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The telome theory.

INTRODUCTION

Morphologists have long been concerned with the fundamental nature of the vascular plant body--with the relationships between root, stem and leaf, and with the origin of reproductive structures. Attempts at analysis on the basis of either formal or phyletic morphology have given rise to a number of theories dealing with the organization of the plant body. Those related to the flowering plants are summarized by Arber (1930) and need not concern us here, since they are of historical interest only. A theory which encompasses a consideration of all vascular plants, and which is based primarily upon studies of the lower Tracheophyta, recent and fossil, has had greater vitality--so much so, indeed, that it has permeated the thinking of many morphologists and paleobotanists to a degree not realized by those who have not followed the literature in these fields. This is the telome theory, perhaps better termed the " telome concept". Among other aspects the telome concept has assisted in a clarification of our ideas on the relationships between root, stem and leaf, and emphasizes that the plant body is an axis, with a descending portion, the root, and an aerial portion, the shoot, whose appendages are merely modified parts of the stem.

Before considering the telome theory itself, it is desirable to point out that, like many another concept, it did not arise de novo, but was foreshadowed by the work of earlier investigators. Reference will repeatedly be made to a fundamental principle advocated by all of these workers which is an essential part of the telome theory itself. This is the principle of overtopping, shown diagrammatically in its simplest form in Fig. 1. This principle involves the belief that the oldest method of branching was dichotomous (a dichopodium). If, at each fork, one of the two branches becomes stronger than the other, a sympodium will result, with an apparent main axis composed of one of the two forks. Continued development of the main axis results in a monopodium which gives off successive axes (lateral branches). This scheme is of course susceptible to many modifications and variations.

[FIGURE 1 OMITTED]

The works of Bower, Lignier, Potonie and others provide a background of the telome theory. Bower early pointed out (1884) that the leaf is fundamentally a flattened branch system--" Thus the stem and leaf would have originated simultaneously by differentiation of a uniform branch system into members of two categories" Potonie (1912) has summarized his views advanced in earlier papers. Briefly, he held that the frequently dichotomous branching of early land plants was derived from forked (dichotomously branched) marine algae, and that both stem and leaf in the higher plants were derived by an overtopping process. Bower later (1916) in a more important paper confirms and elaborates on the views of earlier writers that the leaf of the ferns and other primitive vascular plants is fundamentally dichotomous in organization, and that the monopodial pinnate leaf is derived from dichotomy through a sympodial stage. The broad expanded leaf has resulted from progressive webbing during this development. Primitive distal dichotomy is still found in the leaves of many living ferns but is absent from the leaves of angiosperms because of the early arrest of apical growth, the major part of the leaf lamina resulting from intercalary growth. The findings of Bower are directly contradicted by the work of Wagner (1952) who considers that dichotomy in the leaves of living ferns--both foliar dichotomy and dichotomy in the veins--is a modification of an ancestral pinnately organized system.

The stimulating papers of Lignier (1903, 1908a, 1908b) embody many of the concepts later incorporated into the telome theory. He conceived the sporophyte of the most primitive land plant as a dichotomously branched system of cylindrical cauloids, which bore leaf-like appendages, or phylloids, together with rhizoids. That portion of the cauloid system which was subterranean became transformed into the root system. The cauloids bore terminal sporangia, possibly bivalved and similar to those of Psilophyton. The appendages, or phylloids, were considered to be enations, comparable to the leaves of Lycopodium. From this Psilophyton-like type were derived two lines of evolution--the lycopods, on the one hand, which retained their phylloids and dichotomous cauloids, and, on the other, all other vascular cryptogams, the pteridosperms, all gymnosperms and angiosperms. In this latter evolutionary line the principal cauloids of the plant body became transformed into a monopodium by the sympodial method, and thus a true stem evolved. The groups of terminal cauloids, however, became dorsiventral in symmetry; the cauloids within each cluster fused together, thus forming a flat plate or leaf blade in which the existence of the constituent cauloids was indicated only by the dichotomous venation. The appearance of the megaphyllous leaf brought about the disappearance of the phylloids which had become useless. According to Lignier, then, there is no fundamental difference between stem and leaf, and the root is merely the subterranean portion of the aerial shoot. This, the modern point of view, was advanced by Lignier as early as 1903. Various writers, including Torrey (1932, p. 38) and Bertrand (1947, p. 152), have attempted to visualize, in diagrammatic form, the chief points of Lignier's description of a primitive land plant.

