NUTRITION, STORAGE, AND RESPIRATION 107 



digested, and their substance absorbed into the host plant. Thus 

 the advantage to the Bacillus seems to be only temporary : that 

 to the host-plant is a permanent addition to its store of nitrogen. (See 

 Chap. XI.) 



In addition to this source of combined nitrogen it seems probable from 

 comparison of crops taken from plots of land without, and those with the 

 addition of manure, that a considerable fixation of nitrogen takes place in the 

 soil. On the other hand, certain nitrifying bacteria are known to oxidise 

 ammonia into nitrous acid, and this further into nitric acid. By this means 

 they obtain a supply of energy quite distinct from that derived in green plants 

 by Photo-Synthesis and subsequent respiration. The practical result as regards 

 the nitrogen-supply in the soil is that the products of decay in the form of 

 ammonia-salts are converted into nitrites and nitrates, in which latter lorm 

 combined nitrogen is most readily absorbed by the roots, and used by ordinary 

 green plants. While the nitrifying bacteria thus gain their own physiological 

 ends, they also play an important part in the circulation of nitrogen in organic 

 life. 



In some few plants a supply of combined nitrogen may be obtained 

 by parasitism, or by the still more unusual carnivorous habit. But for 

 the moment it must suffice to mention these. (See Chap. XI.) To sum 

 up : the majority of plants take up their nitrogen in the combined form 

 from the soil, and do not use the free nitrogen of the atmosphere. But 

 some few are able to do this, as in the case of the infected Leguminosae ; 

 while occasionally the supply in combined forfn is wholly or in part based 

 upon a parasitic or a carnivorous habit. 



The supply of sulphur is from salts in the soil. Culture experiments 

 show that its supply in the form of sulphate is satisfactory. It may 

 well be that much of it passes into the plant in the form of calcium 

 sulphate, while the calcium hberated when proteids are formed may 

 be deposited, as those crystals of calcium oxalate so frequently seen 

 in the tissues (Fig. 44, p. 61). The supply of phosphorus is as phos- 

 phoric acid, phosphates being absorbed frojn the soil. Culture experiments 

 show that a supply of calcium phosphate is suitable. 



From such materials, together with carbohydrate resulting from Photo- 

 Synthesis, new proteid is constructed. It is uncertain where this takes 

 place. Probably it may occur in the mesophyll of foliage leaves, but 

 there is no reason to believe that it is restricted to that tissue. The 

 plant supplied thus with carbohydrate, and with proteid in increasing 

 quantity, has at its disposal those materials which are necessary for the 

 groiiDth of its tissues and for the development of new organs. It is thus 

 seen that part of the carbohydrate produced in Photo-Synthesis may 

 l)e used up in the production of proteid. A considerable proportion 



