Chapter 1: The Characeae Plant

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1.2.6 Species of Nitellopsis
Nitellopsis (Fig. 1.11) is a Northern Hemisphere genus, so far absent from Africa, Australia
and South America. The genus occurs in relatively deep water in European lakes (Shubert
and Blindow 2003), as well as in North America. Nitellopsis obtusa (Desv. in Lois.) J.
Groves can be dominant in lakes and frequently co-occurs with the rare charophyte
Lychnothamnus barbatus (Pełechaty et al. 2010). It reproduces sexually as usual, but also
asexually via distinctive star-shaped bulbils (Fig. 1.11d) sometimes called starch-stars
(Fritsch 1948), attached to the lower stems and rhizoids. The capacity for asexual
reproduction could improve the success of long-term culturing in the laboratory. The long,
ecorticate internodes and branchlets can be large enough for physiological studies (Winter
and Kirst 1990; Winter et al. 1999), and it is tolerant of low levels of salinity.
The plant body is relatively simple (Fig. 1.11a) with whorls of elongate branchlets and simple
bract cells. Stipulodes are absent or rare at the base of the whorl, there is no cortication and
few accessory structures (Fig. 1.11c). The oogonia and antheridia are on separate plants
(dioecious), and the oogonia can be calcified, with the oospore surrounded by a gyrogonite
(Fig. 1.11b). Oospores and gyrogonites of Nitellopsis can be among the largest of extant
charophyte reproductive organs.
1.2.7 Species of Nitella suitable for physiological studies
Nitella species are identified on the basis of a coronula of 10 cells on the oogonium (Fig.
1.12b) and furcate or forked branchlets, at least in the fertile parts (Fig. 1.12a, c). The
terminal branchlet segments or rays are called ‘dactyls’ and the arrangement of cells in the
dactyls is used to determine the subspecies and species of Nitella (Fig. 1.4c). The species of
Nitella that have been used for physiological studies in the past are generally characterised by
long internodes, interrupted by few, sparsely forked whorls of branchlets. Species with short
internodes and dense whorls (e.g. Fig. 1.12a) are unlikely to be useful, simply because access
to their large cells would be difficult. In general species of Nitella have branchlets in whorls
of 6 to 9 (Fig. 1.3c, d, Fig. 1.12c), except where there is more than one whorl of branchlets at
the node (a condition termed ‘heteroclemous’ Fig. 1.3d). Branchlets can be once, twice or
more times furcate (Fig. 1.4c). Fertile branchlets can be similar to the sterile ones, or
contracted into ‘heads’ or ‘spikes’. They can sometimes be covered with a jelly-like
substance called ‘mucus’. The reproductive structures occur at the branchlet nodes (forks),