ABSOKPTION OF FOOD MATERIALS 139 



contain many times the percentage of silica that is found 

 in that of the leguminous plants. The grasses accumulate 

 silica in their epidermal cells, while the leguminous plants 

 do not. Hence the absorption of that substance soon 

 ceases in the latter case. 



Again, if a particular soil contains several different 

 salts, a plant growing in it will not absorb them in equal 

 proportions, nor in those in which they exist in the soil. 

 An illustration of this fact is afforded also by marine AlgaB, 

 which accumulate in their tissues much greater amounts 

 of potassic than of sodic salts, though sea-water contains 

 much larger quantities of the latter than of the former. 

 This fact admits of a similar explanation to that given in 

 the case already mentioned. The absorption of a salt will 

 cease as soon as the cell-sap attains exactly the same degree 

 of concentration as the entering stream. In this case 

 there will be no further osmotic action as far as the salt 

 is concerned, though there may be a continuous entry of 

 water into the absorbing cells. 



We have seen that the continuous absorption of water 

 by the root-hair will depend upon certain external condi- 

 tions, such as the temperature of the soil, the activity of 

 transpiration at the time, the degree of illumination the 

 plant receives, &c. These conditions affect also the absorp- 

 tion of the substances in solution. 



The substances which are absorbed by the roots in this 

 way 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. These 

 humble organisms 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 different 



