QUESTIONS TO BE CONSIDEIIED. 57 



is chiefly with a view to the explanation I shall hereafter attempt to 

 give of the nature of this salutary action, that I have thought it neces- 

 sary here to make you acquainted with this add compound of nitrogen 

 and oxygen, in connection with the alkaline compound (ammonia) of the 

 same gas with hydrogen. 



Having thu^ shortly descrihed both the organic elements themselves, 

 and such chemical compounds of these elements as appear to be most 

 concerned in promoting the growth of plants, we are prepared for enter- 

 ing upon the consideration of several very important questions. These 

 questions are — 



1°. From what source do plants derive the organic elements of which 

 they are composed ? 



2°. In what form do plants take them up-^or what proof have we 

 that the compounds above described really enter into plants? 



3°. By what organs is the food introduced into the circulation of 

 plants? In consequence of what peculiar structure of these several 

 parts are plants enabled to take up the compounds by which they appear 

 to be fed ; and what are the functions of these parts, by the exercise of 

 which the food is converted and appropriated to their own sustenance 

 and further growth ? 



4*^. By what chemical changes is the food assimilated by plants, that 

 is — after being introduced into the circulation, through what series of 

 chemical changes does it pass, before it is converted by the plant into 

 portions of its own substance ? 



5°. By what natural laws or adaptations is the supply of those com- 

 pounds, which are the food of plants, kept up ? Animals are supported 

 by an unfailing succession of vegetable crops, — by the operation of what 

 invariable laws is food continually provided for plants ? 



These questions we shall consider in succession 



