50 STATE BOARD OF AGRICULTURE. 



such plants take up no nitrogen from the atmosphere. Next it was 

 found that if leguminous seeds were planted in sterilized soil, devoid 

 of nitrogenous food, and were watered with an infusion of sterilized 

 soil, they manifested two characteristic and peculiar stages of growth. 

 At first the peas sprouted readily and grew vigorously for a very short 

 time, when growth ceased. This period, which the botanist now desig- 

 nates as the stage of nitrogen hunger, was reached as soon as the plant 

 had used up all the combined nitrogen in the seed. If at this stage 

 some of the plants were watered only with sterilized earth infusion,, 

 they did not recover, but continued to waste away and finally died, 

 while those watered with non-sterilized soil infusions soon began to 

 take on a vigorous growth, eventually developed into well nourished 

 plants, and jjroduced an abundant yield. Upon examining the roots 

 of these two sets of plants, it was found that those watered with steril- 

 ized soil infusions showed no tubercles or nodules, while those watered 

 with non-sterilized soil infusions carried these tubercles. These experi- 

 ments quite naturally suggested that the tubercles were formed by the 

 agency of bacteria, and microscopic and cultural studies confirmed this 

 supposition. Soon the characteristic micro-organisms in these tubercles 

 were obtained in artificial culture media and their casual relation to 

 the tubercles was proved by direct inoculation experiments. Moreover, 

 peas were planted in sterilized soils, and watered with these cultures. 

 When this was done it was found that tubercles formed abundantly on 

 the roots of the plants, which, on account of their ability to fix nitro- 

 gen, grew vigorously and produced abundant crops. Additional studies 

 have shown that these bacteria, which are quite widely distributed in 

 the soil, pass into the roots, forming the swelling or tubercle at the 

 place of entrance, penetrate the woody tissue in the form of delicate 

 filaments, and produce a mucilaginous substance which permeates the 

 tissues of the plant. There has been some discussion as to whether it 

 is the bacteria or the plant which fixes the nitrogen, but the only con- 

 clusion which can be justified by the experiments that have already 

 been made is that both of these organisms are essential, both the germ 

 and the leguminous plant are necessary, and they must act together in 

 order to take free nitrogen from the air and combine it into the tissues 

 of the plant. I should state that some experiments indicate that there 

 is a certain amount of nitrogen fixation in the green parts of many 

 plants, but this is so very small compared with the large quantities 

 fixed by the combined action of leguminous plants and the bacteria 

 that it cannot be considered to be of any special importance. In this 

 way it will be seen that there is an association between the plant and 

 the micro-organism which is mutually beneficial to the two, and which 

 enables the two working together to take free nitrogen from the atmos- 

 phere and build it up into tissue which can be subsequently utilized 

 for feeding other plants and even nourishing animals as well. Some 

 experiments indicate that there are different species of the tubercle 

 organism, and that it is necessary in order to get the best results to 

 bring together the special legume and the special tubercle bacillus which 

 best work together. 



"It will be seen from this that bacteriology has been a benefit to 

 the world not only by decreasing sickness and lessening the death rate, 

 but also by pointing out to the fanner a way by which he can utilize 



