The Inflorescence: Introduction.I. Introduction
Traditionally, inflorescences have been seen as morphological structures of the same nature as other organ systems and, therefore, as amenable to description. As similarities were seen among the kinds of inflorescences described, attempts to "classify" inflorescences (i.e., raceme ra·ceme
An optically inactive chemical compound. , cyme Cyme (sī`mē), ancient Greek city of W Asia Minor, on the Ionian Sea and N of the present Smyrna in W Asian Turkey. It was the largest and most important of the 12 cities of Aeolis. In the late 5th cent. B.C. , etc.) were made. Most of these classifications were typological, based on the overall form of the inflorescence inflorescence
Cluster of flowers on one or a series of branches, which together make a large showy blossom. Categories depend on the arrangement of flowers on an elongated main axis (peduncle) or on sub-branches from the main axis, and on the timing and position of flowering. , although others (i.e., Weberling, 1989) attempted to categorize inflorescences according to their purported pathway of development. This analysis moved the typological system from one based on form alone to one based on development. Given the pivotal role of inflorescences in the function and diversification of many plant groups, and recent developments in phylogenetics and developmental biology, it is time to look at inflorescences from a variety of new perspectives. A symposium, The Inflorescence: A New Perspective, was held 6 August 1997 in Montreal, Canada, at a joint meeting of the Botanical Society of Americ a and the Canadian Botanical Association with the American Institute of Biological Sciences. In this introduction, we review some of the relevant background on inflorescences, particularly on inflorescence meristems, because the subject was not covered in individual talks in the symposium. The articles included in the present issue of The Botanical Review will explore diverse approaches and assess new knowledge about inflorescences.
The symposium from which this series of articles was derived had several specific aims, together with the general one of bringing the subject of inflorescences up to date. These aims are expressed first as questions:
1. Is the inflorescence a transitional stage between the vegetative state and the flower, or does it stand as a distinct, morphological entity? Several of the articles address this question, directly or indirectly.
2. How can inflorescence form and development be utilized to explore evolution? The articles on particular plant families or assemblages by James Grimes, by Elizabeth Harris, by Alan Charlton and Usher Posluszny, and the abstract by Andrew Douglas, address the succession of changes during development of inflorescences within related groups of taxa, as well as their evolutionary significance.
3. Do specific genes control inflorescence form and expression? Susan Singer and her co-authors discuss the known genetic evidence for control of inflorescence form.
4. Are inflorescences restricted to angiosperms? What type of inflorescence is most primitive, or ancestral? The abstract of the talk that Dennis Stevenson delivered at the symposium briefly addresses some aspects of these questions.
5. Do intermediates exist between inflorescence and flower? Usher Posluszny and Alan Charlton have addressed this question in some of their publications, and others will be discussed briefly in this introduction.
III. Terms and Definitions
The term "inflorescence" was introduced by Linnaeus (reviewed by Parkin, 1914), who also introduced many of the common terms for types of inflorescences, such as "cyme," "raceme," "spike, and "umbel." The inflorescence was defined by Bentham (1892) as "the arrangement of the flowering branches and the flowers upon them"; he also applied the term to the flowering branch itself. Both meanings, for the pattern and for the actuality, are still used. Many inflorescences, however, defy categorization or may comb e aspects of more than one type. An example is the inflorescence of Gleditsia triacanthos, a system of racemes except that it is terminated by an ebracteate flower, as in a cyme.
Eichler (1878) illustrated ground plans of a huge variety of inflorescences, as well as of flowers, in his definitive descriptions 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 families and their representative genera. Excellent descriptions of the various types of inflorescences can be found in Rickett (1944, 1955), Troll (1964), and Weberling (1989), and brief summaries of types can be found in a variety of sources, such as Stebbins (1974), Heywood (1985), and Harris (1994).
