NITROGEN-BACTERIA AND LEGUMINOUS PLANTS 179 



metre of sterilised water, and a small quantity of the mixture transferred 

 to a test-tube containing about 10 c.c. of the mixture mentioned above. 

 Before the gelatine solidifies the contents of the test-tube are poured 

 into a Petri-dish, which is then set aside until the colonies develop on 

 the plate. It is absolutely necessary to see that no extraneous organ- 

 isms obtain access to the mixture. To avoid this, the outside of the 

 nodule must be cleaned first with water, then placed for a short time 

 in alcohol, and finally in ether. When cut, all the usual precautions by 

 sterilisation must be taken. 



The colonies develop as small shiny spots which do not liquefy the 

 gelatine. Once the colonies are developed they are transferred in the 

 usual way to Agar-tubes, and thus can be studied as pure cultures. 



§5. DESCEIPTION OF BACILLUS EADIGICOLA. 



These microbes are found as either motile or immotile rods. The 

 latter are about 1 ^i in breadth and 4-5 /«. in length, whereas the 

 former are much smaller, beirjg only 0'18 ;u in breadth and 0'9 ij, in 

 length. This diversity in size and motility is one of many instances 

 of the multiplicity of morphological and physiological characteristics 

 assumed by one and the same species. These are also strongly aerobic, 

 and are killed when the temperature reaches 60-70° 0. 



It is doubtful whether all the bacteria to be found in leguminous 

 nodules belong to one species ; and much controversy has arisen on 

 this subject. In the work of recent observers many varieties of this 

 species have been obtained. This of course is only what is to be 

 expected when the conditions of growth are so varied and bacteria so 

 plastic. It has not yet, however, been indubitably proved that these 

 varieties belong to more than one species. 



Side by side with these researches on the nature of the bacteria, 

 much light has been thrown on the effect of the presence of the 

 nodules on the growth of the plants. It has already been mentioned 

 that if the soil be sterilised no nodules are produced on the plants, 

 and the same result is obtained with water cultures of these plants. 

 These experiments were extended until the following facts, established 

 chiefly by Hellriegel, were placed beyond doubt. 



1. Leguminous plants, so far as nitrogen is concerned, behave in a 

 differgnt way to grass plants. 



2. Grass plants obtain their nitrogen entirely from the soil, and their 

 development has a direct ratio to the amount of nitrogen in the soil. 



