SINGLE-CELLED PLANTS AND COLONIES. 9 



nificance of this is that some of the ancestors of the green 

 oscillarias probably had offspring which, instead of living 

 upon food prepared by means of the green coloring matter 

 (^ 185 ff.), learned to use the organic matter in the water, 

 at first perhaps no more than the present oscillarias do ; but 

 gradually they came to live exclusively upon it. As a conse- 

 quence, they lost their green color and became incapable of 

 existing where organic food cannot be had. 



Bacteria. 



14. Fission-fungi. — Along with the loss of color and change 

 of habit went a diminution in size. They have now become 

 so different that they are known as fission-fungi, and popu- 

 larly as bacteria, bacilli, microbes, germs, etc. These plants, 

 probably the descendants of common ancestors with the fis- 

 sion-algae, are the smallest known living things (figs. 8, 9). 

 The diameter of many sorts does not exceed .0005 of a milli- 

 meter. That would allow 1 75 to lie side by side upon the edge 

 of the paper on which this book is printed. Though so minute 

 these plants have the same sort of protoplasm and cell-wall as 

 larger ones. They increase in number rapidly by each cell 

 dividing into two, which separate readily into independent 

 plants. 



15. Gelatin. — In the fission-fungi, as in the fission-algae, 

 considerable masses of jelly-like material are produced, in 

 which the plants may lie embedded. The films, sometimes 

 smooth, sometimes wrinkled, which appear on an infusion of 

 organic matter, such as tea or broth, are formed by masses 

 of bacteria which rise to the surface and become embedded in 

 the gelatinous material they produce {b, fig. 8). 



Demonstration. — Steep a cupful of chopped hay in hot water for fifteen 

 minutes, and set the infusion, loosely covered, in a warm place. After a 

 day or two, show the film of bacteria which covers the surface of the 

 liquid. 



