THE ORGANIZATION OF THE PLANT 77 



again turned to starch by the leueoplasts or amyloplasts, small 

 bodies allied to the chloroplasts. The leueoplasts and starch 

 grains may be easily seen in young shoots of the canna. 



Transpiration. Another important service performed for 

 the plants by the leaves is the transpiration of water. The 

 transpiration stream, passing off through the stomata, not only 

 keeps the cell sap denser than the soil water, thus providing 

 for a continuous inflow of food materials in solution, but the 

 mere evaporation of so much moisture enables the plant to 





FIG. 65. Cells from a dahlia root, showing crystals of iiiulin, a substance 



allied to starch 



keep cool in the midst of the downpour of heat on a summer 

 day. At the end of the growing season most of our broad- 

 leaved plants prepare for the approaching winter by casting 

 their leaves. By so doing they avoid transpiration in winter 

 when most of the moisture in the soil is locked up by the 

 frost. But even in milder climates the leaves are eventually 

 thrown off. In regions of summer drought they may all be 

 cast at once ; otherwise the individual leaves fall one by one, 

 and the tree always has a crown of verdure. One reason for 

 the casting of the leaves is that after a season of food making 

 a considerable amount of useless mineral matter has accumu- 

 lated in the leaf, which impairs its usefulness. The ashes from 



