ELEMENTARY CONSTITUENTS OF THE FOOD OF PLANTS. 623 



remains inactive. The importance of calcimn must therefore be sought partly in 

 its serving as a vehicle for sulphuric and phosphoric acid in the absorption of 

 food-material, and partly in its fixing the oxalic acid which is even 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. 



Si/icon 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 silicificd, 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 docs not take place to any great extent until they are already fully 

 developed. 



The combinations of food-material must be subject within the tissues to progressive 

 changes of position in addition to and in consequence of their chemical transformations. 

 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 

 is the same in the case of every other salt dissolved m 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 w^hen a number of compounds which supply food-material are taken up 

 at one spot such as the root, and are transplanted to another spot as the buds and 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. Only when the 

 force of the pressure drives the whole of the cell-sap to one side is the motion uniform 

 for different substances, 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 in this case also the molecules of different salts are urged forward with a 

 different rate, because the rapidity of filtration of different solutions varies with their 

 composition and degree of concentration. 



