72 NUTRIENT SALTS. 
must at all events come into contact with as great a mass of nutrient soil as 
possible, and this is effected by means of a widely-ramifying system of roots; 
and, in addition, they must assist in making available desirable matter in the 
soil by the elimination from themselves of certain substances. 
In order to explain the remarkable power that plants possess of exercising 
a choice in the absorption of certain food-stuffs from amongst the whole number 
presented to them, we must in the first place assume a special structure to exist 
in the cells which are in immediate contact with the nutrient medium. To 
reach the interior of a cell, the salts must pass through the cell-membrane and 
the so-called ectoplasm. We may look upon these walls, that are to be pene- 
trated, as filters, or, to abide by our previous simile, as sieves, which allow only 
certain kinds of molecules to pass and arrest others. Moreover, just as the 
structure of a sieve, especially the size and shape of its pores, has its effect in the 
separation of the particles of the matter sifted, so also may the structure of a 
cell-wall have a discriminating influence in the absorption of food-salts. It may 
be supposed that the cell-wall in one species of plant acts as a sieve capable of 
letting through molecules of potash but none of alumina, whilst the cell-wall in 
a second species allows molecules of alumina to pass as well, but is impervious to 
those of chloride of sodium. This hypothesis would also explain why the absorp- 
tion of food-stuffs by plants generally takes place through cell-walls, and why 
absorption into the organs concerned by means of open tubes, which would be 
at all events a much simpler method, is not preferred. It is, however, necessary 
to investigate first the nature of the force which causes molecules of the various 
salts to move from the soil to the cell-membranes, which we suppose to be like 
sieves, and through them into the interior of a plant. A force acting in this 
sense from without is inconceivable, and we must therefore look for the motive 
stimulus in the plant itself. 
As has been already stated in connection with the absorption of carbonic acid, 
it is believed that the cause of this movement is the disturbance of the molecular 
equilibrium in the growing vegetable organism. If at one spot in the protoplasm 
of a cell a particular substance is altered, and, let us say, converted into an 
insoluble compound, the previous grouping of molecules appears to be altered, or 
in other words, the molecular equilibrium is disturbed. To restore equilibrium, 
there must be a re-introduction of molecules of the material that has been removed; 
and the attraction of them from the quarter where they occur in a fluid, that is 
to say in a mobile condition, is the more energetic. Supposing, for instance, 
gypsum (ie. sulphate of lime) is being decomposed within a cell, and the lime 
combines with the oxalic acid (set free in the same cell) to form insoluble oxalate 
of lime, whilst the sulphur combines with other elements to form insoluble 
albuminoids, this use of the gypsum occasions a violent attraction of that sub- 
stance from the environment, or, to put it another way, it causes a movement of 
gypsum towards the place of consumption. If this latter place is a cell in imme- 
diate contact with the nutrient substratum, the absorption of the substance 
