NUTRITION 41 



You have been practicing it every instant of your life. 

 You know that in order to keep alive you must take into 

 your body certain substances from the world about you. 

 You know that you must get rid of certain wastes. You 

 know that health depends upon this income and this 

 outgo, and that loss of health results when these proc- 

 esses are not properly performed. You know that 

 too much heat is bad for you, and likewise too little heat. 

 All these are the outward evidences of- nutrition. As 

 they are true for you, they are true also for plants. 



You know that you live at the bottom of a great ocean 

 of gas we call the air, and on the surface of a pleasant solid 

 we call the earth, and that both in the air and in the earth 

 and on it we find an agreeable. liquid called water. You 

 know that from the sun there come to us forms of energy 

 called light and heat, which are of much comfort to our 

 lives. You know that if air, earth, water, light, and heat, 

 or any one of these should be lacking, our lives would 

 soon come to an end. All this is likewise true of plants. 

 Their life, like ours, depends upon the presence of these 

 things, and the great task of their lives is to secure such 

 portions of these things as are essential to their well-being. 

 Your task, just now, is to find how they do this. 



A. Food. — You know that, to live, you must have 

 what we call food. Plants, too, to live, must have what 

 we call food. You know that you get your food from the 

 world outside of you. Plants, too, get food, but they do 

 not get it from the world outside of them. From the world 

 outside they get materials from which food is made, but 

 food itself they make within their own bodies. 



Evidently, to understand what has just been said, you 



