190 



THE ALG^E 



giving rise to four large zoospores (Fig. 182, E), which develop 

 at once into (Edogonium plants. 



Both eggs and sperms are believed to have been derived from 

 simpler ancestral types of ciliated gametes, similar in structure 

 to one another and to the zoospore. These ancestral sexual 

 conditions must have been those of isogamy, somewhat as is 

 at present illustrated in Ulothrix. The originally similar 

 ciliated gametes varied in size. The smaller reduced 

 their chromatophore and food contents because they 

 were formed in large numbers but retained their cilia 

 and thus became the small active sperms. The larger 

 gametes accumulated rich supplies of food, became slug- 

 gish, finally lost their cilia and swim- 

 ming habits, and at last were retained 

 within and protected by the oogonia, 

 thus becoming large nonmotile eggs. 



It is clear that the plant gains very 

 much by differentiating and specializ- 

 ing its gametes as eggs and sperms, for 

 the eggs are protected and richly sup- 

 plied with protoplasm and food, while 

 the sperms are developed very numer- 

 ously and are well adapted to swim FlG - m Draparnaldia, one 

 actively about in the water 4 where they 

 are attracted to the eggs by substances 

 secreted by its protoplasm. The higher 

 sexual condition, as in (Edogonium, 

 where the gametes are eggs and sperms, is called heterogamy 

 because the gametes are dissimilar, in contrast to isogamy (see 

 account of Ulothrix, Sec. 217). 



221. The Chaetophoraceae.* The members of this family, 

 including such common genera as Stigeodonium, Draparnaldia 



* To THE INSTRUCTOR : It does not seem to be generally known that forms 

 of the Chcetophoracece are almost equally good types for the study of zoo- 

 spores as Ulothrix and may be readily substituted for that form. Stigeodonium 



of the Chattophoracece 



zoospores formed singly in 

 the cells 



