158 LIVING PLANTS 



the tree finds its enormous leaf surface throw- 

 ing water into the surrounding dry atmo- 

 sphere much faster than it may be taken from 

 the soil by the delicate absorbing organs. 



The approach of autumn brings cool nights 

 and a consequent great radiation of heat 

 from the soil. The chilled root hairs in the 

 soil are unable to take the necessary suppl^^ 

 of water, and whenever the supply of moisture 

 coursing upward through the sinuous roots 

 and tall stems becomes less than that evapo- 

 rated, adjustment must be made or damage 

 will ensue. The plant is a most delicately 

 self-regulating organism. It cannot increase 

 the water supply, but it may and does decrease 

 the evaporating surface by casting or shed- 

 ding the leaves, a reaction which it exhibits 

 to other conditions as well. Like the true 

 seaman, however, the plant does not shorten 

 sail by cutting away its canvas, but by a de- 

 liberate and well-timed series of processes, 

 withdraws all of the substances from the leaf 

 which may be useful to it, back into its body 

 before it discards the empty sheets of cells and 

 woody fibers of the petiole and lamina. 



Before proceeding to a description of the 

 mechanism of leaf-fall, it may be well to call 

 attention to the popular and erroneous idea 

 that the coloring and casting of autumnal 

 leaves is due to the action of frost. It is true 



