CHAPTER VI 

 THE PHYSIOLOGY OF A TYPICAL PLANT 



IN order to carry on its life a plant must have an income of 

 matter and energy. The problem of energy will be reserved for 

 a later chapter: only a consideration of the relation of plants 

 to their food and its utilization will be given here. 



Plant Foods. The income of an ordinary green plant is de- 

 rived partly from the air and partly from the soil. It consists 

 of:- 



1. Carbon dioxid (CO 2 ), absorbed from the air by the leaves. 



2. Water (H 2 0), absorbed from the soil by the root hairs. 



3. Nitrates or other nitrogen salts, absorbed from the soil by 



the root hairs. 



4. Phosphates, potash salts, and other minerals, in small 



amounts, absorbed from the soil by the root hairs. 



The carbon dioxid and water are absorbed by the plant in 

 enormous quantities and constitute by far the largest proportion 

 of their foods; the soil minerals, although absolutely necessary, 

 are needed only in small quantities. Roughly speaking, the 

 amount of material absorbed from the soil is represented by the 

 ashes that are left after a plant is burned. All of the minerals 

 are dissolved by the waters in the soil and absorbed in this form 

 by the root hairs. 



Ascent of Sap. Since the foods are obtained through organs 

 situated at the opposite ends of the plant, in order that they 

 may be utilized they must be brought together, and since it is 

 in the leaves that they are utilized, the water, containing the 

 dissolved minerals absorbed by the roots, must be carried up the 

 stem to the leaves. This ascent of sap is going on constantly 

 during the activity of the plant and its rapidity is proportional 

 to the activity of the processes going on in the leaves and buds. 



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