Pflanzenfamilien for floras and herbaria may well have increased the influence of Engler's classification on modern taxonomists. It then follows that the Englerian system has become more familiar. Similarities between past and modern authors are multiple: the type of tree used to represent evolution, the rank determination, the assessment of the value of characters and types of characters used, the purposes of classification, the use of intermediates, the constraint of angiosperm monophyly (except for Engler), and the use of a logical language. Stevens (1986) also emphasizes the similarity of many systematic principles between evolutionary and pre-Darwinian classifications.

We wrote that principles of the modern taxonomists were those of Bessey. But, then, on what evidence do modern authors accept those Besseyan principles, when we know that Bessey built his classification first and afterwards proposed his dicta (Cuerrier et al., 1996)? And on which criteria should we establish their use at the different levels of the Linnean hierarchy? One must keep in mind that these are very general principles and are meant to be used at the angiosperm level. Therefore, principles should be handled carefully in establishing the affinities within taxa. Leroy (1993) also mentions that Takhtajan used evolutionary tendencies that were too general when he established the affinity of some orders or families.

We can resume the taxonomic approach of the four modern systematists while quoting Cronquist (1988: 2): "We try to group together the things that are most alike in all respects, and to separate them progressively from things they are progressively less like." In Buffon (1749), Fries (in Lindley, 1826), and de Candolle (1859), from the 18th century to the 20th, taxonomists had the same preoccupation: to group what is similar and to separate what is not. Stevens (1986: 325) wrote that the "correspondence between evolutionary and pre-Darwinian systematic practice is extensive." The difficulties encountered by Bessey at the beginning of the 20th century in establishing the homologies of structures and the affinities of taxa still remain two daunting daunt  
tr.v. daunt·ed, daunt·ing, daunts
To abate the courage of; discourage. See Synonyms at dismay.



[Middle English daunten, from Old French danter, from Latin
 problems even today (Stevens, 1994; Cuerrier et al., 1996).

Classifications have changed little over centuries, even though new data have been discovered and used. This is in part because recent systems are still based on morphological data stemming mostly from the flower, and in part because they were influenced by past classifications. But other complementary explanations further illuminate the resemblance between modern and past classifications. We will discuss below three of these important explanations.

E. OTHER COMPLEMENTARY EXPLANATIONS

1. Typology typology /ty·pol·o·gy/ (ti-pol´ah-je) the study of types; the science of classifying, as bacteria according to type.

typology

the study of types; the science of classifying, as bacteria according to type.
 and Conceptual Constraint

The four modern taxonomists claim explicitly or implicitly to posit their work in the classical concept of the plant and more especially of the flower. Therefore, data are always constrained by a theory that has not changed since Goethe and A.-P. de Candolle. In spite of the works published by Lam, Melville, Meeuse, and Croizat, the classical theory is still accepted without question. And yet, the ideas developed by Croizat (1964) - on strobili, flowers, and inflorescences in general; on the distinction between carpel carpel

One of the leaflike, seed-bearing structures that constitute the innermost whorl of a flower. One or more carpels make up the pistil. Fertilization of an egg within a carpel by a pollen grain from another flower results in seed development within the carpel.
 and ovary ovary, ductless gland of the female in which the ova (female reproductive cells) are produced. In vertebrate animals the ovary also secretes the sex hormones estrogen and progesterone, which control the development of the sexual organs and the secondary sexual ; and on the definition of morphological structures in particular - illustrate that we should not take for granted the foundation of classification systems on a theory of morphology. Similarly, Meeuse (1987, 1992) warns us about such notions as monophyly and flower, while Sattler (1991, 1992) demonstrates the dynamic and continuum notion of the plant. These works, as Leroy (1993) noted, generate numerous ideas and fruitful reasoning. These studies force us to recognize, at least, the elements of constraint often hidden in postulates tacitly acknowledged to be at the very basis of classification systems. Meeuse (1987: 43) wrote: "The published systems of classification of the Flowering Plants purported to be phylogenetic by their originators are purely typological, even if only for the reason that they are exclusively based on certain features of extant plants which were molded into a preconceived pre·con·ceive  
tr.v. pre·con·ceived, pre·con·ceiv·ing, pre·con·ceives
To form (an opinion, for example) before possessing full or adequate knowledge or experience.
 pattern of concepts and interpretations." Indeed, taxonomists of the modern systems try to unravel the angiosperm origin through a comparison of extant plants (e.g., see Cronquist, 1969c; Takhtajan, 1958, 1980, 1991). The resulting classification is more horizontal than vertical. The latter would integrate paleobotanical data and the time aspect. If, as understood by modern systematists, evolution is mostly a vertical process, then a methodology that would lead to a horizontal classification improperly represents the phylogeny. Indeed, Linnean classifications cannot support and illustrate verticality. They hardly formulate the affinity of taxa, let alone their origin.

