PHYSIOLOGY AND HYGIENE 



CHAPTER I 

 PLANT AND ANIMAL LIFE 



Growth. As we watch plants and animals grow, we 

 see how many changes they are making in the world 

 about them. The plants are taking the materials which 

 they need from the earth, air and water, 1 and are build- 

 ing these materials into themselves. The animals in 

 turn are taking the plant substances and are building 

 their own bodies out of them. 



A clover plant starts from a seed within which is 

 hidden a minute plant and food prepared by the mother 

 plant for its use. When placed in the moist warm earth, 

 the plant awakens and by living upon the stored-up 

 food quickly becomes larger and stronger. After it has 

 thrust its head out of the ground and its roots down 

 into the soil, it is able to take a gas known as carbon 

 dioxide from the air and water, with a small amount of 

 soluble material from the earth. From these, with the 

 aid of the sun's light and heat, it gradually builds up 

 a large and strong clover plant. The gas and water 

 are no longer simply gas and water, but are combined 



1 The soil contributes mainly water containing a small amount 

 of salts in solution. The air contributes a colorless gas, carbon 

 dioxide (CO 2 ), which is produced by the burning and decay of 

 wood and other organic materials ; and a small amount of nitrogen. 

 Water (H 2 O) consists of 2 parts of hydrogen to 1 of oxygen. For 

 a further discussion, see Appendix, p. 315. 



