378 EXPERIMENTS WITH PLANTS 



acid (lactic acid) produced by bacteria which change 

 the sugar of the milk into lactic acid (test some sour 

 milk with litmus). After a time this process ceases, 

 long before the milk-sugar is all used up, because the 

 acid checks the activity of the bacteria. If we now add 

 enough lime-water to neutralize the acid, they will 

 begin to grow and form more acid, as you will see on 

 testing with litmus paper a few hours later. Make 

 this test. It is a general rule that both animals and 

 plants are poisoned by their own excretions. 



When bacteria live in the bodies of animals or 

 plants,^ they are dangerous or harmless according as 

 their excretions are poisonous or not to the organism 

 in which they live. It is well known that the human 

 body contains many bacteria which are perfectly harm- 

 less, because their excretions are not poisonous to 

 it. On the other hand, the poisonous excretions 

 (known as toxins) of other sorts may produce death 

 in a few hours (as in lockjaw, etc.). It is not neces- 

 sary that the bacteria should enter the body at all ; if 

 their toxins are introduced into the body the same 

 effect is produced as if the bacteria themselves were 

 present. Bacterial diseases would invariably cause 

 death wherever they obtained a foothold were it not 

 for the fact that the body, when the toxins make their 

 appearance in it, produces antitoxins^ e. g., sub- 



^ Very few bacteria are known which can live when injected into a live 

 plant; in most cases they die in a few hours. 



