PHYSIOLOGY. 117 



Stances which come directly under its influence. These 

 changes are of two kinds. They may be constructive, i.e., 

 they may build up complex substances out of simpler ones, 

 and so fit them for use in repairing the waste caused by the 

 activity of the protoplasm; or they maybe destructive, i.e., 

 they may break down complex substances into simpler, so 

 setting free the energy necessary for the work of the proto- 

 plasm. The substances broken down may be repaired in 

 whole or in part, i.e., may take part in constructive met- 

 abolism. Those in which no repair occurs often undergo 

 further destructive changes by which they become converted 

 into materials useless to the plant, and to be gotten rid of. 

 Metabolism, therefore, includes all the chemical changes 

 by which food is either manufactured or utilized, and by 

 which waste materials are produced and eliminated. 



149. Irritability. — Protoplasm is irritable, that is, it 

 exists in such a state that it is sensitive to external influences, 

 which thereby affect the various functions of the whole 

 plant. By reason of its irritability, it may even transmit the 

 effects of an external stimulus from one part to a distant 

 part. Moreover, it is capable of initiating similar changes 

 without the action of any observable external influences, and 

 is, therefore, not only irritable but automatic. 



150. Contractility. — Protoplasm is contractile, that is, it 

 has the power of altering its form, of shortening in one direc- 

 tion and elongating in another, by virtue of inherent forces 

 whose action is not understood. 



151. Reproduction. — Protoplasm is reproductive, that is, 

 it is capable of so directing' the chemical and physical forces 

 inherent in it that a new organism similar to that of which it 

 forms part may be produced. 



152. Adaptation. — The interrelation of these powers, 

 their harmonious co-working and their variation to suit the 

 varying conditions of the surrounding media (air, water, 



