DIV. I 



THALLOPHYTA 



403 



converted into unicellular germ plants (7), and give rise to several swarm-spores 

 (K), which in turn grow out into new filaments. Under some conditions the 

 planogametes can give rise to new plants parthenogenetically without conjugating. 

 Further, the filaments can, in addition to the swarm -spores with four cilia 

 described above, produce others of smaller size (micro-zoospores) which resemble 

 the gametes. These possess four or two cilia, and as a rule die if the temperature 

 of the medium is above 10 ; below this temperature they come to rest after a few- 

 days and proceed to germinate slowly. This Alga is thus of interest from the 

 incomplete sexual differentiation exhibited by its gametes. 



The genera Oedogonium (**) and Bulbochaete may be quoted as examples of 

 oogamous Ulotrichales. While the thallus of the latter is branched, the 

 numerous species of Oedogonium 

 consist of unbranched filaments, 

 each cell of which possesses one 

 nucleus and a single parietal 

 chromatophore composed of 

 numerous united bands. The 

 asexual swarm -spores of Oedo- 

 gonium are unusually large, and 

 have a circlet of cilia around 

 their colourless, anterior ex- 

 tremity (Fig. 341 B). In this 

 case the swarm -spores are formed 

 singly, from the whole contents 

 of any cell of the filament (A), 

 and escape by the rupture of 

 the cell wall. After becoming 

 attached by the colourless end 

 they germinate, giving rise to a 

 new filament. For the purpose 

 of sexual reproduction, on the 

 other hand, special cells become 

 swollen and differentiated into 

 barrel- shaped oogonia. A single 

 large egg -cell with a colourless 

 receptive spot is formed in each oogonium by the contraction of its protoplasm, 

 while the wall of the oogonium becomes perforated by an opening at a point 

 opposite the receptive spot of the egg. At the same time, other, generally 

 shorter, cells of the same or another filament become converted into antheridia. 

 Each antheridium usually gives rise to two spermatozoids. The spermatozoids are 

 smaller than the asexual swarm-spores, but have a similar circlet of cilia. They 

 penetrate the opening in the oogonium and fuse with the egg-cell, which then 

 becomes transformed into a large firm -walled oospore. On the germination of 

 the oospore its contents become divided into four swarm-spores, each of which 

 gives rise to a new cell filament. In the adjoining figure (Fig. 342) a germinating 

 oospore of Bulbochaete with four swarm-spores is represented. 



In some species of Oedogonium the process of sexual reproduction is more 

 complicated, and the spermatozoids are produced in so-called DWAUF MALES. These 

 are short filaments (Fig. 341 C, a) consisting of but few cells, and are developed 

 from asexual swarm-spores (ANDROSPOIIES) which, after swarming, attach them- 

 selves to the female filaments, or even to the oogonia. In the upper cells of the 



FIG. 339. Enteromorpha compressa. (5 nat. size.) 



Dl 



