THE GENERAL LIFE PROCESSES 325 



structure in living things, because the whole body is com- 

 posed of them, variously fitted together. Through the walls 

 the protoplasts connect with one another by very delicate 

 strands of protoplasm, so that in reality the living substance 

 in a plant or animal body is a continuous substance. 



.Each protoplast is very definitely and intricately 

 organized, but for our purpose we may disregard all other 

 organization except the nucleus and the cytoplasm. The 

 nucleus is a more compact mass of protoplasm, usually 

 near the center of the protoplast. The more fluid proto- 

 plasm which envelops it and which forms a wall about 

 itself is the cytoplasm. In it usually are found floating 

 particles of various kinds, but these are riot living. 



In order to do its work the protoplast must be saturated 

 with water, for every substance that passes into it or passes 

 out from it must be in solution, or at least diffusible in 

 water. In fact the whole plant or animal body may be 

 conceived of as a continuous body of water in which solid 

 particles are arranged. Frequently the protoplasm en- 

 counters conditions unfavorable for its work, such as cold 

 or drought, and then it loses water and passes into what is 

 known as the quiescent condition. When suitable condi- 

 tions return, water is taken in and the customary activity 

 of the protoplasm is resumed. 



To get some general notion of the ordinary work of 

 protoplasm one may follow the activities of a tree from the 

 germination of the seed to the production of seed. A plant 

 is selected because a very important process (food making) 

 goes on in green plants and does not occur in animals, and 

 none of the fundamental processes which occur in animals 

 are omitted in plants. 



