ABSORPTION OF FOOD MATERIALS 437 



up in a certain strength of solution, or, in other words, with 

 each molecule of salt there is a certain invariable quantity of 

 water absorbed. The quantity is not the same, however, for 

 each salt. 



The salts which different plants absorb, in like manner vary 

 as to amount. If two different species are growing in the same 

 soil, side by side, under exactly the same conditions, the amounts 

 of the several salts present in the soil which are absorbed by the 

 two plants will not be the same. In each case the quantity will 

 vary according to the use which the plant can make of it. 



Conversely, if the same soil contains several different salts, 

 a plant will not absorb them in equal proportions, nOr in those in 

 which they exist in the soil. This fact admits of a similar 

 explanation. Again, the absorption of a salt will cease as soon 

 as the cell-sap contains the same amount of it as the fluid out- 

 side the root-hair. In this case there will be no osmotic action 

 so far as the salt is concerned, though there may be a continuous 

 amount of water entering the hair. 



We have seen that the continuous absorption of water by 

 the root-hair will depend upon certain external conditions, such 

 as the temperature of the soil, the activity of transpiration at 

 the time, &c. These conditions affect also the absorption of the 

 substances in solution. 



The substances which are absorbed in this way by the roots 

 are naturally very varied. The most important of them in 

 the metabolism of the plant are the compounds of nitrogen. In 

 the soil these exist in the form of nitrates or nitrites of the 

 metals mentioned, and as compounds of ammonia. Green 

 plants take in little or none of the latter, which are, however, 

 made available for their use by the action of certain bacteria 

 which the soil contains, which have the power of converting the 

 ammonia compounds into nitrites and the latter into nitrates, in 

 which form they are taken up. This process of nitrification 

 is the special property of two of these organisms, one of which 

 forms nitrites from the ammonia compound, and the other forms 

 nitrates from nitrites. 



It is in this way that a normal green plant absorbs aU the 

 nitrogen which it uses for the construction of food substances. 

 The nitrogen of the air is made use only of in very exceptional 

 cases. Certain lowly Algse seem to have the power of using it, 

 but the process is not fully understood. A few plants belonging 

 to the LegmninosEe can also use atmospheric nitrogen, but their 

 power depends upon the association with their roots of certain 



