GROWTH AND CELL DIVISION 



69 



of heat, cold, moisture, and food supply are detrimental to 

 growth, while mean conditions in the case of any given factor 

 are likely to be favorable to it. 



SUMMARY 



Growth in plants comprises three main phases or stages. The first 

 is that of cell division, in which the cells of the entire structure, or 

 of specialized parts termed meristems or cambiums, undergo rapid 

 multiplication by mitosis. 

 The second phase is that 

 of enlargement, during 

 which the new cells 

 formed by mitosis in- 

 crease greatly in size, 

 and so cause the growth 

 of the organ or part con- 

 cerned. The third is the 

 phase of differentiation 

 and maturation, in which 

 the enlarged cells are 

 modified to form the dif- 

 ferent tissues of the per- 

 manent plant organs. 



Plants differ from 

 animals principally in 

 the second and third 

 phases of both cell and 

 organ growth. Plant cells 

 increase in size very 

 largely by the absorp- 

 tion of water, which collects inside the cells in the form of water 

 drops, or vacuoles. In animals the increase in the size of the cells is 

 largely due to actual increase in the amount of living substance. 

 The differentiation and maturation stages of cell growth are also 

 necessarily different in plant and animal cells, on account of the dif- 

 ferent kinds of permanent tissue formed in plant and animal organs. 



Plant organs differ in their methods of growth according to the 

 nature of the organ or part and the length of time during which 

 enlargement continues. Organs which have a short period of growth, 



FIG. 38. Lilac twigs in summer and winter 

 conditions 



and b represent the season's growth of the bud 

 shown in Fig. 37 



