86 THE NATURE AND WORK OF PLANTS 



110. Autumnal leaf fall. — The greater number 

 of the hardy or long-lived plants of the United States 

 are deciduous, or drop their leaves every year. This 

 casting of the leaves is one of the distinctive features 

 of the close of the growing season. It is popularly 

 supposed to be due to the action of cold, but this is 

 a mistake. The leaves of a large tree throw off as 

 much as a barrel of water in the course of a day, and 

 when the plant finds that it is losing water in the 

 dry August days faster than it can take it up from 

 the soil, it begins to get rid of the organs which use 

 most of it. 



111. Separatory layer. — In order to be able to 

 cut off the leaf quickly and economically, a ring of 

 tissue is formed at the base of the stalk, w^hich 

 spreads the other tissues apart as it grows. After 

 this sejjarating layer is fully formed it is very brittle, 

 and the slightest breath of wind will split it apart 

 and allow the leaf to fall to the ground. In com- 

 pound leaves, such as the horse-chestnut, separatory 

 layers are formed at the base of each leaflet also, and 

 in some ivies a piece of the branch is cut off with the 

 leaf. 



If a twig of maple with the attached leaf is taken 



