Book I. FOOD OF THE VEGETATING PLANT. 231 



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 ; tut others are as evidently benefited 

 by it. Trefoil and lucerne have their growth much accelerated by the application of sul- 

 phate of lime, though many other plants are not at all influenced by its action. The 

 parietaria, nettle, and borage vi^ill not thrive, except in such soils as contain nitrate of lime, 

 or nitrate of potass ; and plants inhabiting the sea-coast, as was observed by Du Hamel, 

 will not thrive in a soil that does not contain muriate of soda. It has been thought, how- 

 ever, that the salts are not actually taken up by the root, though converted to purposes of 

 utility, 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 question are found by analysis in the very substance of the plant, and must 

 consequently have entered 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 attract the humidity of the atmosphere. But sulphate of lime is 

 not deliquescent ; and if its action consists merely in accelerating putrefaction, why is its 

 beneficial efiect confined but to a small number of plants ? Grisenthwaite (JSfew Theory 

 of Agnculture, 1819, p. 111.) answers this question by stating, that as in the principal 

 grain crops which interest the agriculturist, there exists a particular saline substance 

 peculiar to each, so, if we turn our attention to the clovers and turnips, we shall still find 

 the same discrimination. Saintfoin, clover, and lucerne have long been known to con- 

 tain a notable quantity of gypsum (sulphate of lime) ; but such knowledge, very strange 

 to relate, never led to the adoption of gypsum as a manure for these crops, any more than 

 that of phosphate of lime for wheat, or nitrate of soda or potassa for barley. It is true 

 that gypsum has been long, and in various places, recommended as a manure, but its uses 

 not being understood, it was recommended without any reference to crop, or indeed to 

 the accomplishment of any fixed object. It is very well known that some particular ingre- 

 dient may be essential to the composition of a body, and yet constitute but a very small 

 proportion of its mass. Atmospheric air contains only about one part in the 100 of 

 carbonic acid ; and yet no one will venture to aflSrm that carbonic acid gas is merely an 

 adventitious and accidental element existing by chance in the air of the atmosphere, and 

 not an essential ingredient in its composition. Phosphate of lime constitutes but a very 

 small proportion of animal bodies, perhaps not one part in 500 ; and yet no one doubts 

 that it is essential to the composition of the bones. But the same salt is found in the 

 ashes of all vegetables ; and who will say that is not essential to their perfection. 



1531. Earths. As most plants have been found by analysis to contain a portion of 

 alkaline or earthy salts, so most plants have been found to contain also a portion of 

 earths : and as the two substances are so nearly related, and so foreign in their character 

 from vegetable substances in general, the same enquiry has consequently been made with 

 regard to their origin. Whence are the earths derived that have been found to exist in 

 plants? Chiefly from the soil. But in what peculiar state of combination do they enter 

 the vessels of the plant ? The state most likely to facilitate their absorption is that of their 

 solution in water, in which all the earths hitherto found in plants are known to be in a 

 slight degree soluble. If it be said that the proportion in which they are soluble is so 

 very small that it scarcely deserves to be taken into the account, it is to be recollected 

 that the quantity of water absorbed by the plant is great, while that of the earth 

 necessary to its health is but little, so that it may easily be acquired in the progress of 

 vegetation. Such is the manner in which their absorption seems practicable ; and 

 Woodward's experiments afford a presumption that they are actually absorbed by die 

 root. 



1532. The proportion of earths contained in the ashes of vegetables depends upon the nature of thesoil in 

 which they grow. The ashes of the leaves of the /Rhododendron ferrugineum, growing on Mount Jura, a 

 calcareous mountain, yielded 4325 parts of earthy carbonate, and only 075 of silica : but the ashes of 

 the leaves of a plant of the same species, growing on Mount Breven, a granitic mountain, yielded two parts 

 of silica, and only 1G75 of earthy carbonate. It is probable, however, that plants are not indebted merely to 

 the soil for the earthy particles which they may contain. They may acquire them partly from the atmo- 

 sphere. Margray has shown that rain-water contains silica in the proportion of a grain to a pound ; which, if 

 it should not reach the root, may possibly be absorbed along with the water that adheres to the leaves. But 

 although the earths are thus to be regarded as constituting a small proportion of vegetable food, they are 

 not of themselves sufficient to support the plant, even with the assistance of water. Giobert mixed 

 together lime, alumine, silica, and magnesia, in such proportions as are generally to be met with in fertile 

 soils, and moistened them with water. Several different grains were then sown in this artificial soil, 

 which germinated indeed, but did not thrive ; and perished when the nourishment of the cotyledons was 

 exhausted. It is plain, therefore, that the earths, though beneficial to the growth of some vegetables, 

 and perhaps necessary to the health of others, are by no means capable of affording any considerable de- 

 gree of nourishment to the plant. 



1533. Supjily of food by manures and culture. With regard to the food of plants derived 

 from the atmosphere, the supply is pretty regular, at least, in as far as tlie gases are con- 

 cerned ; for they are not found to vary materially in their proportions on any part of the 

 surface of the globe : but the quantity of moisture contained in the atmosphere is con- 

 tinually varying, so that in the same season you have not always the same quantity, 



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