76 BOTANY 



the most perishable substances remain unchanged indefinitely. If, 

 however, they are exposed to the air, even for a very short time, the 

 germs which thus are introduced will quickly set up decomposition. 

 The principle of cold storage is keeping perishable substances at a 

 temperature too low for the growth of the .decomposition germs. 



The result of organic decomposition is the splitting of the com- 

 plex organic substances into simpler forms, among which are water, 

 carbon-dioxide, and the simpler nitrogen compounds, of which, per- 

 haps, ammonia is the commonest. 



Nitrogen Bacteria. Water and carbon-dioxide are in condition to 

 be used at once by the green plants, but the available nitrogen com- 

 pounds must undergo further changes before they can be used by 

 them ; and here another group of Bacteria have been recently 

 discovered to be essential. These Nitrogen Bacteria are of different 

 kinds. Some of them are able to utilize free nitrogen, while others 

 assimilate the ammonia and other simple nitrogen compounds, which 

 are changed into forms suitable for absorption by the green plants. 

 Of the forms which can utilize the free nitrogen the best known are 

 the Bacteria (Bacillus radicicola) which inhabit the tubercles upon 

 the roots of most Leguminosse, which are thus rendered quite inde- 

 pendent of any nitrogen in the soil. It is still a question whether 

 in this case the Bacteria themselves assimilate the free nitrogen, 

 which is most likely, or whether by their presence the green plant is 

 enabled to do this. As it has been proved that one species, Clostri* 

 dium Pasteurianum, can independently assimilate free nitrogen, it is 

 highly probable that this is the case also with the Tubercle Bacteria. 



Nitrification. The change of ammonia into nitrites, and these into 

 nitrates which are available for the roots of the higher plants, seems 

 to be the work of two sets of organisms, the so-called Nitroso-bacteria 

 and the Nitrate-bacteria. 



Parasitic Bacteria. It is now a familiar fact that most diseases 

 are due to the attacks of specific "germs," i.e. species of Bacteria, 

 and the modern science of medicine is based upon this fact, the 

 treatment of disease being principally an effort to prevent the intro- 

 duction of such disease germs into the body, as by boiling suspected 

 water, or to find agents which will destroy these germs when they 

 have effected lodgement in the system. 



These disease germs, or " pathogenic " Bacteria, may be true para- 

 sites actually feeding upon the tissues of the host, or the symptoms 

 of disease may be the result of the development of poisonous sub- 

 stances (toxins) which are produced by the growth of the organisms 

 either within living or dead matter. Thus cases of poisoning from 

 eating cheese, milk, or tainted meat are due to poisons resulting 

 from the activity of Bacteria within these substances. 



The whole science of aseptic and antiseptic surgery is also based 



