358 



BOTANY 



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 ( 31 ) and Bulbuchaetc may be quoted as examples <>t 



oogamous Ulotrichales. While tlie 

 thallus of the latter is branched, 

 the numerous species of Oedoijmi in m 

 consist of unbranched filaments, 

 each cell of \vhirh possesses one nu- 

 cleus and a single parietal chromato- 

 phore composed of numerous united 

 bands. The asexual swarm-spores of 

 Oedogonium are unusually large, and 

 have a circlet of cilia around their 

 kinoplasmic, colourless, anterior ex- 

 tremity (Fig. 291 ). In this case 

 the swarm-spores are formed singly, 

 from the whole contents of any single 

 cell of the filament (A), and escape 

 by the rupture of the cell wall. 

 After becoming attached by the 

 colourless end they germinate, giv- 

 ing 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, 

 FIG. 290. Ulothrix zonata. A, Young filament with ,, ,. ., 



rtmoid cellr(x 300); B, portion of i.a,M,nt with Cells ot the Same Or another fila ' 

 escaping swarm-spores ; C, single swarm-spore ; D, ment become converted into an- 

 fonnation and escape of gametes ; E, gametes ; F, G, theridia. Each antheriditim usually 

 conjugation of two gametes ; H, zygote ; J, zygote gives rise to two spermatozoids 

 after period of rest; A", zygote after division into The sperniatozoids are smaller than 

 swarm-spores. (After DODEL-PORT. B-K x 482.) 



the asexual swarm-spores, but nave 



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. 

 292) a germinating oospore of Bulbochaete with four swarm-spores is represented. 



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

 plicated, and the spermatozoids are produced in so-called IHVAKK MAI.KS. These 

 are short filaments (Fig. 291 C, a) consisting of but few cells, and are developed 