Troll (1964, 1969) devised a precise typological framework to describe homologies among the wide range of inflorescences, which Weberling (1965, 1989) extended and translated into English. Inflorescences are divided into monotelic and polytelic types, roughly corresponding to determinate DETERMINATE. That which is ascertained; what is particularly designated; as, if I sell you my horse Napoleon, the article sold is here determined. This is very different from a contract by which I would have sold you a horse, without a particular designation of any horse. 1 Bouv. Inst. n. 947, 950. and indeterminate modes of growth. A monotelic inflorescence (i.e., a cyme in older terminology) has a terminal flower, and each side branch, or "paracladium," can branch repeatedly. Additional terminology distinguishes among variations in the branching pattern of paracladia. A polytelic inflorescence (i.e., a raceme) always has a "main florescence." The side branches, if iterations of the main branch, are called "coflorescences." Side branches (second-order branches, or coflorescences) may produce third-order branches, or "partial florescences," which together make up a coflorescence.
Tomlinson (1983) used a typological system, proposed by Halle and Oldeman (1970), to describe vegetative branching in trees. The models in this system emphasize vegetative growth but also consider inflorescence placement and form. Tree growth reflects position of inflorescences, since these terminate growth of a branch in many cases. Computerized simulations of inflorescences have been devised by Pankhurst (1994). Frijters (1978) proposed mathematical models to simulate the structure and growth of inflorescences.
IV. Evolutionary Trends in the Inflorescence
A particular form of inflorescence typifies some plant families, such as the compound umbel of Apiaceae (Umbelliferae), the ultimate spike of Gramineae, the capitulum capitulum /ca·pit·u·lum/ (kah-pit´u-lum) pl. capi´tula [L.] a small eminence on a bone, as on the distal end of the humerus, by which it articulates with another bone. or head of Asteraceae, the helicoid cyme of Boraginaceae, and the spadix of Araceae. Although each of these character states for inflorescence tends to be stable in its group, many instances of further evolution have occurred in some of these groups. One example is the secondary and tertiary "condensation" of heads in certain Asteraceae (Harris, 1994, 1995, this issue). Among mimosoid legumes, Grimes (1992, 1995, this issue) has found synapomorphies concerning developmental aspects of inflorescences, especially those involving heterochronies. In Proteaceae, flower pairs characterize one large subfamily subfamily /sub·fam·i·ly/ (sub´fam-i-le) a taxonomic division between a family and a tribe.
A taxonomic category ranking between a family and a genus. , Grevilleoideae, and distinguish it from three others. Developmental comparisons among tax a have shown these flower pairs to be two-flowered short shoots, in which the inflorescence apex soon becomes inactive (Douglas & Tucker, 1996a).
In some families, exceptions to the "typical" inflorescence exist. In Fabaceae (Leguminosae), the basic inflorescence type is a raceme, but evolutionary shifts have led to pseudoracemes in five tribes of Papilionoideae (Tucker, 1987) and to cymose inflorescences in a few taxa, including caesalpinioid Dialium and related genera (Tucker, 1998; other cymose legumes are discussed therein). In Gesneriaceae, pair-flowered partial florescences terminating each cyme are characteristic (Weber, 1973), but transitions to "normal" cymes have also been found (Weber, 1978).
The parallelisms in inflorescence architecture that can be seen among unrelated plant families led Stebbins (1974) to speculate about the adaptive value of different types of inflorescence. He asserted that the more specialized types of inflorescence tend to occur in angiospermous an·gi·o·sperm
A plant whose ovules are enclosed in an ovary; a flowering plant.
angi·o·sper trees (even in those of relatively primitive families, such as Platanaceae and Winteraceae), a fact that he attributes to adaptive selection.
Parkin (1914) reviewed the history of work on inflorescences and urged the study of their evolution, a novel idea for his time. His was one of the first of several publications that speculate about whether the solitary flower, or some form of inflorescence, was ancestral among early angiosperms. He favored the solitary flower as ancestral, an idea that supported the hypothesis that woody trees, such as those of Magnoliaceae, were among the most primitive of living families. Developmental arguments could appear to support the idea that a solitary flower is less derived than an inflorescence, because the transition of vegetative apex to the flower (as in Magnoliaceae) is direct. However, the primitiveness of the Magnolia type of flower has been challenged on the basis of its highly complex vasculature vasculature /vas·cu·la·ture/ (vas´ku-lah-chur)
1. circulatory system.
