Chapter 1: The Characeae Plant

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zygote and the maternal spiral cells (Leitch et al. 1990). This specialised zygote is called the
oospore and its multilayered wall allows it to resist desiccation and destruction during
dispersal and dormancy.
1.3.3 The oospore
Characeae oospores are distinctive and easy to recognise (Fig. 1.1c). In all the genera except
Nitella there is sometimes the development of a gyrogonite, or calcified ‘shell’ around the
oospore, deposited during the last stages of development on the parent plant (Fig. 1.15a).
Gyrogonites can gradually decompose over time, or break open when the oospore germinates.
The oospore wall, within the gyrogonite, is formed by both the egg cell (internal layers) and
the oogonium cells (external layers), and is characterised by spiral sutures or lines on its
surface (Leitch 1989). The flat part of the oospore wall is called the fossa, and the sutures are
called the striae. Sometimes the striae have flanges (where the oospore wall is formed on the
side walls of the spiral cells; Fig. 1.15b), and sometimes a thinner structure called a ‘ribbon’
(John et al. 1989) or thicker structures (pachygyra; Fig. 1.15c) are formed. The fossa can be
variously ornamented with grains (typically so for Lamprothamnium) or fibres. The
ornamentation becomes amazingly diverse in the genus Nitella, with verrucae, tuberculae,
reticulae and every combination of wall construction imaginable (Fig. 1.15d, e, f) (John and
Moore 1987). The base of the oospore is impressed with the shape and arrangement of the
basal cell complex. In Chara there is a single basal cell (Fig. 1.15g), in other genera there can
be more (Fig. 1.15h). The apex of the oospore exhibits a ‘w-shaped’ suture where the five
spiral cells join up (Fig. 1.15i). Oospores can be different colours, ranging from pale yellow
to black. In some Nitella species oospore colour is an indication of the maturity of the
oospore, and degree of development of the oospore wall (Casanova 1991). Sometimes
oospore colour is useful for distinguishing species of Chara (Zaneveld 1940). Oospores are
densely packed with starch grains (amyoplasts), the primary reserve for the germinating
plant. Healthy oospores are turgid and resistant to external forces. They can be handled easily
with fine forceps. The distinctive differences among charophyte oospores allows them to be
used in identification of different charophyte species (Haas 1994, Sakayama et al. 2002,
Casanova 2005, Casanova 2009). Cultures of charophytes for physiological studies can be
started by collecting mature oospores, sterilising their surface and germinating them under
controlled or known conditions (see section 1.5.5).