THE EVOLUTION OF PLANT REPRODUCTION 



53 



can grow into a new Ulothrix plant, but a solitary motile cell 

 cannot. This may not seem of momentous consequence, yet this 

 new type of reproductive behavior has crossed the border line 

 between asexual and sexual reproduction. For the essence of 

 sexual reproduction is the union of two cells to form one, from 

 which the new organism develops. Such cells which must fuse 

 before they can develop into new organisms, are known as 

 GAMETES. The gametes of Ulothrix are the simplest and least 



FEMALE GAMETE 



VEGETATIVE 

 CELLS 



^ MALE GAMETES 



Fig. 28. — In an Oedogonium plant certain cells of the filament transform 

 their contents into large female gametes while other cells form smaller male 

 gametes. 



specialized of sexual reproductive cells and the process of their 

 uniting is the simplest form of sexual reproduction. Its possible 

 origin (from zoospores developing the habit of fusing) indicates 

 the probable transition from asexual to sexual reproduction. 



We have seen that spores, by further division of labor, are 

 often produced in special cells or groups of cells, the sporangia. 

 In the same manner gametes tend to become produced in special 

 gamete-forming parts of the plant. These in many cases become 

 definite sex structures and when made up of complex tissues, 

 become sex organs. The gametes themselves also become dif- 