2. any part of the circulatory system.
n. (Stebbins, based on Skipworth & Philipson, 1966) and of the likelihood of pollinator selection (Gottsberger, 1974). Solitary flowers, Stebbins (1973, 1974) pointed out, are usually derived in primitive families; as an example, the solitary flower of Zygogynum is the exception in Winteraceae, in which all other taxa have axillary ax·il·lar·y
Relating to the axilla.
Located in or near the armpit.
Mentioned in: Mastectomy
of or pertaining to the armpit. cymes or dichasia. Rickett (1944) favored a dichasium as the ancestral type of inflorescence, while Stebbins (1974) argued convincingly for a leafy cyme being most primitive, based on its correlation with other primitive character states in the more basal angiosperm families.
Evolutionary trends in inflorescence form were discussed by Stebbins (1973, 1974). He asserted that, because of so much parallelism in inflorescence architecture among angiosperm families, one can identify groups of taxa in which particular types of inflorescence have evolved because of their adaptive value. A possible example is Gottsberger's assertion (1974) that the massive, solitary flower may be maintained in Magnoliaceae because of selection by beetle pollinators. Wyatt (1982) investigated adaptive pressures on some quantitative aspects of inflorescence architecture (size, number of flowers as they influence pollination pollination, transfer of pollen from the male reproductive organ (stamen or staminate cone) to the female reproductive organ (pistil or pistillate cone) of the same or of another flower or cone. , and number of fruits produced). He pointed out that the function of the inflorescence (display for pollinators) often conflicts with that of the infructescence (structural support of sometimes heavy fruits, and positioning of those fruits for seed dispersal). He cited several taxa of Encaceae as examples, in which the inflorescence has pendent flowers, while the fruits are held upright . Another example is that of cauliflory, which Stebbins (1 p74) attributed to avoiding competition for pollinators in rainforest conditions. Endress (1994) attributed the cauliflorous habit to a need for access and support by bats, while Wyatt suggested that it is a response to the need for an adequate support for the often massive or numerous fruits produced.
Comparisons of inflorescence development, variation, and evolution within particular families or orders of angiosperms are of great interest, but it is not possible to review them individually in this article. Significant comparative work on inflorescences includes those on Alismatidae (Posluszny, 1983), Anacardiaceae (Barfod, 1988), Arecaceae (Fisher & Moore, 1977; Moore & Uhl, 1982; Uhl, 1972, 1976; Uhl & Dransfield, 1984; Uhl & Moore, 1978), Chloranthaceae (Endress, 1987), Fagaceae (Macdonald, 1979b), Hydrocharitaceae (Kaul, 1970), Helobiae (Posluszny, 1993), Leguminosae (Tucker, 1987), Loranthaceae (Kuijt, 1978), Myricaceae (MacDonald, 1971, 1978, 1979a, 1980), Myrtaceae (Briggs & Johnson, 1979; Weberling 1988), Potamogetonaceae (Posluszny, 1981; Posluszny & Sattler, 1973, 1974a, 1974b, 1976), Proteaceae (Douglas & Tucker, 1996a, 1996b), Zannichelliaceae (Posluszny & Tomlinson, 1977), Zingiberales pro parte (Kirchoff, 1986, 1988, 1990, 1997, 1998; Kirchoff& Kunze, 1995), several Amentiferous families (Vacdonald, 1971; Sattler, 1973), and Gramineae, Cyperaceae, Juncaceae, and Liliaceae (Barnard, 1955, 1957a, 1957b, 1958, 1960, respectively).
V. Inflorescences in Nonflowering Plants
One speaker at the symposium, Dennis Stevenson, spoke on the topic of inflorescences in nonflowering vascular plants. He emphasized differences between cones and strobili, rather than their aggregation on the plant. Although no article was submitted on this topic for publication, we can offer a few remarks, A strict definition of inflorescence as a aggregation of flowers eliminates possession of inflorescences from all nonflowering plants, However, if an inflorescence is more loosely defined as a cluster or aggregation of sexually reproductive structures, one can then look for homologies between flowering and nonflowering plants.
