106 FUNCTIONS OF NUTRITIVE ORGANS. 



exist, are chiefly concerned in the aeration of the sap, whilst other 

 assimilative processes go on in the upper cells. 



205. Leaves, after performing their functions for a certain time, 

 wither and die. In doing so, they frequently change colour, and hence 

 arise the beautiful and varied tints of. the autumnal foliage. Leaves 

 which are articulated with the stem, as in the Walnut and Horse- 

 chestnut, fall and leave a scar, while those which are continuous with 

 it remain attached for some time after they have lost their vitality, as 

 in the Beech. Most of the trees of this country have deciduous leaves, 

 their duration not extending over more than a few months; while in 

 trees of warm climates, the leaves often remain for two or more years. 

 In tropical countries, however, many trees lose their leaves in the dry 

 season. This is seen in the forests of Brazil, called Catingas. Trees 

 which are called evergreen, as Pines and Evergreen oak, are always 

 deprived of a certain number of leaves at intervals, sufficient being 

 left, however, to preserve their green appearance. Various causes 

 have been assigned for the fall of the leaf. . In cold climates, the de- 

 ficiency of light and heat in winter causes a cessation in the functions 

 of the cells of the leaf; its fluid disappears by evaporation; its cells and 

 vessels become contracted and diminished in their calibre; various 

 inorganic matters accumulate in the texture; the whole leaf becomes 

 dry; its parts lose their adherence; and it either falls by its own weight 

 or is detached by the wind. In warm climates, the dry season gives 

 rise to similar phenomena. 



SECTION II. GENERAL VIEW OF THE FUNCTIONS OF 

 THE NUTRITIVE ORGANS. 



206. In order that plants may be nourished, food is required. This 

 food, in a crude state, enters the roots by a process of absorption or 

 imbibition; it is then transmitted from one part of the plant to another, 

 by means of the circulation or progressive movement of the sap; it reaches 

 the leaves, and is there submitted to the action of light and air, which 

 constitutes the function of respiration; and thus the fluids are finally 

 fitted for the process of assimilation, and form various vegetable pro- 

 ducts and secretions. 



1. FOOD OF PLANTS, AND SOURCES WHENCE THEY DERIVE THEIR NOURISHMENT. 

 CHEMICAL COMPOSITION OF PLANTS. 



207. The nutriment of plants can only be ascertained when their 

 chemical composition has been determined. The physiologist and chemist 

 must unite in this inquiry, in order to arrive at satisfactory conclu- 

 sions. Much has been done of late by Liebig, Mulder, Dumas, Bous- 



