RELATION OF PLANTS TO ENVIRONMENT 5 



stems, and leaves are sensitive to such forces as light and grav 

 ity, and that they have the ability to respond to them by definite 

 growth movements which are dependent upon the nature of 

 the organ concerned. This sensitiveness and response enables 

 the organs of plants to adjust themselves to their environment 

 by placing themselves in proper positions for the absorption of 

 foods and energy. For example, the root system secures a wide 

 distribution in the soil, since its primary and lateral roots are 

 able to respond to the gravity stimulus by growth in different 

 directions with reference to the stimulating force. Similarly, 

 the main stem and its branches respond to gravity and light, 

 and leaves are able to adjust themselves to light coming from 

 different angles. 



Plants are, therefore, sensitive organisms which sustain a defi- 

 nite relation to the forces of their environment, and they are 

 able to use these forces in assuming the various attitudes and 

 positions which are most advantageous to them in a given 

 place and time. 



The energy of the sunlight is also absorbed and transformed 

 into heat and manufacturing energy, to be used in the making 

 of sugar, starch, and cellulose by the green portions of plants. 



The materials. Green plants are likewise able to use the 

 material substances of the air and the soil as no other organ- 

 isms can do, and thereby to build up food for themselves and for 

 the animal world, for we shall find that all animals are ultimately 

 dependent upon plants for sustenance and continued existence. 



This peculiar relation of green plants to the inorganic sub- 

 stances of their surroundings is easily understood from the 

 diagrammatic drawing (Fig. 2). 



The root system of the plant represented in Fig. 1, a, has been 

 admirably distributed through a wide area of soil by means of 

 the sensitiveness of the roots to gravity. From this soil the 

 roots absorb water (composed of hydrogen and oxygen (H 2 O)) 

 and soil salts containing the various substances, such as 

 nitrogen, sulphur, phosphorus, and potash, which the plant 

 uses in building its own foods and its living substance. The 

 leaves take in carbon dioxide (CO 2 ) from the air, and practically 



