18 PROTOPLASM AND THE CELL 



can be decomposed and the energy liberated. This energy- 

 may be employed to change the shape and form of the 

 protoplasm forcibly. The precise mechanism which is 

 employed to turn the chemical energy of food into me- 

 chanical motion by the living substance is not well under- 

 stood and is at best a difficult one to study. 



The Power of Causing Chemical Change. — The 

 energy liberated from foods may be employed to carry 

 out other chemical changes. The sequence of events is 

 somewhat as follows: Energy is stored up in food sub- 

 stances only by the green cells of plants; but once this 

 energy is stored in foods they may undergo further chemi- 

 cal changes. These changes may be of two sorts. In 

 the first place, they may be of the sort which liberates 

 energy. This type of change is of course necessary when- 

 ever energy is to be used for forcible movements. On 

 the other hand, there are changes in which the resulting 

 compound contains more energy than the one out of 

 which it was made. Now, no change of this sort can be 

 brought about without the application of energy from 

 the outside. This energy must then be derived from the 

 decomposition of foods. It will not be possible in a book 

 of this limited scope, to discuss in detail the great variety 

 of chemical changes brought about in living protoplasm. 

 It will be sufficient, perhaps, merely to call attention to 

 the fact that organic substances produced in this way 

 vastly outnumber the inorganic ones. Such various sub- 

 stances as woofl, the textiles, drugs, rubber, gums, and 

 thousands of other articles of commerce, all have their 

 origin in chemical changes which occur in living matter. 



Sensitiveness to Stimuli. — Protoplasm is not only 

 capable of executing chemical reactions and forcible 

 movements but it is also extremely sensitive to outside 

 influences. The changes in the environment to which it 

 responds are called stimuli. They may be of a variety 

 of sorts. Mere contact, the presence of chemical sub- 

 stances of a particular character, electric currents, light, 