2. Parallelism An overlapping of processing, input/output (I/O) or both.

1. parallelism - parallel processing.
2. (parallel) parallelism - The maximum number of independent subtasks in a given task at a given point in its execution. E.g.
 and Convergence

The importance of new data is often muted by using the concepts of parallelism and convergence. They are frequently used to devalue the importance of a character, without recurring to phylogenetic analysis. Thus, these concepts remain superficial. How can one discuss convergence without an organogenetic and ecological study within a phylogenetic frame? For example, Cronquist (1980) and Thorne (1981) disagree with Verb 1. disagree with - not be very easily digestible; "Spicy food disagrees with some people"
hurt - give trouble or pain to; "This exercise will hurt your back"
 Dahlgren's groupings based on phytochemical data. For them, iridoids would have evolved many times during the angiosperm history; therefore a case of parallelism without any analysis. Contrary to Thorne, Dahlgren (1983) reduced to nothing the value of stamen stamen, one of the four basic parts of a flower. The stamen (microsporophyll), is often called the flower's male reproductive organ. It is typically located between the central pistil and the surrounding petals.  centrifugal centrifugal /cen·trif·u·gal/ (sen-trif´ah-gal) efferent (1).

cen·trif·u·gal
adj.
1. Moving or directed away from a center or axis.

2.
 development in some Caryophyllales, since it is merely a case of convergence. Even though Dahlgren's statement seems correct a priori a priori

In epistemology, knowledge that is independent of all particular experiences, as opposed to a posteriori (or empirical) knowledge, which derives from experience.
, a phylogenetic study using flower ontogenetic on·to·ge·net·ic
adj.
Of or relating to ontogeny.
 data should have been done first. For many cases of supposed convergence, Thorne (1958) used his own personal experience in determining the value of characters. Unless this personal experience is based on a character analysis, such a taxonomic judgment constitutes an act of authority, not an accurate research of taxa affinities.

3. Cognitive Constraint and Historical Inertia

Systematists sometimes use their own classifications to determine the value of new characters. For Cronquist (1980: 4), "our general scheme is now good enough to warrant the greatest of caution in accepting radical changes." This assertion suffices for Cronquist and for Takhtajan (1980) to neglect data that do not agree with their classification schemes. Such an approach imparts a static element to classifications. Results of the tests we performed in the present study show that classifications have changed little over a century; therefore, they underline the static element of classifications whether or not such statism stat·ism  
n.
The practice or doctrine of giving a centralized government control over economic planning and policy.



statist adj.
 is imputable to the approach mentioned above. Stevens (1984b, 1986) pointed out that taxonomists try to confirm the existing taxonomic structure rather than to evaluate the one suggested by their data. Taxa formed over two centuries ago encapsulate en·cap·su·late
v.
1. To form a capsule or sheath around.

2. To become encapsulated.



en·cap
 a historical inertia linked to the concept of stability. The usefulness of classifications, and the pressure implicitly exerted to make them remain so, restricts the importance of taxonomic changes that may come from new data. After centuries of utilization, names of taxa are burdened with a historical weight which renders all modifications difficult to apply (Stevens, 1986, 1994). This inertia also explains the similarity of past and modern classifications. For Stevens (1986: 329), "taxa initially recognized seem immune from criticism and provide a pattern to which other taxa must conform." The worldwide use of the Englerian classification might have influenced, in an unconscious manner, taxon delimitations up to now.

F. MOLECULAR BIOLOGY molecular biology, scientific study of the molecular basis of life processes, including cellular respiration, excretion, and reproduction. The term molecular biology was coined in 1938 by Warren Weaver, then director of the natural sciences program at the Rockefeller  AND CLASSIFICATION

Due to the constraints discussed above, it will not suffice in taxonomy to only use new data, as is often the case in morphological and molecular studies, to create the necessary opening for a complete revision of classifications. One should reexamine re·ex·am·ine also re-ex·am·ine  
tr.v. re·ex·am·ined, re·ex·am·in·ing, re·ex·am·ines
1. To examine again or anew; review.

