LEAVES IN SEASONS 163 



laver 



arrive. At some point near the baj^e of the 



leafstalk, the formation of a layer of special 



tissue had begun between the woody cylinder 



in the center and the thin epidermis. When 



the time for the casting of the leaf arrives 



this special tissue grows rapidly, pushing 



apart or cutting the cells which have held the 



leaf rigidly in position, in such manner that 



finally the leaf stalk consists of the brittle 



cylinder of wood surrounded by the loosely uf^^T^^^^ 



adherent cells of this newly formed layer of 



separation . The merest touch or breath of air 



will split the layer of separation, break the 



wood, and allow the leaf to fall to the ground. 



After the layer of separation has been formed, 



a frost or freeze would help to break away 



the fragile strand holding the leaf in place, 



but exercises no other direct influence on the 



process. 



Many plants make provision for cutting 

 away the leaf at more than one point. The 

 vine forms two layers of separation, one at 

 the base of the leaf-stalk, and the other at the 

 upper end below the blade. Layers of separa- 

 tion are formed at the base of the main leaf- 

 stalk and at the base of the separate leaflets 

 in such compound leaves as those of the Vir- 

 ginia creeper, horse chestnut and Ailanthus. 



It is to be remembered, of course, that all 

 plants do not discard their leaves on the ap- 



