ABSORPTION 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 Alga, 
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 ag 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 
