ABSORPTION AND ASSIMILATION OF NUTRIMENT. 547 



ties in their assimilation, and the more suitable are 

 they for the nutrition of the plant. 



Without doubt also the nitrogenous bodies are built Assimilation 

 up to a very large extent in the cells ; and not only 

 those which constitute the protoplasm, but also those 

 which are broken up in the intra-molecular respiration 

 are probably always of more complex structure than 

 the nutrient materials. Even the peptones undergo a 

 transformation during assimilation, and where ammo- 

 niacal salts and amides are the only sources of nitrogen 

 a complex process must result, and more especially a 

 union with assimilation products rich in carbon. The 

 varying expenditure of energy which is necessary for 

 building up the assimilation products according as the 

 material offered as food is closely allied to these products 

 in composition, or differs much from it and is much 

 simpler, explains in part the varying nutritive value of 

 these compounds. The more active the growth and the 

 new formation of protoplasm, and the more heterogeneous 

 the nutrient materials, the greater is the amount of 

 energy which must be set free by the respiratory 

 processes. 



The salts, likewise, do not always appear to be taken Role of the 

 up from the nutrient mixture in the same form in which Sitricnt 

 they are present within the cells. Here and there substances.. 

 transformation and decomposition must occur under the 

 influence of the organic acids which are formed ; further, 

 sulphur, phosphorus, and magnesium, and possibly also 

 calcium and potash, enter into combination with the 

 complex molecules of the proteid materials of the proto- 

 plasm. For the production of the phosphorus, phosphoric 

 acid seems to be alone suitable ; the union of the 

 phosphorus with proteid bodies must occur within the 

 cell. These transformations of the inorganic materials, 

 however, are much less extensive, and require much less 

 energy, than those of the. organic substances. 



In the case of the higher plants the composition 

 of the salts may vary very greatly. Often excessive 

 quantities of the necessary nutrient salts are taken 

 up, so that the relation of the individual constituents 