A particularly relevant article by Endress (1996) analyzed many aspects of development, including "inflorescences" of the reproductive structures of Gnetales. Gnetalean taxa, though usually having unisexual reproductive axes, occasionally have bisexual reproductive structures. For this latter reason, they are more easily compared with angiosperms than are taxa (such as Goniferales) that have male and female structures on different branches or different plants. One common feature among Gnetalean taxa (Ephedra ephedra: see ephedrine. , Gnetum) is the presence of female organs distal to (or above) the male organs in "inflorescences." Efforts to find homologies break down, however, when one looks at the whorled positions of female "flowers," or the even more puzzling arrangement of male flowers in Gnetum.
The dearth of inflorescence-like arrangements of reproductive structures among nonflowering plants may reflect the fact that aggregation into inflorescences (and accompanying reduction of foliar foliar
pertaining to or having the quality of leaves. structures as bracts) is one of the many innovations that arose among angiosperms and made them so successful in competition with nonflowering plants.
VI. Histology and Histogenesis histogenesis /his·to·gen·e·sis/ (-jen´e-sis) the formation or development of tissues from the undifferentiated cells of the germ layers of the embryo.histogenet´ic
n. of the Inflorescence Meristem meristem (mĕr`istĕm'), a specialized section of plant tissue characterized by cell division and growth. Much of the mature plant's growth is provided by meristems.
External changes may include a shift from leaf to bract bract
Modified, usually small, leaflike structure often positioned beneath a flower or inflorescence. What are often taken to be the petals of flowers are sometimes bracts—for example, the large, colourful bracts of poinsettias or the showy white or pink bracts of initiation, more rapid initiation of those bracts, a change in phyllotaxy, and/or elongation ("bolting") of the subtending axis. All of these are controlled by the apical apical /ap·i·cal/ (ap´i-k'l) pertaining to an apex.
1. Relating to the apex of a pyramidal or pointed structure.
2. meristem. As the plant shifts from vegetative growth to the reproductive phase, the vegetative apical meristem ("shoot apex") of a plant usually changes in a variety of ways, reflecting physiological changes. Three contrasting types of apical meristem commonly can be distinguished: the vegetative apical meristem (Fig. 2); the inflorescence apical meristem (Figs. 1, 4, 5); and the floral apical meristem (Figs. 3, 6). Inflorescence apical meristems commonly are wider and higher than vegetative meristems in the same plant, and these size increases at onset of flowering are usually accompanied by changes in cellular configuration (Philipson, 1949; Gifford, 1954; Gifford & Corson, 1971; Hagemann, 1963). Inflorescence apical meristems can have any of three possible cellular arrangements (dep ending on the 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. ): "tunica/corpus," "mantle/core," or "zonate" configuration. These differ basically in planes of cell division in different regions of the apex.
A. TYPES OF APICAL MERISTEMS
Numerous investigations of apical meristem configuration were conducted in a wide variety of angiosperm taxa between 1940 and 1970 (reviewed in Gifford, 1954; Gifford & Gorson, 1971). Vegetative apices of angiosperms commonly display a tunica/corpus arrangement or (rarely) zonation zo·na·tion
1. Arrangement or formation in zones; zonate structure.
2. Ecology The distribution of organisms in biogeographic zones. , while floral apices may have either a tunica/corpus, zonate, or mantle/core cellular configuration. Zonate apical configuration has been reported occasionally in all three types of apical meristems. The apical cellular organization of inflorescence apical meristems varies, depending on the species; the most common is mantle/core (many examples: Gregoire, 1938; Philipson, 1946, 1947b), or zonate (Popham & Chan, 1952; Rauh & Reznik, 1953; Popham, 1964; Tucker, 1980, 1981, 1982).
