NUTRITION. 159 



turgor. If both are turgid, they become curved away from 

 each other so as to increase the size of the opening between 

 them. If they are flaccid, the thick ridges along the inner 

 face of each cell straighten them, and so close the orifice 

 more or less completely (figs. 161, 162). The presence or 

 absence of hairs upon the leaves, the existence of stomata 

 upon one or both surfaces, the sinking of the guard cells 

 below the general leaf surface, the distribution of the stomata, 

 the thickening of the leaves, their inrolling (fig. 357), or 

 revolution (fig. 359), have a decided effect upon the rate of 

 transpiration, and may be adapted to regulate it. (See 



f 434 «r.) 



B. Foods in general. 



211. Foods. — In addition to an adequate supply of water, 

 food is required. Materials consumed by plants as food are 

 either organic or inorganic. Organic materials are those 

 which have been produced in nature by the chemical changes 

 occurring within living bodies. Inorganic materials are those 

 formed in nature by chemical reactions not occurring in con- 

 nection with a living body. A very few of the simplest plants 

 (bacteria) have been grown by the use of inorganic materials 

 alone ; only the minutest quantities of such substances are 

 utilized by most plants as food ; but large amounts are used 

 by all green plants for the manufacture of organic foods. 



Organic foods are of three kinds, carbohydrates, fats, and 

 proteids. 



212. Carbohydrates are substances consisting of carbon, 

 hydrogen and oxygen, so proportioned that there are 6 

 carbon molecules (or some multiple of 6) while the two latter 

 elements are combined in the ratio of two parts of hydrogen 

 to one of oxygen. Well-known examples are sugars and 

 starch. 



