324 



REPRODUCTION IN PLANTS 



Sexual Reproduction by means of Gametes — eggs 

 and sperms. In most algae and nearly all higher plants 

 there is a sexual method of reproduction. Some plants 

 have both methods. At this point it is necessary to de- 

 fine more precisely what is meant by sex. This can best 

 be done by studying a simple form of it. 



Ulothrix has also a sexual method of reproduction (Fig. 

 82). It is particularly suited to illustrate the origin of sex 



Fio. 82. — Ulothrix, sexual reproduction. A, filament in which 

 the cells are forming about 16 motile sex-cells or gametes. B, C, D, E 

 show the gametes fusing in pairs. F, the resting spore developed from 

 the fusion of the two gametes. (7, the germination of the resting 

 spore to produce a new filament. 



because it is easy to observe that the swarm spores some- 

 times take part in sexual reproduction. It will be recalled 

 that in Ulothrix each cell produces either 8 or 16 swarm 

 spores. The more numerous ones are proportionately 

 smaller. Sometimes they appear to be too small to pro- 

 duce a new plant alone and unaided; accordingly they 

 fuse in pairs. The usual sex cells are produced in the same 

 manner as the swarm spores but are still more numerous, 

 16 to 64 in number. These small motile cells are called 

 gametes or sex cells and always fuse in pairs before 