In a tunica/corps arrangement (Figs. 3,5), the tunica consists of one or more meristematic cell layers over the surface, in each of which cell division is only anticlinal, perpendicular to the outer surface. The meristematic corpus is a "block meristem" below the tunica, and its cells can divide in any plane. Representatives of several monocotyledonous monocotyledonous
said of plants the seeds of which have only one cotyledon. families (Barnard, 1955, 1957a, 1957b, 1958, 1960; Endress, 1995) have a tunica/corpus configuration in the inflorescence meristem. Families differ as to the layer in which the periclinal divisions that presage floral apex initiation occur: subsurface layer in Liliaceac (Barnard, 1960); third layer from the surface in Gramineae (Barnard, 1955, 1957a); second and third layers in Cyperaceae (Barnard, 1957b) and Juncaceae (Barnard, 1958).
In a mantle/core configuration (Figs. 4, 6), the mantle is the meristematic portion, coating the surface (with or without a tunica). The core at center of the floral apex is not meristematic, and its cells exhibit aspects of differentiation: enlarged vacuoles, presence of tannin tannin, tannic acid, or gallotannic acid, astringent vegetable product found in a wide variety of plants. Sources include the bark of oak, hemlock, chestnut, and mangrove; the leaves of certain sumacs; and plant galls. , crystals, or thickened walls (Gregoire, 1938; Philipson, 1947a, 1949). Basically, a mantle/core configuration shows a redistribution of the meristem over a broadened, foreshortened dome, while cell differentiation progresses upward at the center at an accelerated rate.
A zonate apical meristem (Fig. 2) resembles the tunica/corpus cellular arrangement in being entirely meristematic, but it has more internal specialization into "zones" (Foster, 1941; Philipson, 1947a). Typically, a zonate apex has a surface meristem in the position of a tunica, but it is not uniform. Instead, it has a centrally located "central mother-cell zone" of randomly dividing cells and a "peripheral zone" in which cells divide anticlinally (perpendicular to the surface) so that layers are maintained but also occasionally divide periclinally. A "rib menstem" zone below the surface zones comprises vertical files of meristematic cells. The zones all are included within the confines of the apical meristem, above any appendages or their initials.
For reviews of systematic distribution of types of apical configuration during ontogeny ontogeny: see biogenetic law.
The developmental history of an organism from its origin to maturity. It starts with fertilization and ends with the attainment of an adult state, usually expressed in terms of both maximal body , see Gregoire (1938), Foster (1941), Philipson (1946, 1947a, 1947c, 1948, 1949), Rauh and Reznik (1953), Gifford (1954), Tucker (1959), and Gifford and Corson (1971).
The fact that inflorescence apical meristems generally possess a characteristic cellular configuration, as distinct from either the vegetative apical meristem or the floral apical menstem, is one of the strongest lines of evidence that inflorescences are distinct entities, rather than merely a transitional stage in the life of the plant.
B. ONTOGENETIC on·to·ge·net·ic
Of or relating to ontogeny. CHANGES IN TYPE OF APICAL MERISTEM
Inflorescence apical meristems arise either by conversion of a vegetative apical meristem, as in species of Piper (Tucker, 1982), Michelia (Tucker, 1960) and members of Asteraceae (Harris, 1995; Horridge et al., 1985; Philipson, 1946, 1947a, 1948), or de novo from buds in the axils of leaves or bracts, as in Acacia baileyana (Derstine & Tucker, 1991) and Drimys winteri (Tucker, 1959). The three kinds of apical meristems (vegetative, inflorescence, floral) may be very similar, as in Vinca Vin·ca
A genus of evergreens usually found in the Eastern hemisphere.
