66 Practical Plant Biology. 



It has already been pointed out that all organic nature is in- 

 debted to green plants for its supplies of food and energy. Ail 

 animals depend on them for their nitrogenous food. The green 

 plants themselves obtain nitrogen chiefly in the form of nitrates 

 and to a lesser extent as ammonia salts from the soil or like 

 Chlamydomonas from the water in which they live. Considering 

 the rate at which plants grow and the quantity of their material 

 which is constantly being used up for the supply of nitrogenous 

 food material for the animal kingdom, we would naturally expect 

 to find large stores of nitrates and ammonia salts in the soil. It 

 is then surprising when it transpires that these substances can as 

 a rule be found only with difficulty in soils supporting luxuriant 

 vegetation. Evidently the supply, which at any one moment is 

 vanishingly small, must constantly be replenished from some con- 

 tinuous source. 



As we have seen, putrefactive saprophytes break down complex 

 proteins into simpler ones and finally produce from their disin- 

 tegration, ammonia, free nitrogen, free hydrogen and sulphuretted 

 hydrogen. These organisms then, seizing on the dead bodies of 

 all animals and plants and their waste-products, pass their nitro- 

 genous substance through their metabolism and give rise to 

 ammonia as a final product. Once this stage is reached putre- 

 factive bacteria can no longer obtain the energy for subsistence. 

 But other bacteria are able to utilise this product as a source of 

 nitrogen and energy. The nitroso-bacteria can oxidise ammonia 

 to nitrous acid and use the energy set free by this oxidation for 

 the assimilation of carbon from carbon dioxide and for the other 

 requirements of life. At the same time some of the nitrogen is 

 assimilated. The third transformation is effected by a third group 

 of bacteria, the nitro-bacteria, which further oxidise the nitrous 

 acid to nitrates. Apparently a large variety of saprophytes par- 

 ticipate in the disintegration of the proteins into ammonia, and 

 parasites assist in the earlier stages preparing the way for the true 

 saprophytes. It is remarkable that not only do the nitroso- and 

 nitro-bacteria not require proteins for their metabolism, but even 

 the presence of proteins seems to be poisonous to many species 

 of them ; so that they only develop in quantity when the proteins 

 are disintegrated. As each step in this process has to wait on 

 the progress of the preceding one, an equable and steady supply 

 of nitrates is liberated in the upper layers of the soil. It is from 

 this supply that green vegetation assimilates its nitrogen, building 

 it up into proteins and protoplasm by means of the light energy 

 absorbed from the sun. 



