PHYSIOLOGY OF VEGETATION. 181 



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 various products) having now been 

 considered, both separately and to some extent in their combined 

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

 general phenomena and results of vegetation ; to inquire into the 

 elementary composition of plants, the nature of the food by which 

 they are nourished, the sources from which this food is derived, 

 and the transformations it undergoes in their system, chiefly in the 

 leaves. 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 mineral, unorganized matter is 

 converted into the tissue of plants and other forms of organized 

 matter (12, 15, 16). From this point of view, therefore, the re- 

 ciprocal relations and influences of the mineral, vegetable, and 

 animal kingdoms may be most advantageously contemplated, and 

 the office of plants in the general economy of the world best under- 

 stood. This portion of general physiology is intimately connected 

 with chemistry, and some knowledge of that science is requisite for 

 the due comprehension of the subject, especially in relation to its 

 exceedingly important applications to agriculture and horticulture. 

 We are here restricted to the bare statement of the leading facts 

 which are thought to be established, and the more important de- 

 ductions which may be drawn from them ; omitting, for the most 

 part, to adduce the evidence by which these general propositions 

 are supported. 



317. Although the organs of vegetation have been considered 

 anatomically and morphologically, or in view of their structure 

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

 connected action in the maintenance of 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 



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