plant genus of Apocynaceae family; contains cardiac glycoside; causes diarrhea; includes V. major (blue periwinkle), V. rosea (Boke v. t. & i. 1. To poke; to thrust. , 1947), but more commonly they exhibit distinct differences. One example of a plant with inflorescence apices that contrast apical configurations of vegetative and floral apices during their ontogeny is Houttynia (Tucker, 1981), in which the vegetative apex (Fig. 2) and early inflorescence apex are zonate but the latter shifts to a mantle/core configuration (Fig. 4) during most of its initiatory in·i·ti·a·to·ry
1. Introductory; initial.
2. Tending or used to initiate.
Adj. 1. initiatory function. Another, Acacia baileyana (Derstine & Tucker, 1991) has floral apices that resemble the inflorescence apices (both have a tunica/corpus configuration) during perianth initiation but, during 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. and 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. initiation, change markedly to a mantle/core configuration. The apical meristems of vegetative shoots fluctuate regularly in size and configuration in plastochronic intervals, as well as in gradual increase in apical size as the plant changes from the seedling to a mature state. The apical meristems of both inflorescences and flowers also undergo change, but these tend to be successional, rather than fluctuating. They are capable of marked shifts in structure during the duration of their activity. Since both types are determinate, their apical meristems eventually cease meristematic activity altogether.
The capitulum or head of Asteraceae is another example of conversion of the terminal vegetative apex into a terminal inflorescence, one that has been particularly well studied in numerous taxa (see the review in Harris's paper, this issue). The huge body of literature on day-length and gibberellic-acid responses of the vegetative apex of Chrysanthemum chrysanthemum (krĭsăn`thəməm), name for a large number of annual or perennial herbs of the genus Chrysanthemum of the family Asteraceae (aster family), some cultivated in Asia for at least 2,000 years. as it converts to form an inflorescence apex was well reviewed in Yahel et al. (1985).
A series of articles by Philipson (1946, 1947a, 1947b, 1947c, 1948) compared histogenetic his·to·gen·e·sis
The formation and development of bodily tissues.
histo·ge·net changes of the apical meristem, based on histological sections, during inflorescence onset and flower initiation in several taxa of Asteraceae, plus taxa of adjacent families Valerianaceae and Dipsacaceae. The vegetative apices were generally zonate in configuration, while the conversion to an inflorescence apex involved increases in height and width and change to a mantle/core configuration.
Harris (1995) provided an extensive review of ontogenetic work on inflorescence formation in Asteraceae, as well as SEM-based comparative inflorescence development of representatives of each tribe of Asteraceae. Of special interest developmentally is the fact that initiation of floral apices on the capitulum is not strictly acropetal a·crop·e·tal
Developing or maturing from the base toward the apex, as in those plant organs in which the younger tissues are nearer the apex.
a·crop , as would be expected. Instead, there is a lagging of initiation of the outermost flowers that will form the rays--in Coriopsis, for example; see Harris (1995: 156-157; this issue)--until after disk-flower initiation has nearly been completed. An extreme example of this delay is seen in Erigeron philadelphicus (Harris et al., 1991), in which the ray flowers are initiated basipetally, yielding a bidirectionally initiated inflorescence.
A so-called intermediate apical meristem (or "transition" apex) develops from the vegetative apex but is flat topped and wider and higher than the latter; it lacks any sign of floral apex initiation (Molder & Owens, 1973). This intermediate stage can be maintained indefinitely under some experimental regimes (Popham & Chan, 1952). The phenomenon is common in taxa of Asteraceae (Aster sinensis, Chrysanthemum segetum, Cosmos) but also has been reported in Chenopodium album (Chenopodiaceae; Gifford & Tepper, 1962a, 1962b), Perilla perilla (p·riˑ·l nankinensis (Labiatae; Nougarede et al., 1964), and Sinapis alba (Brassicaceae; Bernier, 1962). Doubts have been expressed about the "intermediate" stage being merely an "altered physiological state not necessarily related to flowering" (Gifford & Corson, 1971). Harris (this issue) presents more detail on intermediate apical meristems.
Other changes accompanying size increase and changes in cell arrangement include shifts in phyllotaxis (appendage appendage /ap·pen·dage/ (ah-pen´dij) a subordinate portion of a structure, or an outgrowth, such as a tail.
epiploic appendages see under appendix . arrangement), an increased rate of appendage initiation, increased mitotic activity, and appendage initiation relatively higher on the apex (Gifford & Corson, 1971). Cytohistological changes have also been shown to accompany the transition to flowering (Gifford & Stewart, 1965; Gifford & Tepper, 1961, 1962a, 1962b). Although changes of the apical meristems with onset of reproductive phase have been emphasized here, the shifts are gradual and form a continuum during growth, as suggested by Esau (1965: 116).
