NATURE AND SOURCES OF FOOD. 89 



perpetual verdure. Hence it is, that the entire foliage of such 

 trees consists of leaves which have been attached to the stem 

 from one to three or five successive years. 



In the beech and hornbeam, the leaves wither in autumn, 

 and hang on the branches in a dead state through the winter. 

 Such leaves, when examined, will be found to be contiguous 

 with the stem at the base of their petiole, and therefore with- 

 out that articulation or joint which so materially aids in the 

 disruption of the leaf from the stem. These dead leaves fall 

 off when the new leaves expand in the spring. 



Most of the trees of this country have deciduous leaves, and 

 in winter our woods are bare and no longer cast their shadows 

 on the earth ; but the forests of tropical climates are evergreen, 

 and usually retain the same appearance throughout the year. 

 A perpetual shade is thus afforded by nature, which in some 

 measure gives relief against the continuous heat of these 

 regions. 



CHAPTER VII. 



ON THE NATURE AND SOURCES OF THE FOOD ASSIMILATED 

 BY PLANTS. 



THE investigation of the nature and sources of those sub- 

 stances assimilated by the nutritive organs of plants, is neces- 

 sary to a clear understanding of their physiological action, and 

 will very properly close this part of the subject. 



These substances can only be determined by chemical analy- 

 sis. Plants have been examined chemically by Liebig, Mulder, 



