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, an4 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 
eff'ected 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 diff'erent ^. 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. 
189, 
^ [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.] 
