REPRODUCTION IN PLANTS 



97 



significant fact must be attributed the profound importance 

 of sex phenomena in the life history of plants as well as of 

 animals. 



Although sexuality is fundamentally a physiological dif- 

 ference between gametes which 

 leads to their characteristic 

 behavior (zygote formation), 

 even among the lower plants 

 structural differentiations ap- 

 pear. In fact, a series of 

 plants can be arranged show- 

 ing a gradual transition from 

 gametes which are morpholog- 

 ically identical to those which 

 differ so widely that they ap- 

 pear to have little in com- 

 mon. Oedogonium, an un- 

 branched filamentous Alga, 

 will suffice as an example, 

 since it affords an excellent 

 illustration of an intermediate 

 stage in gamete differentiation. 

 One form of Oedogonium 

 gamete, representing an entire 

 protoplast of a greatly enlarged 

 cell, is richly supplied with 

 food materials and chloroplas- 

 tids and remains motionless 

 within the cell wall. The 



other type develops in pairs in small cells with greatly re- 

 duced chloroplastids and food content. Instead of being 

 motionless, each cell is provided with a circlet of cilia by 

 which it leaves its place of origin, swims actively in the 



FIG. 50. Oedogonium, a filamentous 

 Green Alga. A, young filament. B, por- 

 tion of a filament forming gametes (egg 

 and sperm) . Below are two sperm which 

 have just been liberated; above is a large 

 egg with a sperm just coming into con- 

 tact with it to form a zygote. (From 

 Coulter.) 