THE TELOME THEORY

The above viewpoints and others related to them have been synthesized into the telome theory by Prof. Walter Zimmermann, of Tubingen. This is discussed in two chief works (Zimmermann, 1930, 1949) together with a number of separate articles, cited in the bibliography, although the latter does not pretend to include all of Zimmermann's papers on this subject. Brief discussions are also found in Eames (1936), Bower (1935), Stebbins (1950) and Takhtajan (1953). Only the main points of the theory are presented here; an exhaustive account and applications of the theory to the various groups of vascular plants are included in Zimmermann's Phylogenie der Pflanzen (1930). It is convenient to list the main tenets of this theory, as Zimmermann (1945b) has done, under three main heads, namely, (a) the forerunners of primitive land plants, (b) the primitive land plant, and (c) derivation of the plant body of the higher land plants:

THE ANCESTORS OF PRIMITIVE LAND PLANTS. The algal ancestors of the land plants were green algae which lived in the tidal zone of the Cambrian and Silurian seacoasts. The plant body of such forms was undifferentiated, radially symmetrical and composed of dichotomously forked branches (primitive telomes). Internally they possessed a central strand of mechanical tissue (sclerenchyma) useful in their tidal zone habitat. They had isomorphic alternation of generations (Fig. 2), that is, each individual had two kinds of plant bodies, alike except that one, the diploid sporophyte, bore sporangia, while the other, the haploid gametophyte, bore gametangia. Zimmermann thus adheres to the homologous theory of alternation of generations (for further discussion of this vs. the antithetic theory, see Eames, 1936). Zimmermann concedes that it is very difficult to name a definite kind of green alga as an ancestral form. The lower Silurian Codiaceae and Dasycladaceae, for example, should be eliminated because of their high specialization. He suggests that the ancestral type be sought among less well known forms, such as representatives of the genus Oldhamia.

[FIGURE 2 OMITTED]

THE PRIMITIVE LAND PLANT. The telome theory is psilopsidcentered, for the Upper Silurian to Mid-Devonian Psilophytales (Hicklingia, Taenocrada, Zosterophyllum, Rhynia, Horneophyton, Psilophyton, etc.) are visualized as exemplifying the sporophyte of the ancient vascular plants. The sporophyte was derived from algae by the evolution of heteromorphic alternation of generations, in which the sporophyte became much enlarged relative to the gametophyte generation. Zimmermann contends that the gametophyte generation necessarily remained small if fertilization was to be accomplished. The primitive vascular plant was composed only of telomes and mesomes. Telomes are the terminal portions of a dichotomizing branch system. They are of two kinds, fertile, composed of a vascularized stalk and its terminal sporangium, and vegetative, or sterile, termed "phylloids " (this word is used in a different sense from that of Lignier who applied it to enations, or emergences). A telome ends at the first subjacent forking, the mesomes constituting the internodes between each two forkings. In the course of ontogeny each mesome was first a telome, being relegated to the mesome position as growth proceeded. Fertile and sterile telomes, following evolutionary development, may occur in groups composed of either sterile of fertile telomes or mixtures of the two called "telome trusses " (Telomstande).

The sporophyte was at first relatively undifferentiated, with no distinction between stem and leaf, and became erect by negative geotropism. The anatomy was simple, the central strand of sclerenchyma of the algal progenitors becoming a strand of tracheids forming the earliest type of stele, the protostele. The predecessors of sieve tubes were elongated parenchymatous cells which differentiated around the central strand. The aerial portions developed stomates, and the basal portion, rhizoids or root hairs. Zimmermann's conception of the origin of the root as found in the higher plants is essentially that of Lignier--it is descended from the creeping underground branches of an ancient thallus-like plant. The reproductive organs were terminal sporangia which produced aerial spores. Their formation was accompanied by meiosis, as indicated by the findings of spore tetrads in the most ancient fossils of vascular plants. Since gametophytes of the earliest land plants have never been found, it is assumed that, like those of living forms, they were delicate and not readily fossilized.

The mosses also evolved from algae but did not emerge upon the land until much later than vascular plants, probably in the Carboniferous. The sporophyte in this group, however, became parasitically dependent upon the gametophyte and correspondingly reduced to a single fertile telome (Fig. 2). The gametophyte, again presumably because of demands for water for fertilization, remained relatively small and more or less thallophytic in structure. The transformation of isomorphic to heteromorphic alternation of generations is assumed to have occurred at the time of the transition to the land.

