82 The Insides and the Workings of Living Things 



of numerous cells. The bacteria, of which everybody has 

 heard, show hardly any structure at all. The protoplasm 

 is contained within a membrane or cell-wall. In some 

 species delicate vibratile " hairs " project beyond the wall 

 and serve as a means of locomotion. The individual absorbs 

 water from the outside through its wall membrane, and dis- 

 charges wastes in reverse direction in the same way. Most 

 bacteria live in a fluid medium. In the presence of abundant 



moisture some bacteria secrete a 

 ferment which can digest solid 

 food material, and then absorb 

 the fluid containing the digested 

 food. While many bacteria are 

 known to be parasitic and to 

 cause disease in man and in other 

 animals, most of them live upon 

 dead organic matter and cause 

 fermentation or decay. Indeed, 

 they constitute an important 

 link in the cycle of life, since 

 they are responsible for convert- 

 ing most of the dead plant and 

 animal bodies into food which 

 succeeding generations utilize. 

 We say to the dead, " Dust thou 

 art, to dust returneth," but we 

 do not always realize that it is 

 the activity of bacteria that 

 The souring of milk, the cur- 

 ing of hay, the retting of flax, the spoiling of food are also 

 parts of this general process of bacteria keeping themselves 

 alive. 



The greenslime found on the bark of trees and on the 

 shingles of old houses (sometimes erroneously called " moss '') 

 represents another large group of one-celled plants (Fig. 

 i8). Here, as in the bacteria, the individual consists of a 

 single unit of protoplasm surrounded by a cell-wall. In 



Fig. 1 8. Greenslime 



The pleurococcus consists of a single 

 cell, a unit of protoplasm with a defi- 

 nite wall. All the income is absorbed 

 through the wall, in a fluid or gaseous 

 state. The by-products of metabolism 

 are discharged by diffusion through the 

 wall. There are no special organs for 

 taking in or for excreting. When a cell 

 divides into two, the daughter-cell may 

 become separated or they may cling 

 together. From Gruenberg, Biology 

 and Human Life, published by Ginn 

 & Company. 



makes possible the return. 



