700 
CHEMICAL PROCESSES IN THE PLANT. 
food-material, and partly in its fixing the oxalic acid which is poisonous to the 
plant, and rendering it harmless. The elements just named are taken up by the 
plant when they are offered to it in the form of phosphates, sulphates, nitrates, or 
chlorides. 
Silicon finally is taken up by a very large number of plants in the form of a 
very dilute aqueous solution of silicic acid ; by some in larger quantities than all the 
other constituents of the ash. By far the larger part of the silicic acid passes into 
the insoluble state within the cell-walls, and remains behind after the destruction 
of its organic substance together with calcium (magnesium and potassium ?) as a 
skeleton possessing the structure of the cell-wall. In land-plants it accumulates 
chiefly, though not exclusively, in the tissues exposed to evaporation, and espe- 
cially in the cuticularised walls of the epidermis. In Diatoms, the cell-wall of which 
is very strongly silicified, this arrangement of course does not exist. Since it is 
possible to cause, by artificial feeding, plants which usually contain abundance of 
silica (like Maize) to grow almost entirely without it, and without any obvious 
departure from their normal structure, silicic acid appears to be of very subor- 
dinate importance for the chemical and organic processes; and its deposition in 
the cell-walls does not take place to any great extent until they are already fully 
developed. 
The compounds serving as food-material must be subject within the tissues to pro- 
gressive changes of position in addition to and in consequence of their chemical trans- 
formations. The equilibrium of diffusion is disturbed by the decomposition of a salt ; 
immediately round the spot where this takes place the fluid of the tissue contains fewer 
molecules of the compound; and the more distant molecules of the same salt in a 
state of solution move therefore towards the spot where they are wanted. Every cell 
therefore which decomposes any particular salt acts as a centre of attraction upon the 
fluids of the tissue surrounding it, and the salt in question is drawn towards this 
centre. But this process does not affect any other salt dissolved in the same fluid. 
If, for example, calcium sulphate is decomposed in a cell and crystals of calcium oxalate 
formed, this itself supplies a cause for the more distant molecules of sulphate to be 
drawn towards that cell ; but it aff'ords no reason for the molecules of potassium nitrate 
which are also present to move in the same direction. Every substance dissolved in 
the cell-sap is set in motion only in so far as the equilibrium of diff"usion and the uniform 
distribution of its own molecules is disturbed. It follows therefore clearly that there 
can be in general no such thing as a continuous uniform motion of a so-called '■ nutritive 
sap.' It is only when a number of compounds which supply food-material are taken up 
at one spot such as the root, and are decomposed at another spot such as the buds or 
green leaves, that the direction of movement is nearly the same for all ; but even in this 
case the rapidity with which the molecules of each particular salt move will vary, because 
this depends on the rapidity of consumption at the point towards which the movement 
is directed, and on the special rate of diffusion of each compound. It is only when some 
external pressure drives the whole of the cell-sap in one direction that the motion of 
diff"erent substances is uniform, provided that the fluid moves in open channels such as 
the laticiferous vessels or sieve-tubes ; but if the pressure causes filtration through closed 
cell-walls, then the molecules of different salts are urged forward with a different speed, 
because the rapidity of filtration of different solutions varies with their composition and 
degree of concentration. 
The same principles hold good also for the absorption of combinations of food- 
material from without into the absorbing organ. It has already been shown in the 
previous paragraph how the decomposition of carbon dioxide in a cell containing chloro- 
