14 PLANT-LIFE 



nitrogen-fixing Bacteria, enrich the soil in which they 

 grow, and it is not uncommon for agriculturists to plough 

 in crops of leguminous plants in order to increase the fer- 

 tility of the soil. The fact that Bacteria are agents in the 

 purification of sewage should also be placed to their credit. 

 Bacteria are practically ubiquitous. If we ascend the 

 hills, they are there, for they have certainly been re- 

 corded at an altitude of 10,000 feet. They abound in 

 soil and water, even in the water we drink, which, by 

 the way, would be decidedly " wersh " without their 

 presence. They also float about in the air, and are 

 distributed amongst the dust which we raise with our 

 feet as we walk. When we inhale the fragrance of 

 flowers, we may draw Bacteria from the petals into our 

 nostrils. When we kiss, we may " exchange microbes." 

 When we stroke a dog, we may scatter a host of these 

 organisms. It has been calculated that no less than 

 50,000,000 of Bacteria may be contained in so little 

 as an ounce of roadside dust. Their size is so exceed- 

 ingly small that in the case of such a germ as that of the 

 Bubonic Plague 250,000,000 would cover about a 

 square inch of soil, and be able to thrive on it. The 

 average short diameter of the Bacteria is said to be about 

 3 5 "^ 00 inch. These organisms are not easily killed; 

 they can survive great extremes of temperature. In an 

 experiment a number (forty-four) were exposed to the 

 intensely cold temperature of - 210° C, yet were able 

 to survive. Dry spores resist high temperatures; they 

 have survived a ten minutes' exposure to 150° C. Nor 

 need we think that mere boiling will kill certain spores, 

 for some are known to be able to survive even after 

 being boiled at a temperature of 100° C. for several hours. 



