ALG.E 107 



It has been observed that these small swimming cells come 

 together in pairs and fuse (Fig. 97, C, d), each pair thus 

 forming one new cell (Fig. 97, C, e). The cell thus formed 

 passes through a resting period (usually during winter), 

 then begins to grow (Fig. 97, ), and finally produces four 

 swimming spores (Fig. .97, F), each of which is able to pro- 

 duce a new filament of Ulothrix. Here is evidently a third 

 method of reproduction, which is peculiar in the fact that 

 two special cells unite to form the spore that produces the 

 new plant. These two special cells are gametes (sexual 

 cells); their act of fusion is fertilization; the spore thus 

 formed is the oospore (egg-spore); and this kind of re- 

 production is called sexual reproduction.* It should be 

 observed that the swimming spores and the oospores of 

 Ulothrix do not differ in what they are able to do, but in 

 the method of their formation, one being formed by cell- 

 division and the other by cell-fusion; but to distinguish re- 

 production by spores from sexual reproduction by oospores, 

 the former is called asexual reproduction, and the spores are 

 often spoken of as asexual spores; although when the word 

 "spore "is used it generally implies an asexual spore. 



The three methods of reproduction found in Ulothrix 

 may be summarized in the following graphic way: 



(1) Vegetative multiplication is indicated by P P 

 P P , in which P stands for the plant, there being a 

 succession of plants arising directly one from the other 

 without the interposition of any special cells. 



(2) Reproduction by asexual spores is indicated by 

 P o P o P o P , indicating that new plants are 

 not produced directly from the old ones, but that between 

 the successive generations there is the asexual spore. 



* It does not seem wise to multiply terms at this point, and hence 

 the more general terms "fertilization" and "oospore" are used as in- 

 cluding the more special terms "conjugation" and "zygospore." 



