124 FUNCTIONS OF NUTRITIVE ORGANS. 



takes place by a folding inwards of the tissue at the point where the 

 leaf joins the stem or branch, and this gradually extends ; complete 

 separation then takes place, and the leaf 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 op the 

 Nutritive Organs. 



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

 in a crude state, enters the roots by a process of absoi-ption or imbibi- 

 tion ; 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 

 products and secretions. 



1. — Food of Plants and Sources whence they derive their Nourishment. 

 Chemical Composition of Plants, 



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 

 conclusions. Much has been done by chemists to aid the botanist in 

 his investigations, and to place physiological science on a sound and 

 firm basis. It is true that many processes take place in plants which 

 cannot as yet be explained by the chemist, and to these the name of 

 vital has been applied. This term, however, must be considered as 

 implying nothing more than that the function so called occurs in 

 living bodies, and in the present state of our knowledge cannot be 

 fully explained by chemical or physical laws. A greater advance in 

 science may clear up many difficulties in regard to some of the vital 

 functions, while others may ever remain obscure. 



Plants are composed of certain chemical elements, which are com- 

 bined in various ways, to form organic and wiorganic compounds. The 

 former are composed of carbon, oxygen, hydrogen, and nitrogen or 

 azote, with a certain proportion of sulphur and phosphorus ; while 

 the latter consist of various metals, combined with oxygen, other metal- 

 loids, and acids. In aU plants there is a greater or less proportion 

 of water, the quantity of which is ascertained by drying at a temper- 

 ature a little above that of boiling water. By burning the dried plant 

 the organic constituents disappear, and the inorganic part is left in 



