4 NATURE OF THE LIVING SUBSTANCE 



heat the carbon has a stronger attraction for the oxygen than 

 the iron has. Consequently the oxygen is drawn away from the 

 iron and unites with the carbon, forming a new combination of 

 carbon and oxygen, and leaving the iron free. This decomposi- 

 tion and re-combination is only a part of the chemical change 

 that takes place, but it will serve to illustrate the nature of many 

 chemical reactions. 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 com- 

 posed of a great variety of "chemical substances that are very 

 readily decomposed by gases and other substances which it ab- 

 sorbs. Light, heat, gravitation and moisture are 'even more 

 powerful in producing 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 vari- 

 ous 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 per- 

 form any duty, "ft 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 furnished 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 influenced by this stimulus so that a growth is 

 aroused that bends the stem into an upright position. The proto- 

 plasm 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 



