516 MANUAL OF BOTANY 



plant. The gradual appearance of the sporophj'te can be observed 

 still m the group of the Algae. In ffidogonimn the fertilised cell 

 does not grow out into a new filament, but produces in its 

 interior four zoospores which escape from it, and after a period 

 of rest germinate and produce new plants. The fertilised cell 

 here represents the sporophyte, which is reduced to a single 

 sporangium. In Coleocha;te the zygote is invested with a cover- 

 ing derived from the adjacent cells, and after sinkmg to the 

 bottom of the water it germinates, producing inside its coating 

 a small mass of cells, each one of which liberates a zoospore. 

 There is here a sporophyte of a slightly higher type than that of 

 CEdogonium. Somewhat more complex sporophytes occur among 

 the Ehodoph^^ceae. An indication of the sporophyte may perhaps 

 be seen in Spirogj^ra, where the nucleus of the fertilised cell 

 divides into four, though no definite ceUs are formed. On ger- 

 mination of the zygote, however, only one filament grows out. 



It was suggested above that probably the sexual cells were 

 originally derived from asexual ones. A studj^ of such forms 

 as Hsematococcus and Ulothrix leads us to this view. In the 

 former two forms of zoogonidia are produced, which differ from 

 each other only in size. In Ulothrix the same thing is seen, but 

 there is a difference in their behaviour. The larger ones are asexual, 

 but the smaller generally conjugate or fuse in pairs. They repre- 

 sent, therefore, the sexual cells or gametes. This is the more likely, 

 as the cells which fuse are generally, if not always, produced 

 hj different plants. The sexuality is not, however, well pro- 

 nounced, for if one does not become fused with another, it can 

 still germinate independently. 



The gradual differentiation of the gametes into definite 

 male and female mdividuals has already been traced in the 

 section on the Thallophj^ta. When completely differentiated 

 the male cell, which is known as an antherozoid, is a small 

 free-swimming body, furnished with cilia. This form, which is 

 the most perfect known, is not, however, of universal occurrence. 

 In the Fungi the male gamete is almost always an undiffe- 

 rentiated mass of protoplasm which sometimes becomes free 

 from the cell in which it is produced, but is then clothed with 

 a cell-wall and has no power of locomotion. Sometimes it is 

 not set free until it actually passes into the oogonium. In the 

 KhodophycesB it is similarly unciliated, though at first it is a 

 naked cell. The same want of differentiation is seen in all the 

 Phanerogams, where the male gamete is represented by a 

 portion of the protoplasm of the pollen tube. 