2. Law To question (a witness) again after cross-examination.
 the meaning of "classification," and try to avoid evaluating data with existing classifications as a standard. The choice of taxa entering the input matrix for cladistic analyses should be based on more than one classification. For example, the analyses of Dicotyledons and Monocotyledons by Chase et al. (1993) and Qiu et al. (1993) rely on Cronquist (1981) and Dahlgren et al. (1985), respectively. For Donoghue and Sanderson (1992:359), "it is also ironic that even those who are most wary of morphological data nevertheless lean on it heavily in designing their own research, namely in choosing which groups to work on, which subgroups to sample, and so on." The use of traditional classifications in cladistic analyses remains a type of constraint, based on taxon definitions and on a scheme of homology homology (hōmŏl`əjē), in biology, the correspondence between structures of different species that is attributable to their evolutionary descent from a common ancestor.  that is hardly explicit (see also Meeuse, 1982). Since the four modern classifications are similar in their structure and the content of their taxa, as well as in their foundation and their taxonomic approach, it would be preferable to take into account opinions and phylogenetic hypotheses formulated by unorthodox authors (Hughes, Krassilov, Lam, Meeuse, Croizat). Molecular systematics offers new data; the latter are welcome, but other types of data (morphological, serological serological

pertaining to or emanating from serology.


serological test
one involving examination of blood serum usually for antibody.
, phytogeographical, etc.) should not be excluded. Donoghue and Sanderson (1992; 342) rightly wrote: "Our argument is not against the use of molecular data; rather, it is against ignoring relevant morphological evidence."

Perhaps the importance of molecular studies in comparison to the morphological ones can be more readily appreciated at the level of preconceived ideas that Meeuse (1987) has so often denounced. For him, the angiosperm phylogeny is based "on the repetition of ingrained tenets or the use of the phrase: established ideas" (Meeuse, 1987: x). Molecular phylogenies have a chance to bypass a priori ideas from which morphological structures suffer. They can be used to examine the evolution of morphological structures (see Soltis & Soltis, 1995).

Molecular studies have already shown that the series of cladograms obtained from rbcL sequences not only resemble the classifications of Thorne (1992b) and Dahlgren et al. (1985) but, moreover, differ from the classification of Cronquist (1988). One should keep in mind, however, that Thorne's (1992a, 1992b) work follows the development of molecular systematics, which might explain its similarity with the study of Chase et al. (1993). Also, it seems that the divergence between the cladogram based on molecular data and Cronquist's classification is caused in part by the simplicity and the clarity that spring from the latter classification. Indeed, Cronquist's classification responds to a need for convention (didactic); it is thus a logical tool. Therefore, it is hardly surprising that it differs from a cladogram based on different conventions. It follows that Cronquist's classification and Chase et al.'s cladogram are two different objects. For example, the Ranunculales and Magnoliales of Qiu et al. (1993) are not conceptually those of the modern taxonomists. A quantitative comparison between modern classifications and a classification taken from the cladogram of Chase et al. (1993) would help to establish whether the resemblances mentioned above are statistically significant and, therefore, whether Thorne's classification is really similar to the cladogram (and that Cronquist's classification is really dissimilar).

VI. Conclusion

The results of the statistical tests used in this study are in agreement with the opinion stating that the modern classifications do not differ from those published at the end of the 19th and beginning of the 20th centuries. In spite of what authors have written, not only do modern classifications resemble one another, but also their content of orders has remained broadly that of the past taxonomists (Bessey, Engler, Gobi, Hallier). Modern classification systems of the flowering plants are Besseyan only in their adoption of an angiosperm ancestor near the Bennettitales; they are not Besseyan because their classifications have followed the classificatory scheme and taxon circumscription of Bessey. Thus, the genealogies of systems drawn by Lu (1981) and Woodland (1991) are of limited value only. They can be viewed as a complementary viewpoint to ours, which renders the affinity among classifications as a network [ILLUSTRATION FOR FIGURE 13 OMITTED]. There is complementarity in the sense that the network has been based on comparisons involving the classificatory structure and/or the content of orders, whereas Lu's and Woodland's genealogical trees have been founded on the origin of the Angiosperms expressed by taxonomists.

This study reveals a historical inertia (classification relying on morphological structures) as well as the idiosyncrasies of all modern authors of classification systems. This means that modern taxonomists started their classification from existing classifications, modifying it as works and studies by morphologists, anatomists, organogeneticists, phytochemists, and others became available. At times, their own classification is used as a reference for establishing the value of "new" characters. It hardly comes as a surprise, then, that results of statistical analyses do not show an important amount of difference among the classifications compared, since circularity is therein inevitable. It is difficult to separate what creates the "stable" in the stability of classifications: either taxa described a century ago are still viewed as good taxa in general (they were already well-circumscribed) or taxa are maintained because the conceptual approach to taxonomy has remained identical over years. One explanation may be better than the other or both may have explanatory powers, yet they both imply that past classifications have had an undeniable influence on those of today.

