30 Plant Life and Evolution 



assimilation being accompanied by the evolution of 

 free oxygen. The cell increases in size until it 

 reaches its full development and then by division 

 two cells are formed, which constitute new indi- 

 viduals. 



Such simple unicellular plants react promptly to 

 the environment. Deprived of light, photosynthesis 

 at once ceases; changes in temperature materially 

 effect the activity of nutrition and growth which 

 only can be maintained within certain often decid- 

 edly small limits; should the temperature of the 

 water in which the plant is growing be raised above 

 a certain point, death will result. Most of the lower 

 plants are much more resistant to low temperatures 

 and may often be frozen solid without injury. 

 Withdrawal of water does not necessarily destroy 

 the plant. Many of them simply become dormant, 

 remaining inert so long as they are dry, but absorb- 

 ing water quickly when moistened, and soon resum- 

 ing activity. Retention of moisture is often facili- 

 tated by the gelatinous character of their walls, 

 which hold water very tenaciously. 



Most Plants are Multicellular. While there are 

 many plants which are unicellular and some of these 

 are quite highly specialized, the greater number of 

 plants are multicellular. The simplest multicellular 

 plants are filamentous algae, rows of often slightly 

 coherent and quite uniform cells. Such a plant 

 might be considered as a chain of unicellular indi- 

 viduals rather than a multicellular individual, and 



