The Algae 241 



from each spore has divided and produced the first two cells 

 of a new filament. 



The protoplasm of other cells of the same or other filaments 

 continues to divide until 16, 32, 64, or more bodies have been 

 formed. These are called gametes. They are similar to the 

 swimming spores but much smaller, and each possesses two 

 cilia for swimming. Like a swimming spore, each of them 

 leaves the old cell through an opening in the wall. The 

 gametes swim about for some minutes and then unite in pairs. 

 They are attached at first only by the cihated ends, but later 

 the two gametes fuse. The body thus formed may grow 

 directly into a new filament, or it may produce swimming 

 spores from each of which a new filament is formed. 



Asexual and sexual spores. Spores of Microspora and the 

 swimming spores of Ulothrix are formed directly from vegeta- 

 tive cells or by the division of the contents of these cells. The 

 spore which is formed by the two gametes of Ulothrix is pro- 

 duced by the union of tw^o cells. Since cell division takes 

 place in all parts of plants while cell union occurs only in the 

 sexual process, it is plain that wx have in Ulothrix a simple 

 type of sexual reproduction. The spore that is formed by 

 the union of the gametes is, therefore, a sexual spore. The 

 swimming spores and the resting spores are formed without 

 the union of cells, and these are called asexual spores. The 

 gametes of Ulothrix look alike, but physiologically they are 

 probably different. One of them corresponds to the male 

 reproductive cell of the higher plants, and the other to the 

 female reproductive cell (page 205). 



(Edogonium. (Edogonium is another filamentous alga 

 that flourishes in ponds and streams. In early life the fila- 

 ments are attached, but large masses of them will often be 



