INTRODUCTION. 145 



180. The fundamental powers of protoplasm arc tour ; it 

 is metabolic, irritable, contractile, and reproductive. 



181. Metabolism. — Protoplasm is metabolic, thai is, it is 

 capable of initiating a series of chemical changes in itself and 

 in substances 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 may be destructive, i.e., 

 they may break down complex substances into simpler, so 

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

 toplasm. The substances broken down may be repaired in 

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

 tabolism. 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. 



182. 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 

 organism 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. 



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

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

 direction and elongating in another, by virtue of inherent 

 forces whose action is not understood. 



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

 it is capable of so directing the chemical and physical forces 



