PLANTS AND ANIMALS 29 



respiration and goes on only under favoring conditions. 

 At night all living things are consuming oxygen and giv- 

 ing forth carbon dioxid. The same reactions occur by 

 day but the constructive activities of the higher plants 

 are then so remarkable that we are prone to forget the 

 undercurrent which is still setting in the opposite direc- 

 tion. Sometimes it is said that carbon dioxid and water 

 are the principal foods of green plants, but it is more ac- 

 curate to say that such plants have the power to make their 

 foods from simple raw materials. 



Attention should be called to the fact that light is not, 

 on the whole, a factor that is favorable to life. It is 

 highly destructive to the lower organisms and its value 

 to man is indirect. It is helpful to him because it de- 

 stroys some of his enemies, because it keeps up the food 

 supply of the world, and because of its relation to his 

 intellectual interests. Light is probably always detri- 

 mental rather than beneficial to defenseless living matter. 

 This is to say that all transparent organisms are subject 

 to injury through its influence. An important function 

 of the pigment (chlorophyll) in those plants which profit 

 by the light is probably to turn back from the leaves most 

 of its searing rays and to admit only selected ones to the 

 laboratories where the photosynthesis goes on. Our 

 experiences with sunburn remind us that light may harm 

 the human skin, while the development of tan suggests a 

 protective reaction. It is not unreasonable to say that 

 the chief reason why we are not more damaged by light 

 is that we are too thick; it does not pierce to the seat of our 

 vital processes. 



Recognizing this, we find the synthetic application of 

 light all the more remarkable and the scale on which the 

 action proceeds is too vast for the imagination. The 

 annual harvest of all the nations and the cut of timber 

 give us an inkling of it. These immense returns from the 

 vegetable world, we must remember, are produced only 

 to a small extent from the soil but very largely from the 

 air. This was the fact which surprised Van Helmont as 



