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 affords 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 diffusion 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 

 different 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- 