DERIVATION OF HIGHER LAND PLANTS. The higher land plants evolved from the telome trusses of the primitive forms along three parallel lines, as indicated in the following table:
 Primitive Land Plants

 Lycopsida Sphenopsida Pteropsida

Examples Selaginella Equisetum Phanerogams
 Lycopodium Ferns
 Lepidophyta

Transitional Protolepido- Hyeniales Protopteridales
 Groups dendrales


Among the Protolepidodendrales Zimmermann lists Protolepidodendron, Barrandeina, Duisbergia, Baragwanathia, Drepanophycus and doubtfully Asteroxylon (or Thursophyton). Included in the Protopteridales are Protopteridium, Rhacophyton, Condrusorum, Aneuropteris, Cephalopteris, Cladoxylon and Pseudosporochnus.

According to Zimmermann, the shoot axis and leaves of the higher Tracheophytes developed from the earliest vascular plants by the operations of a few important kinds of morphological specializations termed " elementary processes" because they may have proceeded independently of one another. Frequently only one or another elementary process became manifest, especially in the transition groups. The following, according to Zimmermann, are the chief kinds of elementary processes:

a). Overtopping. In this process two sister branches of a fork (mesomes or telomes), equal in most primitive forms, become unequal. The stronger branch became vertical and formed the axis, while the weaker branch was pushed aside and became overtopped. Thus the shoot and its appendages, the leaves, became differentiated through equal dichotomy to a sympodial and finally to a monopodial system (Fig. 3 A). Overtopping is also involved in the evolution of the leaf. In a pinnately veined leaf, for example, the overtopping mesomes formed the midvein, and the overtopped mesomes constituted the lateral veins. As a natural consequence of the formation of the shoot by overtopping, leaves and pinnae were primitively alternate instead of opposite in position.

b). Planation. Branching in more than one plane is replaced by fan-shaped dichotomy, bringing all telomes and mesomes into one plane (Fig. 3 B). This applies especially to the evolution of the leaf.

c). Webbing or Fusion (Fig. 3 C). This may involve only the formation of parenchymatous tissues between telomes and mesomes (webbing) or webbing accompanied by the fusion of vascular tissues. This concept is believed to afford an explanation of the origin of both the leaf and the stele of the stem. In the simplest cases the phylloids became united by the development of parenchymatous tissues betwen them, thus forming a foliar appendage with open dichotomous venation. Webbing accompanied by overtopping, as described above, led to a leaf with pinnate venation. If fusion of vascular bundles also occurred, the leaf venation became reticulate. Similarly in the shoot axis a purely parenchymatous webbing led to the polystelic condition, as in Selaginella. Anastomoses of steles also occurred (Fig. 3 F), producing the various types of steles found in vascular plants.

d). Reduction of a telome system may lead to the formation of a single needle-like leaf (Fig. 3 D).

[FIGURE 3 OMITTED]

This last principle involves the origin of the microphyllous leaf of the Lycopsida. It is widely accepted that primitively large leaves (pteropsid) represent major branch systems that have become flattened and appendicular; extreme reduction of such a major branch system may of course also lead to a needle-like leaf. Similarly there is little disagreement that the sphenopsid leaf is derived from a branch system, with the difference that the number of telomes entering into the composition of such a leaf is smaller than in the case of the macrophyllous leaf. With respect to the origin of lycopsid (Lycopodium, etc.) leaves, however, there has been much disagreement (Browne, 1935). Zimmermann (1938) contends that in the " Lycospross " small dichotomous lateral branch systems were reduced to a uni-nerved leaf following webbing and reduction. The lycopod leaf is considered by others to represent an enation, that is, a leaf arising de novo from a naked axis, and secondarily developing a single vein. Eames (1936), for example, suggests that the resemblances between the leaves and sporangium position of Asteroxylon, Drepanophycus and Baragwanathia, on the one hand, and of Lycopodium, on the other, together with stelar structure, support the enation hypothesis. The view that lycopod leaves are enations, instead of being telomic in origin, is also accepted by Takhtajan (1953).

Fertile telomes have become arranged in various ways by elementary processes similar to those which operated to produce the vegetative structures. Among these transformations are aggregation, fusion, overtopping, reduction, recurvation (Fig. 3 E) and incurvation. The sporophyll, according to the telome theory, is composed of one or more telomes or both telomes and mesomes.

