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24 E. Imbert Fig. 4. The relationship between presence/absence of heterocarpy and biogeographical origin (mediterranean and deserts vs. others) in Crepis according to the phylogenetic relationship among the 27 sections (from Babcock 1947). “H” symbolizes the presence of at least one heterocarpic species in the section. Box with bold lines, all species occur in either mediterranean or desert areas; box with full lines: all species occur in other areas; box with broken lines: the section contains species from mediterranean or desert areas (M) and from other areas (O). Numbers represent section numbers (cf. Fig. 1). marium (Asteraceae) fruits are dispermic and seeds in the upper position are the heaviest (Shull 1911; Thornton 1935; Maun & Payne 1989; Zhang 1993). The capitulum of Asteraceae is supposed to directly derive from a spike, resulting from condensation of the axis (Harris 1995), and flower development is thus centripetally. Following the ontogenic arguments presented above, we expect peripheral achenes to be heavier than central ones. Several authors have reported Perspectives in Plant Ecology, Evolution and Systematics (2002) 5, 13–36 that in Tragopogon dubius, where seed size decreases linearly from the periphery towards the centre (McGinley 1989; Maxwell et al. 1994; see also Zohary 1950), and consistently in most heterocarpic Asteraceae, morphological differentiation between achene morphs is related to position within the capitulum, peripheral achenes being heavier than central ones. However, in Carduus pycnocephalus, C. tenuiflorus, Centaurea solstitialis (Olivieri & Berger 1985), Picris radicata (Ell-
ner & Shmida 1984) and Bidens pilosa (Rocha 1996) central achenes are heaviest. Gardocki et al. (2000) found no relation between flower position and seed morph in Calendula sp., and all seed types can be produced by peripheral flowers. Because the capitulum is a very condensed structure, constraints for space are not the same in the periphery as in the centre. For instance, the package of central achenes implies that they are straight, while peripheral achenes can be curved-shaped, as observed in Crepis sancta. Furthermore, in some species (e.g. Crepis leontodontoides, Hedypnois cretica, Picris echioides, Picris galilea), peripheral achenes are enclosed in involucral bracts. Several authors (Zohary 1950; McEvoy 1984; Venable 1985a) have argued that the condensed inflorescence of Asteraceae favours heterocarpy. The position of seeds within the individual can also influence the chemical composition, in particular of the seed coat (Gutterman 1983; Gonzalez-Rabanal et al. 1994; Gutterman 1994; Maxwell et al. 1994). Jaimand & Rezaee (1995) reported that the chemical composition of achenes of sunflowers varies according to their position within the capitulum. This observation is consistent with the variations in the concentration in germination inhibitor of some heterocarpic Asteraceae. Developmental constraints can easily explain bimodality of seed size, and the curved shape of peripheral achenes in the Asteraceae, but are not sufficient to explain the tremendous morphological differentiation observed in certain species. Whatever the ontogenetic process initialising the differentiation among seed morphs, there must exist, as for phenotypic plasticity, a genetically controlled mechanisms that either inhibits or enforces morphological differentiation (Bachmann 1983). For instance in Malva moschata (Malvaceae), leaf morphology varies with leaf position within stems, and esterase activities show striking differences between leaf types (Bachmann 1983). The tremendous morphological differentiation among seed morphs necessarily implies the production of at least one chemical component. Bachmann (1983) proposed a model largely based upon observations on the ontogeny of the capitulum: Assuming that there (1) is only one hormone involved in the differentiation between peripheral and central achenes, (2) C i is the concentration of this hormone at time i, and (3) the hormone is only produced when the meristem is in a vegetative stage or quiescent. When environmental conditions (i.e. temperature or photoperiod) are suitable for plants to reproduce, the vegetative meristem transforms into flower buds. The concentration of the hormone has thus an initial and maximal value (C 1). Peripheral parts of the meristem stop mitotic activity and start to differentiate into florets. Therefore, in these peripheral cells C i has the value C 1, or less if the hormone is not time-stable. Conversely, central Consequences and ontogeny of seed heteromorphism 25 parts of the bud still show mitotic activity, and C i is divided by two for each cell division. Such a model leads to a gradient in hormone concentration from the periphery towards the centre of the capitulum. Bachmann (1983) suggested that this model can also be applied to floret differentiation (ligulate in periphery and tubulate in centre) and sexual differentiation. This model is based upon a morphogen gradient and it may also apply for other types of inflorescence, where the flower buds in the lower part having higher hormonal concentrations. For instance, in the Araceae, sexual morphs vary along the inflorescence: in the lower part, flowers are female, median flowers are hermaphroditic and male in the upper part. Such variation may be due to a similar morphogen gradient (Boubes & Barabé 1996; Barabé & Jean 1996). Furthermore, it is important to note that the model leads to a cell-specific development, i.e. the gradient of the morphogen is independent of primordium boundaries (Bachmann 1983). For instance in Carduus species, the outer side of achenes of intermediate morphology have a peripheral-like morphology while the inner side is morphologically close to central achenes (Olivieri et al. 1983; see also Fig. 4 in Bachmann 1983). Finally, we have to note that in many examples of heterospermy and heterocarpy, the morphological differentiation relies upon the importance of development of a particular structure (beak in Hypochoeris glabra, wing in Spergularia marina, pappus in several Asteraceae species etc.), which can be the result of heterochrony. Heterochrony has been invoked to explain other differentiation such as evolution of corolla shape in the Delphinium genus (Guerrant 1982) or differentiation between gamopetaly and sympetaly (Stebbins 1974). Therefore, the hormonal factor could be a growth factor (Bachmann et al. 1984). Note that the model could also work with an inhibitor agent. This model explaining the ontogeny of differentiation between seed types leads to the conclusion that morphological differentiation is independent of any paternal contribution. Indeed, the differentiation among morphs only results from maternal effects, and there is no genetic difference among seed morphs that control for morphological differentiation. For instance, in Calendula arvensis (Asteraceae), differentiation between peripheral and central ovules started before anthesis (Pomplitz 1956). The same observations have been made in Carduus sp. (I. Olivieri, pers. comm.). Consequently, differences among morphs are non-genetic maternal effects (Imbert et al. 1999). Furthermore, seed heteromorphism is considered as intra-individual variation, i.e. a variation within a single plant, and a relevant way to interpret the character is to consider the respective proportions of each seed type produced by a single individual (e.g. Imbert 2001). Perspectives in Plant Ecology Evolution and Systematics (2002) 5, 13–36
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Page 3 and 4: 250,000 angiosperm species, the pre
Page 5 and 6: melina benghalensis (Commelinaceae;
Page 7 and 8: quence of seed heteromorphism (Harp
Page 9 and 10: and presence/absence of seed hetero
Page 11: Table 3. Number of heterocarpic and
Page 15 and 16: Bachmann K & Price HJ (1979) Variab
Page 17 and 18: Gutterman Y (1994) Long-term seed p
Page 19 and 20: Ruiz de Clavijo E (1994) Heterocarp
Page 21 and 22: Appendix Consequences and ontogeny
Page 23 and 24: Family Species References Consequen

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