Chapter 1: The Characeae Plant

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annuals (i.e. reproducing once in their lives), germinating in either Spring or Autumn,
growing, producing sexual organs once, then dying. These species are highly dependent on
oospore production and storage in a seed bank for survival (Casanova and Brock 1994; Bonis
and Grillas 2002). Other species are polycarpic perennials (i.e. reproduce several times in
their lives) which can survive inclement periods either through dormancy or production of
vegetative reproductive structures (bulbils). Such species are less dependent on oospore
germination for the maintenance of populations. It is likely that many species maintained for
physiological studies are perennial species, because these can persist in culture. Whether a
species is monoecious (with both sexes on a single plant) or dioecious (with separate male
and female plants) can modify the timing of life history events such as germination and
timing of reproduction (Casanova and Brock 1999a). Monoecious annual species can
germinate and grow rapidly, completing their life cycle in a relatively short period of time.
Dioecious annuals can be slower to germinate and grow, and dioecious perennials can have
very specific germination requirements. All these life history types maximise reproductive
success in a variety of habitats (Casanova and Brock 1999a).
1.5.7 Population ecology
Studies into the ecology of characeae populations relies on examination of the transition rates
between the states of propagule dormancy, germination, growth, reproduction, perenniation,
senescence and death. The processes of inter- and intra-specific competition and loss to
predation or herbivory can modify the development of populations. In established populations
charophytes deposit their oospores into the soil or sediment, forming a bank of viable
propagules that can become both diverse and dense. Oospores can be dispersed to new
habitats by water birds, through their digestive systems (Proctor 1962) or attached to the feet
and coverings of animals and people. Oospores in the seed bank can be readily germinable
(Casanova and Brock 1990), or more frequently, dormant (Skurzynski 2009). The dormancy
can be innate or enforced, depending on the adaptations of the species, and the type of habitat
in which they occur. Enforced dormancy is defined as a maintenance of viability when
conditions are not adequate for germination. Innate dormancy is defined as the failure of a
viable oospore to germinate when exposed to conditions that are adequate for germination
(Harper 1980).