Inflorescences may be determinate or indeterminate, depending on the activity of the terminal inflorescence apical meristem. Determinate ones undergo conversion of the inflorescence apex into a terminal flower, which may be followed by branching from axils of bracts and bracteoles below the terminal. The direction of flower initiation is basipetal, at least on ultimate branches. This pattern is typical of cymes, dichasia, monochasia, and related forms.
Indeterminate inflorescences have continuous prolonged activity of the inflorescence apex, which produces flowers in acropetal succession on each axis, with the oldest ones at the base. This pattern of activity produces a raceme, which can be modified as a spike or an umbel.
Although indeterminate inflorescence apical meristems technically could continue as meristems indefinitely, in practice they usually eventually decline in activity. In inflorescences of Piper (Tucker, 1982) and Peperomia (Tucker, 1980), the inflorescence meristems decline in height, diameter, and volume, and then either they terminate as spines, they abort after formation of abscission zones, the component tissue matures, or they simply die and dry up in place.
Briggs and Johnson (1979) introduced terminology in response to a problem with the morphological definitions of determinate and indeterminate inflorescence axes. The problem is that an axis may be determinate either due to conversion of a terminal meristem to a flower or due to death of the inflorescence meristem itself, with no terminal flower formed. They proposed the terms "anthotelic" for those meristems that convert to a terminal flower and "blastotelic" for those that remain vegetative. Vegetative meristems may, in turn, be of two types: "auxotelic" meristems are those that die after some period of growth (determinate, but no terminal flower); "anauxotelic" meristems are those that resume vegetative growth after some period of flowering. The branches of many of the Myrtaceae studied by Briggs and Johnson (1979), such as species of Callistemon Bottlebrush (Callistemon) is a genus with 34 species of shrubs in the family Myrtaceae. The majority of Callistemon species are endemic to Australia; four species are also found in New Caledonia. (bottlebrush), have alternating phases of flowers and vegetative growth.
Order of flower initiation generally differs in determinate and indeterminate inflorescences: basipetal in the former; acropetal in the latter. Although order of flower anthesis in an inflorescence and order of initiation are usually the same, they may differ. In many mimosoid legumes, for example, flower initiation is acropetal, but floral development is synchronous, so that all flowers of an inflorescence reach anthesis at the same time (Ramirez-Domenech & Tucker, 1989; Tucker, 1988). The inflorescences of Drimys winteri have acropetal floral initiation, but the direction of anthesis among flowers is basipetal (Tucker, 1959). Likewise, the "condensed" inflorescences of Asteraceae display an uncoupling between initiation, maturation, and anthesis of the flowers (Harris, 1995, this issue).
Inflorescences are basically branching systems, with ultimate inflorescence form determined by the number of branching orders involved, by the positions of the contributing axillary buds, and by other factors, such as the relative timing of branch expansion and the differential elongation or suppression of internodes.
VII. Genetic Bases for Inflorescence Form
Several genes that affect inflorescence form profoundly in the pea are described by Singer et al. (this issue). Genes that affect aspects of inflorescence architecture and development have been reviewed recently by Evans and Barton (1997). Mutations are known that increase or reduce numbers of axillary branches or that control formation of accessory branch buds. The "bracteomania" gene causes lateral-branch formation in place of single flowers in leaf axils in Antirrlzinum (Huijser et al., 1992). In Zea mays, different genes control terminal (staminate) and lateral (carpellate) inflorescence formation.
A single gene (TELl) causes terminal flower formation in normally indeterminate inflorescences of Arabidopsis. Shannon and Meeks-Wagner (1991) point out that the apical meristem that is changing from an inflorescence to a floral apex has fewer anticlinal divisions and more periclinal divisions in subsurface cells than in a typical inflorescence apex of the same species. Subtle shifts in planes of cell division in the apical meristem, under the control of a single gene, thereby have profound effects on inflorescence architecture. Adaptive pressures must usually prevent establishment of a terminal-flower gene in racemose racemose /rac·e·mose/ (ras´i-mos) shaped like grapes on their stem.
