PLANTS WHlCa VAUtiE DECAY 385 



containing the tubercle - forming bacteria, they soon 

 form tubercles and recover, while the control - plants 

 which are without bacteria do not recover. It is 

 furthermore found that if the roots of such control- 

 plants are pricked ^vith a needle covered with the 

 bacteria, the tubercles develop at the points pricked. 



It has been found that certain kinds of Beans fail to 

 grow well in certain localities until the soil there is 

 infected with the proper bacteria Ijy bringing soil from 

 another locality where the Beans in question flourish. 

 A practically pure culture of one species of tubercle 

 bacteria is sold under the name '' Nitragin." Its use 

 has been very satisfactory in some cases, but not in 

 others, which may depend on the fact that it is adapted 

 to certain kinds of leguminous plants, but not to others, 

 and perhaps also on the character of the soil, etc. 



In order to preserve foods, the bacteria of decay 

 must be kept in check. This may be accom- 

 plished by: 



(a) Drying. — Bacteria cannot grow in dry sub- 

 stances. Their growth ceases, as a rule, when the 

 water- content falls below 25 per cent. Seeds are not 

 subject to decay as long as they are dry. Hay and 

 dried fruits are further illustrations. Dried meat is 

 commonly smoked as well as dried; the smoke has 

 both a drying and a germicidal action. 



(&) Preservatives. — The most important of these is 

 common salt, which is so extensively used in the 



