4 NATURE OF THE LIVING SUBSTANCE 



has a stronger attraction for the oxygen than the iron has. Con- 

 sequently the oxygen is drawn away from the iron and unites 

 with the carbon, forming a new combination of carbon and oxy- 

 gen, and leaving the iron free. This decomposition and re-com- 

 bination is only a part of the chemical change that takes place, 

 but it will serve to illustrate the nature of many chemical re- 

 actions. The energy that was required to hold the oxygen and 

 iron together begins to be set free as soon as the decomposition 

 starts and it contributes to the rise of temperature during the 

 reaction. The living substance of the plant is composed of a 

 great variety of chemical substances that are very readily de- 

 composed by gases and other substances which it absorbs. Light, 

 heat, gravitation and moisture are even more powerful in pro- 

 ducing these changes. These substances and forces are often 

 referred to as stimuli (sing, stimulus) because they start the 

 chemical changes referred to above. So the various substances 

 and forces in nature stimulate or cause chemical changes in the 

 living substance of the plant and the energy thus set free is used 

 by the protoplasm in the performance of its work. The cells of 

 a seed have no power of their own to grow or perform any duty. 

 It is not until a certain amount of heat, moisture, gases or other 

 stimulating forces have acted upon the protoplasm or substances 

 contained in it and so aroused chemical changes that it is fur- 

 nished with the requisite energy to begin growth. Furthermore, 

 these forces exert a very definite influence upon the protoplasm 

 and cause it to accomplish very definite results. This is due to 

 the fact that not all portions of the protoplasm of the plant body 

 are equally influenced by, or we may say equally sensitive to 

 heat, light, and other forces and consequently the various parts 

 of the plants do not respond alike. The force of gravity, for 

 example, acts upon some stems and the living substance is in- 

 fluenced by this stimulus so that a growth is aroused that bends 

 the stem into an upright position. The protoplasm in the cells 

 of the root, however, is so constituted that the stimulus of gravity 

 causes a growth that bends the root down into the soil. Light 

 stimulates the mustard plant in such a way that the stem grows 

 toward the light and the root away from it. So the living sub- 



