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, hut it will serve to illustrate the nature of many 



chemical reactions. The energy that was required to hold the 

 n ami iron together begins to be set free as SOOH as the 

 decomposition starts and it contributes to the rise <>f temperature 

 during the reaction. The living substance of tin- plant is com- 

 posed of a great variety of chemical substances that are verj 

 readil) decomposed 1>\ gases ami other substances which it ab- 

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

 powerful in producing these changes. These substances ami 

 forces are often referred to as stimuli i 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 energ) 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- 

 forin 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 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 dm- to the fait that not all portions of the protoplasm 



of the plant body are equally influenced by, or we ma 

 equall) sensitive to heat, light, and other forces and consequently 



the vrariors part- of tin- plants do not respond alike. The torn 

 of gravity, for example, acts upon some steins and the living 



snbstan.c j- influenced b) 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 s,, constituted that the 



stimulus of gravity cans., a growth that bends the root down 

 into the soil. Light Stimulates the mustard plant in such a way 



