ELEMENTARY CONSTITUENTS OF THE FOOD OF PLANTS. 625 



the cell-sap of the root by means of the water imbibed by the cell-walls of the root- 

 hairs. In this manner it is possible for the root to suck up the water of the soil; as 

 this water enters the points of contact, the equilibrium of the strata of water that 

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

 capillary attraction is set in motion towards the 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 are not 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 the absorbed food-materials occur as an extremely 

 fine coating over the particles of soil, and can therefore only be taken up together 

 with them by the root-hairs at the points of contact; 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 absorbed particles of substance which 

 come directly into contact with the root-hairs are dissolved and sucked up. 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 ex- 

 tremely 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 into 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 both the soluble constituents of the soil as well as those insoluble in 

 pure water, the absorption 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 explaia^d 

 in the case of absorption from a solution. Here also it is the consumption, the decom- 

 position 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 

 nutriment from the same soil may be altogether different \ 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 contra- 



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

 (See Toum. Roy. Agric. Soc. vol. VIII, p. 496 ei seg., 1S47; Journ. Chem. Soc. vol. X, p. 1, icSg; ; 

 Report Brit. Assoc. 1S61 and 1867.) Their latest publication, « Report of Experiments on the growth 

 of Barley for twentv 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. — Ed.] 



s s 



