THE president's ADDRESS. 461 



production of this substance the presence of decomposing animal 

 matter and carbonate of potassium are necessary. The bacteria 

 first liberate ammonia from the animal matter, this is by stages 

 converted into nitric acid, which in turn takes the place of the 

 carbonic acid in the carbonate of potash, and saltpetre, or 

 nitrate of potash, results. The nitre bacterium is one of the 

 smallest of bacteria known, and is the only organism known that 

 can assimilate carbonic acid in darkness. 



It has been known from very early times that leguminous 

 plants, peas, beans, tares, etc., added nitrogen to the soil in 

 which they gi^ew ; whereas all other plants impoverished the 

 soil of this important element of plant food. This property 

 of leguminous plants is now known to be due to bacteria living 

 in the roots. These bacteria are present in the soil, and when 

 peas or beans are growing, the delicate root-hairs liberate a 

 certain amount of asparagin, by which the bacteria are chemo- 

 tactically attracted. The tips of the root-hairs then become 

 partly dissolved, and allow the bacteria to penetrate their 

 interior, from whence they pass in a glairy stream into the 

 thicker roots, and, increasing rapidly in numbers, form the well- 

 known nodules, or warts, present on the roots . of leguminous 

 plants. 



During the first period of existence in the root, the bacteria 

 are provided with an ample supply of food by the host-plant, 

 in the form of soluble carbohydrates, etc. After a time the 

 bacteria present in the nodules of the roots begin to absorb 

 free nitrogen, which accumulates in the nodules to the extent of 

 six to seven per cent, as time goes on. When the plant is in 

 bloom, the bacteria, which hitherto have been liberally supplied 

 with food as already stated, are now victimised by the plant, 

 which secretes certain enzymes that practically dissolve the 

 bacteria, and their store of nitrogen is appropriated by the plant,, 

 and mostly transferred into its own seeds. 



When this period is reached, the nodules on the roots graduallj^ 



