PHYSIOLOGY OF VEGETATION. 177 



CHAPTER VI. 



OF THE FOOD AND NUTRITION OF PLANTS. 

 Sect. I. The General Physiology of Vegetation. 



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

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

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

 action, we are i^i'epared to take a compreliensive 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 transfoi*- 

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

 or, to use a better temi, 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 

 \ie\\, 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 Avith chemistry, and some knowledge of that sci- 

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

 the bare statement of the leadinij facts Avhich are thought to be 

 established, and the more important deductions which may be di'awn 

 from them. 



317. While the oi-gans of A^egetation liaA'e been considered ana- 

 tomically and morphologically, or in view of their structure and 

 dcA'elopment, still the leading points of their physiology, or connected 

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

 been explained or assumed. 



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

 comprise a A^ai'iety of distinct processes, are reduced to the greatest 

 degree of simplicity in A'egetables. Imbibition, assimilation, and 

 growth essentially include the Avhole. 



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

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



