ADJUSTMENT TO TEMPERATURE 



109 



growth. Light apparently exerts only an indirect effect through 

 the dependence of growth upon the food supply furnished by 

 photosynthesis. Strong sunlight has been supposed to exert a 

 retarding influence upon growth, but this is very doubtful. In 

 diffuse light and darkness, plants grow taller than in sunlight, 

 but this result seems to be due to other causes, and only indirectly 

 to light. Humidity acts indirectly but powerfully upon growing 

 plants by controlling transpiration, and through it affecting absorp- 

 tion. Soil influences growth by its control of water content, air 

 content, and nutrient salts. Except for its occasional mechanical 

 action upon the growth of woody plants, wind, like the remaining 

 physical factors, affects growth only through other factors. 



132. Regions of greatest growth in various organs. The 

 growth in length of roots, stems, and leaves is normally due to 



YiQ_ 07 Seedling of the horse-bean, Vina faba, showing the amount and 



location of the growth in A after b hours (/i) and after 24 hours (C). 

 (After Pfeffer.) 



the apical meristem. In some cases layers of cells at one end 

 of the internodes of the stem or near the bases of the leaves re- 

 tain the power of growing, and thus produce intercalary growth. 

 In all regions of growth, and especially so in apical ones, there 

 are two more or less distinct zones. In one the cells divide actively, 



