22 SUMMARY. [CHAP. II. 



and give off carbonic acid gas. This may be regarded as a 

 respiratory process. It is not improbable that the green 

 parts of plants also may at all times, but especially in the 

 dark, absorb oxygen and give off carbonic acid in the same 

 way, but in these parts the amount of carbon fixed greatly 

 predominates over that which is liberated. The mutual 

 relations, however, of these processes are as yet very 

 imperfectly understood. 



9. It is not yet perfectly clear from what immediate 

 source the plant obtains its nitrogen ; not that the element 

 is scarce, since it forms four-fifths of the atmosphere, but, 

 the precise state in which it enters the plant, whether 

 uncombined (which is not probable) or in combination, as 

 in ammonia and nitrates, is still a matter of inquiry and 

 discussion among scientific men. 



TO. The processes which we have briefly described of 

 absorption, transpiration, assimilation, and respiration, we 

 find mainly concern the root, the stem, and the leaves. 

 These are the food-providers and preparers of the plant. 

 Hence we call them collectively the 7iutritive organs; the 

 root being, as we have shown, the organ of absorption, the 

 foliage-leaves specially of transpiration, assimilation, and 

 respiration. The stem, when green, assists the leaves in 

 their work ; but, speaking generally, it may be regarded 

 merely as the support of the leaves, maintaining their con- 

 nection with the root. 



In our next chapter we shall inquire into the mutual 

 relations and functions of the leaves which compose the 

 flower, deferring further reference to the chemistry of the 

 organs until we speak of their minute structure. 



