PHYSIOLOGY OF VEGKTATION. 177 



C PI AFTER YI. 



OF THE POOD AND NUTRITION OP PLANTS. 



Sect. I. The General Physiology of Vegetation. 



316. The Organs of Vegetation or Nutrition (those by which 

 plants grow and form their various products) having now been con- 



. sidered, both as to their structure and to some extent as to their 

 action, we are prepared to take a comprehensive survey of the 

 general results of vegetation ; to inquire into the elementary com- 

 position of plants, the nature of the food by which they are nour- 

 ished, the sources from which this food is derived, and the transfor- 

 mations it undergoes in their system. It is in vegetable digestion, 

 or, to use a better term, in assimilation, that the essential nature of 

 vegetation is to be sought, since it is in this process alone that min- 

 eral, unorganized matter is converted into the tissue of plants and 

 Other forms of organized matter (1, 12-16). From this point of 

 view, therefore, the reciprocal relations and influences of the min- 

 eral, vegetable, and animal kingdoms may be most advantageously 

 contemplated, and the office of plants in the general economy of the 

 world best understood. This portion of general physiology is inti- 

 mately connected with chemistry, and some knowledge of that sci- 

 ence is requisite for understanding it. We are here restricted to 

 the bare statement of the leading facts which are thought to be 

 established, and the more important deductions which may be drawn 

 from them. 



317. While the organs of vegetation have been considered ana- 

 tomically and morphologically, or in view of their structure and 

 development, still the leading points of their physiology, or connected 

 action in the life and growth of the plant, have from time to time 

 been explained or assumed. 



318. The functions of nutrition, which, in the higher animals, 

 comprise a variety of distinct processes, are reduced to the greatest 

 degree of simplicity in vegetables. Imbibition, assimilation, and 

 growth essentially include the whole. 



319. Plants absorb their food, entirely in a hquid or gaseous form, 

 by imbibition, according to the law of endosmosis (40), through the 