The axillary position of the sporangia in the Lycopsida is derived by Zimmermann as shown in Fig. 4 A-D. An aggregation of fertile and sterile telomes followed by reduction of supporting mesomes and reduction of sporangium number leads to the condition in which but one phylloid and one sporangium remain. Further reduction would lead to fusion of the sporangium with the shoot axis, thus becoming cauline in position, as is considered by Zimmermann to be true of some living species of Lycopodium. The sporophyll of the Sphenopsida is believed to have arisen by a downward bending of the sporangium toward a common axis (recurvation, leading to anatropy, Fig. 4E-H), followed by fusion and expansion of telomes and mesomes into a peltate structure. The sporophylls of the Pteropsida (ferns and seed plants) are visualized as formed by overtopping, fusion, and reduction, which lead finally to sporangia or sori along the margins of a pinnate sporophyll. Incurvation (Fig. 4 M-Q), or unequal growth on the two opposite flanks of an organ, often leads to shifting of the sporangia to the lower surfaces of the sporophylls. Further incurvation leads to the simple carpel, and fusions of carpels give rise to the various types of ovaries (Zimmermann, 1930, Fig. 231). Zimmermann holds that the flower started with radial sporangial clusters, the sporophyll arising, following planation, from aggregations of mesomes. This is assumed to have occurred previous to any aggregation of sporophylls into a strobilus.

[FIGURE 4 OMITTED]

SOME APPLICATIONS OF THE TELOME THEORY

A theory is useful if it allows known facts to be explained. On this basis a few selected fields will be explored to see whether morphological facts have been clarified or may be interpreted usefully in the light of the telome theory.

FERNS AND SEED FERNS. The telome theory offers one explanation for the nature of the aerial portion of the plant body of the Ophioglossaceae. This is commonly interpreted in terms of the more highly specialized leaves of the leptosporangiate ferns as composed of a single frond; it has apparently an anterior sterile and a posterior fertile segment. The fertile segment is held by some authors (Eames, 1936; St. John, 1952) to be composed of two fertile basal pinnae which have fused and become erect. Zimmermann (1930, 1942) believes, however, that the fertile branch of Botrychium represents one of the two limbs of an ancient dichotomy, the sterile segment being the other. This viewpoint is also accepted by Bower (1935) who regards Ophioglossum, with its reticulate venation, as a derived type. Zimmermann's interpretation of the nature of the Botrychium shoot is supported by Chrysler (1945) who finds that the vascular system of the aerial portion of Botrychium may be interpreted as composed of a series of modified dichotomies (sympodium). The first of these dichotomies externally visible is of course found at the level where the fertile and sterile branches separate. He concludes that the shoot system is composed of an aggregation of telomes and mesomes that have arisen by repeated dichotomies, and that the Ophioglossaceae may well have arisen more or less directly from the Psilophytales.

The anatomy of some species of the Medutlosaceae supports certain aspects of the telome theory. Stewart and Delevoryas (1952) have shown that the vascular system of the stem in Medullosa heterostelica consists of a dichotomizing system of branches. Some of these fuse in the central axis, while others continue to dichotomize in the leaf bases. It is suggested that the nature of the primary vascular system indicates that this plant had a relatively low degree of organ differentiation and that it constituted a primitive branching axis exhibiting an early stage of foliar and cauline differentiation.

The telome theory is also involved in the interpretation of the nature of the plant body in the coenopterid ferns. The complex branch systems arising from the stems of the plants of this group have been referred to as "fronds ". Delevoryas and Morgan (1953), however, have discovered true expanded foliar structures in Botryopteris. It is postulated that these foliar organs have arisen from branch systems by a process of planation in the ultimate segments followed by webbing, while the lower axes retain the primitive character of three-dimensional branching.

It is probable that a considerable measure of agreement could be obtained for the thesis that ferns and' pteridosperms developed from similar Psilophyton-like ancestors with terminal fertile telomes'. In the course of evolution the fern sporangia shifted in position from terminal on rachis branches to marginal and finally superficial on the laminae of well-developed pinnules (Halle, 1935). Fern pinnules, in turn, were originally formed by the fusion of sterile telomes in a single plane. The reproductive structures of the pteridosperms were likewise phylogenetically terminal; later the microsporangia became aggregated into synangia and the seeds were borne on modified or unmodified portions of the frond. Baxter (1949) suggests that the development of seeds in the pteridosperms, at least in some cases, may have preceded the evolution of fern-like leaves, that is, the pteridosperms may be early seed plants which later developed leaves through evolutionary processes similar to those which operated in the true ferns. While the male reproductive organs of the pteridosperms have been considered the result of aggregation of fertile telomes, Schopf (1950) has presented evidence that in the medullosan Dolerotheca the male fructification is an elaborate campanulate "septangium", composed of radially arranged and paired rows of sporangia. It is suggested that this structure is not a synangium but is comparable to a single fertile telome divided by numerous septa. This septation hypothesis is further extended to include all pteridosperms, thus providing a basic evolutionary difference between the seed ferns and other groups.

