344 A TEXTBOOK OF THEORETICAL BOTANY 



mode of life and are so successful that they are man's most serious com- 

 petitors. If we regard Man as the apex of that line of evolution which has 

 stressed the development of relatively small numbers of large and complex 

 individuals, we might also say that the Bacteria represent the extreme 

 expression of the opposite trend towards the greatest possible numbers of 

 the smallest individuals. 



Cell Structure 



Leeuwenhoek (Fig. 332), who discovered the Protozoa, was also the 

 first to observe Bacteria, in 1683, but he thought they were animalcules, 



Fig. 332. — Portrait of Anton van Leeuwenhoek. 



(By courtesy of Dr. C. Dobell.) 



owing to their vigorous powers of locomotion. The movements of Bacteria 

 are not entirely due to their own activity. Some are self-motile, but all 

 alike show Brownian movement, which is purely physical and is exhibited 

 by all small particles in suspension. This movement is caused by the 

 bombardment of the particles by the vibrating molecules of the surrounding 

 medium, which has no visible effect on large objects but sets up rapid 

 oscillations in particles which are sufficiently small. 



The cell wall is a rigid layer, apparently chiefly polysaccharide in 

 nature, though fatty substances and proteins are also important con- 

 stituents. True cellulose is of rare occurrence, but chitin is occasionally 

 present. The outer wall is sometimes modified into a thick, mucilaginous 

 capsule, as in the diplococcus of pneumonia. These capsules protect the 

 enclosed organism against desiccation, and no doubt permit the dispersal 

 through the air of the organisms which possess them. 



Little is known as yet with certainty about the cell contents. The 



