188 THE 



develop from such gametes are generally smaller and weaker 

 than those which come from the usual zoospores. For these 

 reasons it seems evident that the gametes of plants arose from 

 zoospores, or motile cells similar to zoospores, which, adopting 

 the habits of fusing in pairs, became sexual cells. Such types 

 as Ulotlirix and Ulva have an especial interest because they 

 illustrate the general conditions which must have been present 

 with the origin of sex in any group of plants. 



220. (Edogonium. (Edogonium (family (Edogoniacece) is one 

 of the best illustrations in the green algae of the higher sexual 

 condition where the gametes become differentiated and specialized 

 as eggs and sperms. 



The species are unbranched, filamentous, fresh-water forms, 

 attached by a disk-like development from the lowest cell (Fig. 

 182, A) called a holdfast. The cells have large chromatophores 

 of irregular form, containing pyrenoids. There are sets of curi- 

 ous lines called caps across the ends of many of the cells 

 (Fig. 182, B, c), structures peculiar to this family, which 

 result from a method of cell division too complicated to be 

 described here. 1 Zoospores are developed singly in the cells, 

 and are large protoplasts with a circle of cilia at a colorless end 

 (Fig. 182, Z>).- After swimming about for a short time the 

 zoospores settle down on the ciliated end, develop the hold- 

 fasts, and grow at once into new filaments. 



The sexual cells, or gametes, of (Edogonium are eggs and sperms. 

 The eggs are developed singly in enlarged cells, which are the 

 female sexual organs (Fig. 182, B] and are called oogonia 

 (singular, oogonium). The entire protoplast of the oogonium 

 becomes the egg (Fig. 182, B, e), which remains within the oogo- 

 nium as a naked, motionless cell, without cilia, and is richly 

 supplied with chromatophores and food material. The sperms 

 are developed in pairs in short, disk-shaped cells, which are 

 found in small groups, forming the male sexual organ, or 



1 See Goebel, Outlines of Classification and Special Morphology of Plants, 

 p. 44. 