CONIFERS AND GINKGO. The most extensive and fruitful application of the telome concept is found in the studies of Dr. Rudolph Florin on the evolution of the vegetative and reproductive structures of the cordaites and early conifers. His chief publications in this field (Florin, 1938-45) have been summarized in part (Florin, 1951).

The male reproductive organs of the Cordaitales consisted of elongated clusters of strobili, each strobilus composed of spirally disposed sterile scales and fertile scales, or microsporophylls. The later were intermixed with the sterile scales or confined to the apex of the axis of the dwarf shoot. The vascular bundle of the microsporophyll was dichotomized at the apex, and the sporophyll terminated in a cluster of four to six upright sporangia. These microsporophylls were primitive organs, developed from a small dichotomous branch system terminated by telomes bearing erect, elongate and cylindrical sporangia. Their dichotomy was cruciate, indicating a primitive radial symmetry. The "sporophylls", therefore, are not metamorphosed leaves, but evolved, like the female sporophylls, parallel with foliage leaves. They retained the primitive features characteristic of early land plains, cruciate dichotomy and terminal sporangia. The male reproductive organs of the early conifers, on the other hand, resembled those of modern conifers, such as the Pinaceae. The dorsiventral microsporophylls bore two sporangia on the lower surface and were arranged in a close spiral on the axis of a strobilus, sterile scales being absent. The transition from the cordaitean to the early conifer condition cannot be traced in the fossil record, but Wilde (1944) has described a reduction series in the living podocarps which parallels the evolutionary changes leading to the simple male coniferous cone. She concludes that the male cone of the modern conifers is homologous with the entire seed-scale complex of the female cone. In the Taxaceae, viewed by Florin as distinct from the true conifers, the peltate microsporophyll of Taxus is considered primitive for the group and derived from a simple dichotomized branch system with terminal erect sporangia.

The female reproductive structures of the Cordaitales were borne in lax clusters of strobili. These strobili were arranged in two vertical rows in the axils of alternating or subopposite bracts. They were short shoots, composed of a radially symmetrical axis carrying spirally arranged appendages, some of which were fertile and bore terminal ovules. These short shoots were the forerunners of the seed-scale complex of the modern Taxodiaceae and Pinaceae. The most primitive ovule-bearing organs were in Cordianthus pseudofluitans of the Upper Paleozoic. Each organ was a cruciately dichotomized terminal telome system carrying two or more pendent ovules at the apex (Fig. 5). Each ovule, in turn, was derived from three telomes, a central fertile one (megasporangium) and two lateral phylloids, which became modified into an integument, free from the sporangium except at the base. Other, younger, species of Cordianthus exhibited reduction from the above condition in that the telome system, composed of mesomes and telomes (termed by Florin a "megasporophyll") was much shortened, unbranched, and bore but one ovule at the apex. Certain of the sporophylls had also become sterilized. Florin holds that these cordaitean dwarf shoots were differentiated from the telome systems of primitive vascular plants by the processes of overtopping, aggregation and fusion.

[FIGURE 5 OMITTED]

The Upper Carboniferous and Lower Permian conifers also possessed various modifications of a radially symmetrical fertile dwarf shoot in the axil of a bract. These shoots were aggregated into groups more cone-like than those of the Cordaites and were arranged spirally, not in two rows. In Lebachia the seed-scale complex in the axil of a bract was composed of a short axis with spirally disposed scales, all but one of these sterile (Fig. 6). In Ernestiodendron (Fig. 7) the axis carried two to 30 scales, including four to six " megasporophylls " with terminal inverted ovules. The entire cluster was somewhat flattened.

