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 

 coenocytic body, such as characterizes the Siphonales (see p. ^t,). 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. 



(b) 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). 



Oedogonitim. — 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 ciHa (figs. 74-76). Other cells of the fila- 

 ment become enlarged and very conspicuous, forming the oogonia, 