Having or growing in a branching manner that resembles a bunch of grapes. Used of glands. taxa. But in a few taxa-for example, Gleditsia triacanthos (Tucker, 1991); species of Clethra, Juglans, Digitalis digitalis (dĭj'ĭtăl`ĭs), any of several chemically similar drugs used primarily to increase the force and rate of heart contractions, especially in damaged heart muscle. The effects of the drug were known as early as 1500 B.C. , Convallaria, Campanula campanula (kămpăn`yələ): see bellflower. , and Pyrola (Eames, 1961 )--racemes usually have a terminal flower, whereas others in some of the same genera (G. amorphoides) do not.
VIII. Anomalous Flower/Inflorescence Intermediates
Recurring through the morphological literature is a theme that some reproductive structures are transitional between an inflorescence and a flower. For example, inflorescences of Triglochin (Juncaginaceac) and Potamogeton (Potamogetonaceae) are said to resemble individual flowers (Eames, 1961). Sattler (1965) asserted that the reproductive axis of Potamogeton richardsonii is somewhat intermediate between a flower and an inflorescence. However, Posluszny and Sattler (1974a) pointed out that the presence of bracts between the whorl whorl
1. A form that coils or spirals; a curl or swirl.
2. A turn of the cochlea or of the ethmoidal crest.
3. An area of hair growing in a radial manner.
4. of stamens and that of carpels supports identity as an inflorescence. More obviously false "flowers" or pseudanthia are found in the highly reduced inflorescences of Euphorbiaceae (Sattler, 1973; Weberling, 1989: 303-307). The individual flowers are highly reduced, often to a single stamen or carpel, with or without minute bracts. Burger (1977) hypothesized that trimerous flowers (as in monocots) are derived from a fusing of three subunits, each of which resembles a chloranthoid flo wer. He suggested that the flower of Scheuchzeria resembles a stage in this transition. Posluszny (1983), in a developmental study of the latter, refuted this argument by showing that Scheuchzeria flowers develop similarly to other typical trimerous monocot flowers. The only unusual feature is that the terminal flower of the inflorescence lacks a subtending bract.
Aggregations of extremely reduced individual flowers may cause difficulty in determining whether a structure is a flower or an inflorescence. One such example is the female reproductive axis in Cercidiphyllum japonicum (Cercidiphyllaceae). One to 13 free carpels are aggregated, but with their ventral sutures facing outward, rather than inward as in most apocarpous flowers. No perianth is present. Swamy and Bailey (1949) and Endress (1993) determined that this aggregation is an inflorescence, based on the suture positions and also on the fact that the carpels are in an approximately decussate de·cus·sate
To cross or become crossed so as to form an X; intersect.
Intersected or crossed in the form of an X.
1. to cross especially in the form of Y.
2. crossed especially like the letter Y. arrangement, not at the same level, and that each has a membranous membranous /mem·bra·nous/ (mem´brah-nus) pertaining to or of the nature of a membrane.
1. Relating to, made of, or similar to a membrane.
2. bract at the base. The male flowers are even more difficult to delimit de·lim·it also de·lim·i·tate
tr.v. de·lim·it·ed also de·lim·i·tat·ed, de·lim·it·ing also de·lim·i·tat·ing, de·lim·its also de·lim·i·tates
To establish the limits or boundaries of; demarcate. , according to Endress (1993), because they lack both perianth and subtending bracts.
The following articles should inform workers in various disciplines concerning the potential usefulness and significance of inflorescences and, we hope, will lead to more collaboration on inflorescences between disciplines.
The authors acknowledge support from NSF NSF - National Science Foundation grant DEB-9596281 to the first author and that from the Royal Botanic Gardens, Melbourne For other places with the same name, see Royal Botanical Gardens (disambiguation).
The Royal Botanic Gardens, Melbourne are botanical gardens located near the centre of Melbourne, Victoria, Australia, on the south bank of the Yarra River. , Australia, to the second author.
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