THE EVOLUTION OF PLANT REPRODUCTION 51 



Such reproduction by fission is also characteristic of the entire 

 group of the bacteria. Because of the simplicity of the process, it 

 can take place at frequent intervals, sometimes several times an 

 hour. This accounts for the rapid increase in bacterial popula- 

 tions when these unicellular heterotrophic plants become estab- 

 lished in a favorable nutrient medium such as blood. 



Reproduction in Multicellular Thallus Plants 



In the transition to multicellular life, some plants attempted 

 to retain the individuality of every cell, not only in the accom- 



FiG. 26. — Ulothrix cells can undergo division to form small motile bodies known 



as zoospores. 



plishment of all maintenance activities (cf. p. 27) but also in the 

 ability of every cell to reproduce the entire plant. In such cases 

 each cell undergoes a modified sort of fission, its contents break- 

 ing up into one or more units known as spores, which are asexual 

 reproductive structures possessing the ability of growing into 

 new organisms upon separation from the parent. Some spores 

 have become highly specialized for their reproductive function, 

 being protected by thick cell walls which aid in keeping the germ 

 of protoplasm alive during unfavorable environmental condi- 

 tions, as excessive heat or cold, or dryness. Under suitable condi- 

 tions of temperature, food supply, etc. the spore bursts its re- 



