SCHIZOPHYTA 459 



IOO C., at least for a time. For complete sterilisation it is necessary 

 after boiling to incubate the culture at a favourable temperature of 

 37 C. for 48 hours, during which time the spores will all pass into 

 the active but vulnerable state. Then a second boiling will com- 

 pletely sterilise the fluid. This method is commonly used in the 

 preparation of media for the culture of Bacteria. 



It is important to realise the great rapidity of multiplication of 

 Bacteria. Under favourable conditions B. subtilis is found to divide 

 once in about 20 minutes. If this pace be continued by all the 

 progeny for 8 hours the result from a single Bacillus would be over 

 1 6 millions. It is not, however, the rapid multiplication and easy 

 transfer of these minute bodies alone which gives the Bacteria their 

 importance. A still more interesting feature is the variety of their 

 physiological powers. Being as a rule parasitic and saprophytic plants, 

 they depend for their. supply of food and energy upon breaking down 

 more complex into simpler compounds : and they do this in the most 

 various ways. The end of all such changes is ultimately carbon- 

 dioxide and water. In fact, Bacteria are the great scavengers of the 

 world, restoring organic material to the sources from which it came. 

 But in the course of the process many steps may intervene : and bye- 

 products may be produced which are sometimes useful, though often 

 harmful to other organisms, and to Man. Thus the Bacteria that 

 cause various diseases release into the blood or tissues, toxins or 

 poisons which give the several diseases their character. On the other 

 hand changes wrought by Bacteria may be beneficial in various ways : 

 for instance, the nitrifying Bacteria convert ammonia into nitrous, 

 and finally into nitric acid, present in the soil as nitrates ; acetic- 

 acid-bacteria may convert alcohol into vinegar ; and Rhizobium may 

 fix free nitrogen in the root-tubercles in which it grows (p. 204). 

 Again, the flavours of cheese, butter, or tobacco, and of many other 

 commodities, depend for their market-value upon the exact type and 

 conduct of the partial decomposition of their constituents by bacterial 

 action. 



Where suitable material is available Bacteria may multiply 

 indefinitely. But there are important external checks which control 

 them. Many Bacteria are susceptible to injury by light. This has 

 been shown for the Anthrax-Bacillus, by growing it on culture- 

 plates, partly exposed to light and partly shaded; and the results 

 have been verified in various others. The destructive effect 

 lies in the blue-violet end of the spectrum. After incubation in the 

 dark for three or four days, the area of a plate exposed for a time to 



