140 PLANT BIOLOGY 



A. Bacteria 1 



147. Microscopical appearance and size of bacteria. 

 Every one is familiar with the fact that if a bouquet of flowers 

 is left for some time in a vase of water, the stems decay and 

 disagreeable odors are given off. This is a common example 

 of the action of bacteria, for all decay is due to the work of 

 these organisms. When we come to examine the flower-stems 

 or the putrid water, we find a slimy scum. If we put a drop 

 of this scum on a slide, cover with a cover-glass, and examine 

 with the highest powers of the microscope, usually we would 

 see many different forms of living things. Some of them 

 would probably appear relatively large, and these, as we shall 

 see later (Chapter IV, " Animal Biology "), are single-celled ani- 

 mals. A closer examination will disclose countless numbers 

 of very minute, colorless organisms ; these are the bacteria. 

 A careful study of many kinds of bacteria shows that they have 

 several characteristic shapes (see Fig. 71) by means of which 

 they can be roughly classified. Some are rod-shaped (like a 

 firecracker), some are spherical, or egg-shaped, and still others 

 are spiral-shaped. Each bacterium is a tiny bit of translucent 

 protoplasm, inclosed in a cell wall of cellulose. Thus far no 

 nucleus has been discovered in any kind of bacteria. Be- 

 cause of their cellulose walls, and because of their likeness 

 to certain low forms of green plants, biologists now regard 

 these organisms as plants rather than animals. 



Some of the rod-shaped bacteria have one or more long, 

 hairlike projections from the ends, called cil'i-a, which give 

 the germs still further resemblance to firecrackers. These 

 cilia lash about rapidly, and thus drive the cells through the 



1 Because of the importance of bacteria in relation to sanitation, 

 it may be found advisable to consider this whole topic in connection 

 with human biology. Sections 148-154 will therefore be repeated 

 in the book on human biology. 



