SECT. I 



CRYPTOGAMS 



361 



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 temperatare 

 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 gajuetes. 



The genera Ocdogonium (•^'') and 

 Bulbochaetc may be quoted as ex- 

 amples of oogaraous Ulotrichales. 

 "While the thallus of the latter is 

 branched, the numerous species of 

 Ocdogonium consist of unbranched 

 filaments, each cell of which possesses 

 one nucleus and a single parietal chro- 

 matophore composed of numerous 

 united bands. The asexual swarm- 

 spores of Oedogoniuni arc unusually 

 large, and have a circlet of cilia 

 around their kinoplasmic, colour- 

 less, anterior extremity (Fig. 282, B). 

 In this case the swarm -spores are 

 formed .oiugly, from the whole con- 

 tents of any single cell of the fila- 

 ment {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, wliile 

 the wall of the oogonium becomes 

 perforated by an opening at a 

 point oppo.site 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 autheridium usually gives rise to two spermato- 

 zoids. 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. 283) a germinating oospore of Bulbochaetc with four swarm -spores is 

 represented. 



In some species of Oedogomum the iirocess of sexual reproduction is more 



Fio. 281. — Ulothrix zonata. A, Young filament with 

 rhizoid cell r (x 300) ; 11, portion of filament with 

 escaping swarm-spores ; C, single swarui-spore ; D, 

 formation and escape of gametes ; E, gametes ; F, G, 

 conjugation of two gametes ; H, zygote ; J, zygote 

 after period of rest ; K, zygote arter division into 

 swarm-spores. (After Dodel-Port. B-K x 482.) 



