NITROGEN-FIXING BACTERIA. 141 



forms o-f bacterial life that are concerned with the transformations of 

 the soil nitrogen it is necessary to consider separately some distinct 

 phases of the nitrogen question. The plant tissues from which life 

 had departed hold in them the nitrogen that had once moved in soluble 

 form through the soil. The nitrogen in the dead plant tissue can not, 

 however, again become a part of the food for other plants; not until 

 it has again been changed into simpler soluble forms. This locking 

 up of the nitrogen in forms slowly decaying, and therefore slowly 

 available, is a wise provision, otherwise the nitrogen would soon 

 be washed out of the soil. Thus we see that the soil nitrogen is con- 

 tained in an insoluble form in the remains of former plants, and, no 

 matter how much of it the soil contains it is inaccessible to the plant 

 growing upon it until it has been first changed into the simpler forms. 

 Now, as to the agents that produce this transformation. Bacteriology, 

 in general, and soil-bacteriology, in particular, are subjects to which the 

 attention of the scientific world has turned very recently. Of the 

 many hundreds of different species of bacteria living in the soil, but 

 few are known. Nevertheless, even at the present time, enough has 

 been learned to enable us to form a conception, at least, of the changes 

 that take place there. The nitrogen of organic substance, whether plant 

 or animal, usually exists in the form of albuminoids, more frequently 

 termed proteids. These proteid molecules are seized by the soil 

 bacteria and are utilized by them for the formation of their own 

 bodies. Being saprophytic by nature, that is, unable to build up or- 

 ganic substance from the simpler materials, as is done by higher plants, 

 they must derive their energy from the tissues that chlorophyl-bearing 

 plants had fashioned with the aid of sunlight. In availing themselves 

 of this potential energy for their own purposes, they break down the 

 complex molecules ; to use a popular expression, they cause decay. Tn 

 order to gain their end, that is, to secure the food contained in the pro- 

 teid molecules, the bacteria must first change it into a readily diffusible, 

 soluble form. For this purpose the chemical ferments known as en- 

 zymes are produced. With the aid of these enzymes, the albuminoid 

 substances are ' peptonized.' In the laboratory such organisms are de- 

 scribed as gelatin-liquefying bacteria. A part of the food thus made 

 accessible is appropriated by the microorganisms and in their physio- 

 logical processes is still further simplified. A part of the carbon is 

 oxidized and escapes into the atmosphere as carbon dioxide, gaseous 

 hydrogen or oxygen is set free, or the two are combined to form 

 water. The nitrogen with which we are here concerned is subject 

 to many changes. In the course of its migration it forms a part of 

 the amid molecules; from these it is split off in the form of am- 

 monia, and this again may be destroyed and gaseous nitrogen set free, 

 or seized by another distinct class of organisms, and oxidized to nitrites 



