THE GROWTH OF THE PLANT 93 



moisture be present in sufficient quantity and that they 

 contain enough of dissolved nutrient salts to provide 

 an abundant supply of mineral food elements for the 

 growing plants. 



The principal function of the stem is to act as a 

 framework on which to hang the leaves and display 

 them properly to the action of air and sunlight. It 

 also serves as a reservoir of moisture and its vessels 

 and tissues provide for the transmission of the water 

 and mineral salts absorbed by the roots to the leaves 

 in which it is elaborated into plant food. It also 

 serves for the distribution of the prepared food from 

 the leaves to nourish all parts of the plant. In some 

 cases, too, stems like roots become swollen and serve 

 as special reserve food storehouses. 



The leaves are the laboratories where the crude 

 mineral food elements are combined with the car- 

 bonic acid of the air and so elaborated that they can 

 be utilized for the nutrition and growth of the plant 

 cells. It is this power to utilize mineral elements as 

 food that chiefly serves to distinguish plants from 

 animals. All green-leaved plants can do this, but 

 animals must depend for food on materials that have 

 already been elaborated by plants. In almost all 

 other respects their life processes are identical. 

 Every living cell, whether of plant or animal, must 

 have proteids and carbohydrates for food and must 

 breathe or absorb oxygen and give off carbonic acid 

 as the result of those chemical changes from which 

 its vital energy is derived. The plant can make its 

 proteids and carbohydrates, the animal cannot. 

 Carbonic-acid gas always exists in the atmosphere. 



