THALLOPHYTES 27 
These changes from the filamentous to the palmella form and back again have 
been brought under experimental control (see Part III). 
Cladophora. This is one of the most common of the green algae, 
being a profusely branching and anchored filamentous form, the branches 
arising from the upper ends of the elongated cells (fig. 63). The cells 
are remarkable in containing many nuclei, such multinucleate cells 
being called coenocytes (fig. 64). A filament composed of a series of 
coenocytic cells suggests a structure on the way towards a completely 
coenocytie body, such as characterizes the Siphonales (see p. 33). In 
addition to the nuclei, the Cladophora cell contains many chloroplasts 
in the peripheral layer of cytoplasm, and numerous pyrenoids in the 
plastids. The cell division in this case is by means of a plate that begins 
as a ring at the cell wall and growing centripetally cleaves the protoplast 
In enlarged apical cells (usually) a very large number of biciliate zoo- 
spores are formed (fig. 65), and biciliate gametes may be formed in any 
cell. The zygospore in germination develops a new filament directly. 
(6) Heterogamous forms 
Sphaeroplea. This form is a simple filament, with multinucleate 
(coenocytic) cells, as in Cladophora, and there is the same abundant 
development of zoospores. In sexual reproduction, however, two kinds 
of gametes are produced; one being the usual biciliate, zoospore-like 
cell (the sperm) ; the other being a larger cell, with no cilia (the oosphere 
or egg). The cell producing the numerous sperms is an antheridium 
(fig. 66), and the cell producing the several eggs is an oogonium (fig. 67); 
but in this case antheridia and oogonia are vegetative cells, unchanged in 
form. In the same way, the cells producing zoospores may be called 
sporangia, but it is not usual to apply these names until these cells become 
distinct in appearance from the vegetative cells. The sperms find en- 
trance into the oogonium and fertilize the eggs, the resulting oospores 
being protected cells (figs. 67-70). In germination the oospore produces 
a number of zoospores (figs. 71, 72), which in turn give rise to new fila- 
ments (fig. 73). 
Oedogonium. This very common alga is a simple anchored fila- 
ment of uninucleate cells containing a single peripheral chloroplast. 
Certain of the vegetative cells produce zoospores, which are large, usually 
solitary, and with a crown of cilia (figs. 74-76). Other cells of the fila- 
ment become enlarged and very conspicuous, forming the oogonia. 
