218 



the plant through the net work of cellular tissues to be finally re* 

 deposited in the leaf. 



The same statement may be made with regard to the other purely mine- 

 ral foods of plants. It is quite certain that they do not become a part of 

 the plant organisms in the form in which they are found in the soil or 

 applied fertilizers. In phosphorus, for instance, is found one of the 

 most important mineral foods of plants. This substance exists in the 

 soil almost exclusively as mineral phosphates, or is applied as such in 

 fertilisers. Nevertheless, the phosphorus which is found in plants and 

 especially in the seed of cereals, exists largely in organic combination, 

 showing that the original mineral phosphates have been entirely de- 

 composed by the process of digestion to which they have been subjected. 

 Even the mineral phosphates which are found in plants are not those 

 which pre-existed in the soil. Soil phosphates are chiefly those of lime, 

 iron and alumina, while plant phosphates are chiefly those of potash. 



Solution of soil particles. 



At the present moment it is supposed that the purely mineral mat- 

 ters mentioned above pass into solution under the influence of the 

 secretions and vital forces of plant rootlets. It is not probable, how- 

 ever, in view of the knowledge we already possess of independent soil 

 organisms, that there may be a class of such bodies especially active in 

 the disintegration of mineral particles and the preparation of them for 

 plant digestion. Naturally, the first organisms which would act upon 

 a bare rock would be those which could subsist upon a purely mineral 

 environment. Such organisms could draw their nourishment solely 

 from the mineral itself and from the air. One of the most important 

 of modern discoveries is the fact that the nitrifying organism of the 

 soil, the nature of which will be explained further on, and which is the 

 chief instrument in providing and digesting nitrogeneous nutriment 

 for plants, is capable of subsisting and flourishing in a purely mineral 

 medium, It is believed, therefore, that in the primary decay of bare 

 rocks, especially at high altitudes, the nitrifying organism plays a 

 highly important part and prepares the surface of the rock for the first 

 growth of lichens and other low vegetable organisms from which the 

 the first traces of humus are formed. While these organisms are said 

 to subsist in a purely mineral environment, it must be understood that 

 the carbon dioxide and traces of ammonia, which the air may contain 

 belong to this category. It has been shown that these bacteria can be 

 developed by absorbing from tbe ambient atmosphere traces of am- 

 monia and other bodies which may be present in the air. They even 

 assimilate the carbon of the carbon dioxide much in the same manner 

 as vegetables which contain chlorophyll. Thus, even in the denuded 

 rocks of high mountains, the conditions for the development of all 

 these inferior organisms exist. In examining the particles produced by 

 attrition from such rocks it is easily established that they are uniformly 

 covered by a layer of organic matter, evidently formed by microscopic 

 vegetations. There is thus discovered in the very first products of the 

 attrition of rocks the characteristic element of vegetable soil, viz. 

 humus, the proportion of which increases rapidly with the process of 

 disintegration, until finally the decaying mass is capable of sustaining 

 chlorophyll-bearing plants. 



