332 



GENERAL SCIENCE 



ducing Diph- 

 theria. 



a 



for them. Bacteria on the roots of clover, peas, alfalfa, 

 and similar plants take nitrogen from the air and convert 

 it into plant food. Most plants require some 

 particular kind of bacteria for proper growth. 

 Food may be protected from destructive bac- 

 teria by sealing it or by using as a preserva- 

 tive some substance in which the bacteria 

 cannot grow. 



Mosses and Ferns. Mosses and ferns are 

 plants of higher development than the algae 

 and fungi. They contain chlorophyll bodies, and there- 

 fore are able to manufacture starch. 



Mosses will grow in extreme climates with little soil, 

 and therefore have probably had con- ^ A 



siderable part in the formation of soil 

 from rocks. 



Ferns form a class of plants slightly 

 higher than mosses. True ferns grow 

 best in damp, shady places. They 

 flourish in the densely wooded regions 

 of the tropics, where they grow to 

 immense sizes (Figure 296). 



The reproductive organs of the 

 common ferns are called sori. 

 They appear as little brown dots 

 on the under surface of the leaf. 

 Distribution of Plants. The 

 distribution of plants over the 

 earth's surface is determined by 

 the soil and the climate. It some- 

 times happens that a soil is rich 

 enough, but the climatic con- 



FIG. 295. Bacteria from a ... . 



Healthy Mouth Magnified. ditions are unfavorable to the 



FIG. 294. a, Bac- 

 teria of Pneumonia. 

 b, Bacteria of Tuber- 

 culosis. 



