TIIK FOOD OF VEGETABLES. 



117 



iiiixL'd with lime-water, carbonate of lime was 

 precipitated, though not in a quantity much ex- 

 ceeding that of its precipitationby spring-water 

 in general. 100 cubic inches of the solution 

 yielded by experiment an air containing two cubic 

 inches of carbonic acid gas. This is, no doubt, 

 a small proportion: but it appears from a variety 

 of considerations, that the quantity of this gas 

 taken up by the roots of plants is not great ; 

 consequently they do not require a great supply 

 from the soil. 



Salts. On analysing vegetable substances, 

 certain proportions of sahne matters are found 

 in their composition, such as the nitrate, muriate, 

 and sulphates of potash and soda. Now it may 

 either be that the assimilating powers of vege- 

 tables, are such as to form these substances out 

 of their primary elements, derived from the soil 

 and atmosphere ; or, that these saline matters, 

 in a state of solution, are absorbed from the soil. 

 Perhaps in the economy of plants, both these 

 processes are adopted. Facts are not wanting 

 to show that plants do absorb by their roots 

 considerable quantities of salts in solution. M. 

 Saussure prepared 10 different solutions, consist- 

 ing each of 40 cubic inches of distilled water, 

 together with 12 grains of the peculiar salt or 

 other substance on which the experiment of ab- 

 BOi-ption was to be made. The first solution 

 contained muriate of potash; the second, muriate 

 of soda; the third, muriate of lime; the fourth, 

 sulphate of soda; the fifth, muriate of ammonia; 

 the sixth, acetate of lime, the seventh, sulphate 

 of copper ; the eighth, crystallized sugar ; the 

 tenth, vegetable extract. Plants of polygonum 

 persicaria, and bidens cannabina, were then im- 

 mersed in each of these solutions with the fol- 

 lowing result: — In the solutions of muriate of 

 potash, muriate of soda, sulphate of soda, nitrate 

 of lime, and extract, the former species vegetated 

 in the shade for five weeks, developing their 

 parts ; but in the other solutions, they died in 

 the course of a few days. The latter species 

 succeeded or failed in nearly the same way. It 

 was afterwards found that a portion of the salts 

 had been taken up along with the water by which 

 they were held in solution; and if we suppose 

 the quantity contained in each of the solutions 

 to be divided into 100 parts, the ratio of their 

 absorption may be shown as follows : — In con- 

 suming one half of the water assigned to the 

 experiment, plants of the polt/gonum had ab- 

 sorbed 14 parts of muriate of potash, 13 of muriate 

 of soda, 4 of nitrate of lime, 14 of sulphate of 

 soda, 12 of muriate of ammonia, 8 of acetate 

 of lime, 47 of sulphate of copper, 9 of gum, 

 27 of sugar, and 6 of extract. Plants of the 

 bidens had absorbed the several salts in propor- 

 tions not very different. But without minutely 

 regarding proportions, the fact is thus clearly 

 ascertained that plants are capable of taking up 



salts by the root, at least when presented to 

 them in a state of artificial solution; and if so, 

 there is then reason to presume that salts are 

 also taken up by the roots of plants vegetating 

 even in their natural habitats. 



But if salts are thus taken up by the root of 

 the vegetating plant, does it appear that they 

 are taken up as a food? Some plants, it must 

 be confessed, are injured by the application of 

 salts, as is evident from the experiments of 

 Saussure ; but others are as evidently benefited 

 by it. Trefoil and Lucern have their gi-owth 

 much accelerated by the application of sulphate 

 of lime, though many other plants are not at all 

 influenced by its action. The parictaria, nettle, 

 and borge, will not thrive except in such soils 

 as contain nitrate of lime, or nitrate of potash: 

 and plants inhabiting the sea coast, as was ob- 

 served by Du Hamel, will not thrive in a soil 

 that does not contain muriatic of soda. 



It has been thought, however, says Dr Keith, 

 that the salts are not actually taken up by the 

 root, though converted to purposes of utihty, 

 by acting as astringents or corrosives, in stopping 

 up the orifices of the vessels of the plant, and 

 preventing the admission of too much water: 

 but it is to be recollected that the salts in ques- 

 tion are found by analysis in the very substance 

 of the plant, and must consequently have en- 

 tered in solution. It has been also thought 

 that salts are favourable to vegetation only in 

 proportion as they hasten the putrefaction of 

 vegetable substances contained in the soil, or at- 

 tract the humidity of the atmosphere. But sul- 

 phate of lime is not deUquescent ; and if its ac- 

 tion consist merely in accelerating putrefaction, 

 why is its beneficial effect confined but to a 

 small number of plants? 



Lastly, some writers have contended that the 

 salts which are found in vegetables, are merely 

 accidental in their occurrence, and not necessary 

 to the health or perfection of the individual; 

 because they are found to exist in but a very 

 small proportion, both in the soil and plant: but 

 as there are many species in which some salts 

 are to be met with constantly and uniformly, at 

 least, if they have vegetated in a soil in which 

 they are found to thrive, we can scarcely regard 

 their occurrence as being merely accidental, or, 

 as producing no beneficial effect upon the plant. 

 But the proportion of salts lodged in the soil is 

 not so small as is generally believed. Re-agents 

 do not indeed detect a great quantity in general; 

 but that is because the alkaline salts of mould, 

 like the alkaline salts of vegetables, are to be 

 discovered chiefly in the remains of combustion ; 

 and because the ashes of the greater part of 

 vegetable moulds do not readily part with their 

 salts in boiling water. This difficulty of solu- 

 tion is thought by Saussure to be owing to a 

 semivitrifaction that takes place in the mould 



