Lecture IX. 67 



From this short account of the circulation of nitrogen you will 

 realise that every organism in the animal and vegetable kingdoms 

 depends for its existence on the functioning of each link in the 

 complex chain. On the efficiency of each link depends the 

 amount of living matter possible at any one time on the earth. 

 Here I would like to draw your attention to the fact that, as a 

 result of putrefaction, some of the nitrogen of the proteins is set 

 free from the cycle. This constitutes a leak as it were in the 

 system. And so far as it is concerned, it does constantly act so 

 as to reduce the amount of life possible on the earth. Fortu- 

 nately, however, there are other processes at work restoring the 

 balance and introducing nitrogen into the system. Foremost 

 among these are the activities of a very peculiar group of bacteria. 

 These organisms are found in the soil in the roots of leguminous 

 plants (the group of flowering plants to which the pea, clover and 

 acacia belong). These bacteria enter the roots and there give 

 rise to nodules or tumours, and supplying themselves with energy 

 from the oxidation of the carbohydrates of their host, they are 

 able by some wholly unknown mechanism of their joint metabolism 

 to build up atmospheric nitrogen into proteins. The result is 

 that the supply of nitrates in the soil is reinforced when the host 

 and parasite are ultimately nitrified by the bacteria of the nitrogen 

 cycle, and thus nitrogen is introduced into the system. 



We will now turn to the consideration of a wholly different 

 organism Spirogyra. There are several different species of this 

 plant. One of the commonest is Spirogyra porticalis. They are 

 usually to be found in ponds or very slowly moving water, where 

 they form a mass of green. When taken up into the hand the 

 mass has no tendency to form a mat, but readily separates into 

 numerous filaments of great fineness. This ready separation is 

 due to the nature of the material and to the fact that the filaments 

 are unbranched so that they very easily move over each other and 

 do not tangle together. 



For microscopic examination a few of the filaments should be 

 mounted in a drop of water and covered. Inspection even with 

 a low power discloses an organism of extreme beauty. Down 

 each filament of crystalline transparency runs one or more spiral 

 bands of the purest green. Attentive observation shows that, in 

 addition to these bands, each filament carries a series of transverse 

 markings which subdivide it into a succession of uniformly sized 

 portions, and sometimes a darkening and irregularity of the green 

 bands marks the middle of each of these portions. 