[FIGURES 6-7 OMITTED]

The female cones of the Upper Permian and Mesozoic conifers, however, approach more closely to those of living forms. In Pseudovoltzia, for example (Fig. 8), the axis of the short shoot had become rudimentary and bore five distal sterile scales and two or three basal stalk-like megasporophylls, the whole, with the subtending bract, corresponding to the bract and ovuliferous scale of a modern pine cone. The fertile portion of the dwarf shoot had thus become basal, and the entire seed-scale complex had developed dorsiventral symmetry. In other forms the sterile scales were fused, and with them were incorporated the stalk-like megasporophylls. The seed-scale complex of modern conifers is thus a modification of a radially symmetrical dwarf shoot and its accompanying bract, the sector facing the bract having become totally suppressed. The term "sporophyll" obviously cannot be applied to this seed-scale complex.

[FIGURE 8 OMITTED]

The female reproductive organs of the Taxaceae are not arranged in cones. The shoot axis in Taxus terminates in a single erect ovule, the aril being a secondary outgrowth from the axis. The ovule represents a single fertile telome which had attained its terminal position after a process of overtopping. The reproductive structures of this family are therefore not derived from the Paleozoic Cordaitales or conifers.

One of the most clearly defined phylogenetic relationships involving the telome concept was worked out by Florin (1949) between the Lower Permian Trichopitys heteromorpha and the living Ginkgo biloba. The female reproductive structures of Trichopitys were produced on a long shoot which was seated in the axil of a stout leaf (Fig. 9A). This subtending leaf, about 12 cm. long, branched by repeated dichotomies into four to eight narrow segments, and is regarded by Florin as a scarcely flattened branch system--the forerunner of the broad photosynthetic leaf of the living ginkgo. The long shoot bore finely dichotomously dissected leaves, each subtending an ovule-bearing complex (megasporangial truss). This axillary short shoot (Fig. 9 B) was composed of an axis bearing three to 20 fertile branches, each terminated by a single inverted ovule, and with an ovule terminal on the axis itself. Such a megasporangial complex is believed to have arisen by overtopping, fusion and reduction from a radial, uniform and regularly dichotomizing branch system with terminal sporangia. Each ovule-bearing branch, in turn, is considered to be a small telome system with one central fertile telome and several sterile ones which form the integument. According to Florin, "the leaf and the fertile complex developed in its axil are obviously homologous structures, each representing a branch system which has been specialized for a different function. Originally each system probably constituted part of a larger and more complicated lateral telome system, which became differentiated into a dorsal vegetative and a ventral reproductive part. By reduction of the basal mesomes and their incorporation in the axis of the long shoot, the axillary position of the fertile complex was gradually acquired".

[FIGURE 9 OMITTED]

Florin believes Trichopitys to be of the greatest importance in the interpretation of the female reproductive structures of Ginkgo biloba. These have been interpreted variously; most commonly, perhaps, as a strobilus composed of a stalk and two megasporophylls, the collars, each bearing an ovule. But ovule-bearing peduncles of Ginkgo have been found bearing up to 15 (most commonly a maximum of ten) stalked ovules (Fig. 10). These are believed by Florin to be of the same nature as the female complexes of Trichopitys, the type with only one or two sessile ovules being reduced. The collar is considered to be a secondary feature, formed by the arrest in growth of the tip of the ovular stalk; it is modern in origin and has no phyletic significance. The view that the modern Ginkgo biloba has no megasporophylls was also held by Zimmermann (1930). The ovule-bearing complex of Ginkgo is, instead, an axillary short shoot, bearing fertile telomes, each invested with an integument developed from sterile telomes. The axis or peduncle upon which the ovules are borne was originally dichotomized into uni-ovulate sporangiophores.

[FIGURE 10 OMITTED]

ANGIOSPERMS. The telome theory, largely founded upon paleobotanical evidence, has been accepted as a working hypothesis by many morphologists and paleobotanists concerned with vascular plants other than the angiosperms. Application of the concept to this latter group has encountered objections, and morphologists--students of floral anatomy in particular--are divided in their point of view, some regarding it with favor, while others are doubtful or opposed. As far as vegetative organs are concerned, attempts have been made (Lam, 1948; Van der Hammen, 1947-48) to find traces of ancient dichotomies in the venation of angiosperm leaves, and, a number of plants are listed which exhibit this type of venation. That these dichotomies are actually ancestral retentions is uncertain. Referring to the above papers, Foster (1950) has expressed the view : "But the 'typical' venation of the leaves of angiosperms is 'closed' and it seems highly doubtful to the writer that the 'forked' veins which frequently occur in pinnately and palmately veined leaves, should be interpreted as 'traces of ancient dichotomies'". The telome concept is also involved in a bizarre classification by Lam (1948, 1947-48) of the " Cormophyta " into two large groups, the Stachyosporae, in which the sporangia are essentially terminal on fertile telomes, and the Phyllosporae, in which the sporangia are borne on many-telomed fronds. The angiosperms are also divided into stachyosporous and phyllosporous families. Lam's papers have been discussed by Eames (1950) and so thoroughly refuted that no further consideration need be given them here.

