LEAVES IN SEASONS 159 



that the phenomena of autumnal leaf-fall are 

 due to low temperatures, but as may be seen 

 from the above, the defoliation of the plant is 

 not a reaction to the cold, but is an adjust- 

 ment to the limited water supply furnished by 

 the chilled roots. The reduction of the water 

 supply and the beginning of the processes 

 leading to defoliation occur a long time before 

 the temperature of the air is depressed to the 

 freezing point, or the formation of frost. The 

 influence of low temperatures upon the plant 

 is illustrated by the manner in which leaves of 

 tobacco and melon plants blacken and die as 

 the result of cool nights before the occurrence 

 of frost. These plants transpire a relatively 

 large amount of water from the broad leaves, 

 and if the temperature of the soil descends to 

 forty degrees Fahrenheit, the roots are unable 

 to take up the necessary supply of water, and 

 the leaves are literally dried out, though the^^ 

 are incorrectly described as frozen or frosted 

 by gardeners. 



The casting of the leaf is not a sudden and Withdrawal of 

 quick response to any single depression of the leaf-substance 

 temperature, but is brought about by a 

 complex interplay of processes begun days or 

 perhaps weeks before any external changes 

 are to be seen. The leaf is rich in two classes 

 of substances, one of which is of no further 

 benefit to it, and another which it has con- 



