Ecological consequences and ontogeny of seed ... - Accueil du site

14 E. Imbert
ent types (or morphs) of seeds or fruits can be defined.
This variation is associated with heteromorphism,
which is an example of phenotypic variation as it
refers to within-individual variation. Therefore, seed
heteromorphism can be defined as the production of
different types of seeds by a single individual.
Plant ecologists have neglected intra-specific variation
in seed size for a long time, because such variation
was considered negligible compared to that occurring at
the interspecific level (Harper et al. 1970; Fenner 1985).
Conversely, early botanists recognised this feature as
an important character for species diagnosis, and
intra-individual variation in seed morphology has even
been used to name some genera and species. Dimorphotheca
in the Asteraceae, (named by C. Moench
1744–1805; biographic data of botanical authors are
from Mabberley 1997) and Heterotheca (named by
A.H.G. Cassini 1781–1832) for example, include in
their names the Greek word theke, meaning seed and
clearly refer to different forms of seeds. M.C. Durieu
de Maisonneuve (1796–1878) used the production of
different fruits to name the species Ceratocapnos heterocarpa
(Fumariaceae), carpos meaning fruit.
Several words or expressions can be found in the literature
to describe this character. In his Population Biology
of Plants, Harper (1977) used the expression
“somatic polymorphism” to signify that the phenotypic
differentiation among seeds is “not a genetic segregation
but a somatic differentiation” (Harper 1977,
p. 69). However, the term polymorphism commonly
refers to a differentiation among individuals, in particular
“genetic polymorphism”, thus Venable (1985a)
suggested the use of the term “heteromorphism”.
Hannan (1980) suggested “heterospermy” for differentiation
among seeds and “heterocarpy” for differentiation
among fruits. Mandák (1997) proposed a more
complete classification of seed heteromorphism based
on diaspore morphology and other features. In the
present study, seed heteromorphism is used in its
broad sense, and heterospermy and heterocarpy are
distinguished when necessary.
The review deals with both the ecological consequences
of differentiation among seed morphs and the
ontogeny of seed heteromorphism in angiosperms.
However, before considering these topics in detail, it is
important to describe the nature of the differentiation
among morphs, and in particular to distinguish continuous
variation and heteromorphism. For most species
classified as seed heteromorphic, the differentiation
among morphs is obvious. For instance, the variation
of achene shape in Calendula sp. is a well-known example
of heterocarpy, and in many Calendula species
(C. arvensis, C. stellata for instance), three or four achene
morphs are present (Heyn et al. 1974). However,
plant species commonly show intra-individual varia-
Perspectives in Plant Ecology, Evolution and Systematics (2002) 5, 13–36
tion in seed size, either mass or length. This variation
can also be observed for other structures as pappus or
wing. Therefore, the distinction between continuous
variation and heteromorphism can be difficult. In such
cases, I propose to associate heteromorphism to a clear
bimodal (for dimorphism) or multimodal distribution.
Venable (1985a) defined seed heteromorphism as
“the production by single individuals of seeds of different
form or behavior”. Behaviour means ecological
behaviour, i.e. mainly dispersal ability and germination
requirements. There are several examples of differentiation
in ecological behaviour without any morphological
difference. For instance, in the Cistaceae,
seeds are protected by a very hard seed coat, and seeds
can only germinate when high temperatures, provoked
by fire, destroy this seed coat. However, some seeds
are morphologically identical to the previous ones but
lack a hard seed (Vuillemin & Bulard 1981). This
leads to “germination heterochrony”, a character that
is probably be very common in plant species (Harper
1977; Westoby 1981; Silvertown 1984; Fenner 1985).
Burke (1995) also described a case of seed heteromorphism
sensu Venable in the Asteraceae Geigeria alata.
In this species from the Namib desert, plants produce
seed heads at the base and along the main stem, but
there are no morphological differences between the
achenes according to the position of the seed head
(Burke 1995). The only difference is in relation to dispersal
ability, since basal achenes are less dispersed
than aerial ones (Burke 1995). These two examples of
ecological differentiation in absence of apparent morphological
differences illustrate the idea of “cryptic
heteromorphism”. This character is supposed to be
very common in angiosperms (Silvertown 1984; Venable
1985a) although underestimated. In the present
review, some cases of well-described cryptic heteromorphism
are included.
The taxonomic distribution of seed
heteromorphism and its nature
The collection of data is based on an extensive literature
survey. To avoid synonymous species names, I checked
each species using the Global Provisional Checklist created
by the International Organization for Plant Information
(www.bgbm.fu-berlin.de/IOPI/GPC/default.htm; last
updated 27 November 2000). This checklist includes
information from Flora Europaea, the USDA Plants
Database and the Med-Checklist. I included 218 species
with either heterocarpy or heterospermy (Appendix;
note that only 170 are referenced in the Global Provisional
Checklist); the number of species is similar to
the one proposed by Flint & Palmblad (1978). Because
it is not feasible to check the seeds of the ca.