The telome concept as applied by students of floral anatomy to the stamen (Wilson, 1937, 1942, 1950; Reece, 1939) and the carpel (Hunt, 1937) has recently been reviewed by Purl (1951). An extensive discussion of the nature of the stamen and carpel is also given by Lawrence (1951). Concepts of the fundamental nature of the stamen and carpel have bad a long and involved history. As Arber (1937, 1950) points out, Goethe and, De Candolle recognized that carpel and stamen are equivalent to foliage leaves in an abstract sense--they did not derive floral structures from foliage leaves. Following Darwin's Origin of Species, and even earlier, Goethe's concepts were forced into an evolutionary frame, and the idea became general that floral organs are derived phylogenetically from leaves. This is reflected in recent textbooks in such statements as the carpel is an ovule-bearing floral leaf, the pistil is a specialized leaf, and that each of the parts of a flower is of foliar origin. Surely there is no evidence that stamens and carpels are derived from leaves, although Puri (1947) states that he " considers the filament and connective equivalent to petiole and leaf blade, respectively".

A strict return to the "classical" theory--the concepts of Goethe and De Candolle--requires that we view the stamen and carpel as phyllomes--foliar organs in the abstract sense, with no phylogenetic meaning. In modern terminology the stamen and carpel may be interpreted as specialized leaf-like appendages which evolved with the leaf along parallel lines, but which were never primarily photosynthetic organs. The opponents of the telome concept, if this reviewer interprets their position correctly, would insist that all consideration of the nature of the carpel and stamen stop at this point. For example, Canright (1952) states that he regards the broad expanded types of microsporophylls found in certain Ranalian families as relatively unmodified fertile phyllomes--that is, presumably, fertile leaves or leaf-like organs. Those favorably disposed toward the telome theory, on the other hand, while accepting it as a fact that stamens and carpels resemble leaves in their anatomy and in other ways, would go further and apply the concept to the origin of these specialized leaf-like structures and possibly also to the origin of the foliar organs of angiosperms.

According to the telome concept, the proangiosperm might be visualized as having a plant body built up of mesomes and differentiated into a vegetative and a reproductive region. The vegetative portion terminated in branch systems composed of phylloids; these systems originally branched dichotomously in three planes, but later, after such processes as overtopping, webbing and planation, they became converted into expanded foliar appendages. The spore-producing members were also originally dichotomously branched systems composed of mesomes and fertile telomes which early became aggregated into some kind of primitive " flower". The carpel may have arisen from telome trusses composed of fertile telomes and their subjacent mesomes. These became converted into a leaf-like structure following planation, webbing and monopodial growth. This structure is leaf-like because it evolved parallel with the foliar organ itself and may long have retained the photosynthetic capacity of the units of the plant body from which it arose. Other, perhaps smaller, aggregations of telomes may have given rise to the stamen. This structure probably evolved in various directions; in many, perhaps most forms, following sterilization, reduction and fusions a dichotomous branch system became reduced to a single stalk with four terminal sporangia. Later these sporangia became marginal in position. In other forms the terminal telomes may have undergone the same processes as in the carpel, and a broad expanded microsporophyll resulted, as in the Magnoliaceae (Can, right, 1952). Carpels and stamens may therefore be regarded as the highly modified descendants of ancient telome branch systems--they are probably not sporophylls, although this term may be retained for purposes of convenience. This point of view is, of course, open to discussion, to criticism and to modification. The telome theory may even eventually be discarded, in whole or in part, but it will have served a useful purpose, as it already has done, in suggesting new avenues of approach and in stimulating further investigations on the origin of the vegetative and reproductive structures of the vascular plants of the land.

LITERATURE CITED

1. ARBER, AGNES. 1930. Root and shoot in the angiosperms: A study of some morphological categories. New Phyt. 29. 297-315.

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CARL L. WILSON

Dartmouth College, Hanover, New Hampshire
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