702 CHEMICAL PROCESSES IN THE PLANT. 



cover contiguous particles of earth is disturbed, and the water of the soil retained by- 

 capillary attraction is set in motion towards these points of contact. This process 

 spreads centrifugally from every root, and thus gradually makes the most distant 

 parts of the soil subserve the nutrition of the plant. If salts, such as calcium 

 sulphate, are present in solution in the enveloping strata of water, these salts follow 

 the movements of the water, and finally enter at the points of contact with the root- 

 hairs. 



But a large portion of the food-material, especially compounds of ammonia, po- 

 tassium, and phosphoric acid, occur in the ground in a fixed condition, or, as it is 

 generally termed, absorbed; they cannot be extracted from the soil even by very large 

 quantities of water ; the roots nevertheless take them up out of it with ease. It may 

 be supposed in these cases that these food-materials occur as an extremely fine coating 

 over the particles of soil, and can therefore only be taken up at the points of contact 

 of the root-hairs with these particles; and they are there rendered soluble by the 

 carbon dioxide exhaled by the roots. This action of the root is limited to the points 

 of contact ; only those particles of substance which come directly into contact with 

 the root-hairs are dissolved and absorbed. But since the number and length of the 

 roots is very considerable in all growing land-plants, and since also they are continually 

 lengthening and forming new root-hairs, the root-system comes gradually into contact 

 with innumerable particles of earth, and can thus take up the necessary quantity of the 

 substance in question. This power of the roots of taking up, by means of the acid sap 

 which permeates the walls of even their superficial cells, substances which are insoluble 

 in pure water, presents itself in an extremely evident manner, as I was the first to show, 

 when polished plates of marble, dolomite, or osteolite (calcium phosphate) are covered 

 with sand to the depth of a few inches, and seeds are then sown in the sand. The roots 

 which strike downwards soon meet the polished surface of the mineral and grow upon 

 and in close contact with it. After a few days an impression of the root-system is found 

 corroded in rough lines on the smooth surface ; every root has dissolved at the points of 

 contact a small portion of the mineral by means of the acid water which permeates its 

 outer cell-walls ^. 



In taking up those constituents of the soil which are insoluble in pure water, the 

 solution is therefore first of all accomplished by the plant itself ; and it is at the point 

 where solution takes place at the surface of the root that absorption inwards is also 

 effected by endosmose. But in spite of this complication the same principles hold 

 good for the absorption of material from the soil as have been explained in the case 

 of absorption from a solution. Here also it is the consumption, the decomposition 

 of the compounds in the plant, that regulates the absorption of the material. The 

 quantitative composition of the ash has therefore no resemblance to that of the soil ; 

 and the ash of plants of different kinds growing side by side and deriving their nutri- 

 ment from the same soil may be altogether different 2. But the composition of the 

 soil is important to the plant in a secondary degree ; since plants of the same kind, if 

 they grow for example on a soil rich in lime, will take up a greater quantity of lime 

 than if the soil contained but little of it. This is obviously not in contradiction to the 

 principle laid down, but only shows that the decomposition of a salt in the plant will 

 take place more largely the more easily it is enabled to take it up. 



For a more detailed account see the Handbook of Experimental Physiology, 1865, p. 



^ [Messrs. Lawes and Gilbert's long series of experiments on this subject are of especial value, 

 (See Journ. Roy. Agric. Soc. vol. VIII. p. 496 et seq., 1847 ; Journ. Chem, Soc. vol. X. p. i, 1857 ; 

 Report Brit. Assoc 1861 and 1867.) Their latest publication, 'Report of Experiments on the growth 

 of Barley for twenty years in succession on the same land' (Journ, Roy. Agric. Soc, second series, 

 vol. IX) contains much information as to the power possessed by plants of extracting different 

 substances from the soil,] 



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