ANIMAL BIOLOGY 



Energy in the Organic World. - - We see animals exerting 

 energy; that is, we see them moving about and doing 

 work. Plants are never seen acting that way; yet they 

 need energy in order to form their tissues, grow, and raise 

 themselves in the air. 



Source of Plant Energy. - - We notice that green plants 

 thrive only in the light, while animal growth is largely in- 

 dependent of light. In fact, in the salt mines of Poland 

 there are churches and villages below the ground, and 

 children are born, become adults, and live all their lives 

 below ground, without seeing the sun. (That these people 

 are not very strong is doubtless due more to want of fresh 

 air and other causes than want of sunlight.) 



The need of plants for 

 sunlight shows that they 

 must obtain something 

 from the sun. This has 

 been found to be energy. 

 This enables them to lift 

 their stems in growth, and form the various structures 

 called tissues which make up their stems and leaves. (See 

 Part I, Chap. XIII.) It is noticed 

 that they take in food and water 

 from the soil through their roots. 

 Experiments also show that green 

 plants take in through pores 

 (Fig. i), on the surface of their 

 leaves, a gas composed of carbon 

 and oxygen, and called carbon 

 dioxid. The energy in tJie sunligJit 

 enables the plant to' separate out the 



carbon of the carbon dioxid and 



. FIG. 2. A LEAF STORING 



build mineral and water and carbon ENERGY IN SUNLIGHT. 



FIG. i. SURFACES OF A LEAF, 

 magnified. 



Carbonic^Acid Gas 

 in the Air going 

 into the Leaf 



