46 THt LEAF. 



preserve their green foliage during the winter, and through 

 the year. 



These species throw off their leaves gradually, one after 

 another, while new ones spring forth to supply their places. 

 All plants, therefore, whether annual, perennial, or evergreen, 

 shed their leaves. In annual, or deciduous plants, these 

 organs are produced only for the season, and having per- 

 formed the functions for which they were designed, decay 

 and fall to the ground ; while in evergreen plants, these parts 

 being designed for a different, or more extensive purpose, 

 are supplied with new leaves as fast as the old ones decay. 

 There is, however, a difference in this respect, depending on 

 the climate where the same species are cultivated. Thus 

 some plants which are deciduous, or lose their leaves in our 

 climate, become evergreens when removed beyond the reach 

 of frost, or are protected in a green-house. The Kidney 

 bean, (Phaseolus,) becomes an evergreen when protected 

 from the frost. 



Botanists have offered various reasons with respect to the 

 cause which produces the fall of the leaf. The opinion of 

 Sir James E. Smith, on this subject, is perhaps the most sim- 

 ple and satisfactory, and at the same time has the advantage 

 of being easily understood. He supposes that this phenome- 

 non arises merely from the sloughing of the diseased or dead 

 part, on the same principle that a separation takes place be- 

 tween dead and living flesh. Every one knows that if he 

 crushes the end of his finger, so as to destroy the vitality of 

 that part, a separation will take place between the dead and 

 living flesh. This effect is produced by the vital action of 

 the sound flesh, which thus removes the lifeless and offensive 

 part, and finally restores the injury by producing a new part 

 in the place of that which was dead. The same effect ap- 

 pears to take place with respect to the dead leaf. A separa- 

 tion takes place between the lifeless petiole, and the living 

 branch of the tree, by the vital action of the latter, and in 

 consequence of which, the leaf is cast off, to be renewed 

 again the next year. That this is the true solution, seems to 

 be proved by the circumstance, that if a tree be cut down, or 

 its life otherwise destroyed, when covered with leaves, no 

 separation takes place, because the vital action of the tree 



Do evergreen trees cast off their leaves ? What is said of the effect of 

 climate in producing the iall of the leaf ? What is the opinion of Sit 

 James E. Smith with respect to the cause of the fall of the leaf? When 

 the life of a. tree is destroyed in the summer, whv do not its leaves fall ? 