The use of intermediates and of evolutionary tendencies and the importance of constructing taxon sequences by modern taxonomists are reminiscent of the approach followed more than a century ago by past taxonomists. The idea of tendency in itself is teleological tel·e·ol·o·gy  
n. pl. tel·e·ol·o·gies
1. The study of design or purpose in natural phenomena.

2. The use of ultimate purpose or design as a means of explaining phenomena.

3.
 (Bernier, 1984); it refers back to ideas like progression and progress, orthogenesis, creative force (i.e., Bildungstrieb). Many of these notions have more than a century of existence. Also, in the writings of modern taxonomists, one can still find a whiff of scala naturae. Indeed, notions like progress and perfection are to be found in the works of Cronquist (1969a), Takhtajan (1958, 1959, 1973), and Thorne (1992a, 1992b). For instance, these authors apply the metaphor of evolutionary scala when discussing the phylogenetic part of their work. In short, the taxonomic language has changed very little since the 19th century (Stevens, 1994).

Recent advances in molecular biology have proven important for taxonomic works, but a classification system is more than just a heap of data - it is also principles, methods, and a theory. The revision of the modern classifications which should spring from molecular works (as well as from any taxonomic studies), must run the risk of questioning the actual foundation of classifications. Also, molecular studies must keep clear of the constraints imposed by recent systems. The point of view still in use remains that of the classical or traditional taxonomy from Goethe, A.-L. de Jussieu De Jussieu, the name of a French family which came into prominence towards the close of the sixteenth century, and was known for a century and a half for the botanists it produced. , and A.-P. de Candolle to Cronquist, Dahlgren, Takhtajan, and Thorne. To paraphrase a Bachelardian idea (Bachelard, 1972): One could write that, in taxonomy, prudence has became an epistemological e·pis·te·mol·o·gy  
n.
The branch of philosophy that studies the nature of knowledge, its presuppositions and foundations, and its extent and validity.



[Greek epist
 obstacle. With molecular data, taxonomy is on the threshold of renewal, because these data are independent of ideas and notions molded by a long history which has burdened the terminology used in morphology (notions like what is a flower, what is primitive, etc.). Therefore, results of molecular analyses could help to reinterpret re·in·ter·pret  
tr.v. re·in·ter·pret·ed, re·in·ter·pret·ing, re·in·ter·prets
To interpret again or anew.



re
 the evolution of morphological characters and to reorient Re`o´ri`ent   

a. 1. Rising again.
The life reorient out of dust.
- Tennyson.

Verb 1.
 fruitful organogenetic studies. To rethink plant classifications from their basis will obviate the epistemological obstacle laid down by history more than a century ago.

VII. Acknowledgments

Numerous articles needed for the completion of this manuscript were made available by S. Madrinan (Universidad de Los Andes Universidad de Los Andes (Spanish: "University of the Andes") may refer to:
  • University of the Andes, Colombia
  • University of the Andes, Chile
  • University of the Andes, Venezuela
  • Los Andes Peruvian University in Peru
) and P. F. Stevens (Harvard University Harvard University, mainly at Cambridge, Mass., including Harvard College, the oldest American college. Harvard College


Harvard College, originally for men, was founded in 1636 with a grant from the General Court of the Massachusetts Bay Colony.
). Dr. Stevens also read and commented on an earlier version of this manuscript. The English was kindly revised by J. St. John Winter. We acknowledge V. Perreault of the Inter-Library Loan Service (Universite de Montreal) for his ability to find so many articles from obscure periodicals. We thank F.-J. Lapointe for contributing his knowledge of tree comparisons, and J. Podani (L. Eotvos University [Budapest]) for kindly sending the MATTEST and MTCOM programs of his software SYN-TAX V; without his help, results would not have been so complete. Members of the IRBV IRBV Institut de Recherche en Biologie Végétale (Plant Biology Research Institute - Canada)  Ecological Laboratory and H. Veronneau were kind enough to lend me their computers. J. Brisson has developed and made available his application permitting matrix conversions needed for all the different analyses. This paper has been written with the support of a FCAR grant (to AC) and a FCAR group grant (to LB and DB).

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Author:Cuerrier, Alain; Brouillet, Luc; Barabe, Denis
Publication:The Botanical Review
Date:Oct 1, 1998
Words:11579
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