Book I. FOOD OF THE VEGETATING PLANT. ' flsi 



culiar 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 those 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 affirm 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 ? 



1505. 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 

 to vegetable substances in general, the same enquiry has consequently been made vvdth 

 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 the 

 root. 



1506. The proportion of earths contained in the ashes of vegetables depends upon the nature of the soil in 

 which they grow. The ashes of the leaves of the rhododendron ferrngineum, growing on Mount Jura, a 

 calcareous mountain, yielded 43'23 parts of earthy carbonate, and only 075 of silica. But the ashes of 

 leaves of the same plants, growing on Mount Breven, a granitic mountain, yielded two parts of silica, and 

 only 1675 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 atmosphere. 

 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. 



1 507. Supply 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 the 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, 

 though in the course of the year the deficiency is perhaps made up. From the atmo- 

 sphere, therefore, there is a regular supply of vegetable food kept up by nature for the 

 support of vegetable life, independent of the aid of man : and if human aid were even 

 wanted, it does not appear that it could be of much avail. But this is by no means the 

 case with regard to soils ; for if soils are less regular in their composition, they are at 

 least more within the reach of human management. The supply of food may be in- 

 creased by altering the mechanical or chemical constitution of soils ; and by the addi- 

 tion of food in the form of manures. The mechanical constitution of soils may be 

 altered by pulverisation, consolidation, draining, and watering; their chemical properties 

 by aeration and torrification ; both mechanical and chemical properties, by the addition 

 of earths or other substances ; and manures, either liquid or solid, are supplied by irri- 

 gation and distribution of dungs and other nourishing matters, with or without their 

 interment. (See Book III.) 



1 508. Soils in a state of culture, though consisting originally of the due proportion of 

 ingredients, may yet beco7ne exhausted of the principle of fertility by means of too frequent 

 cropping ; whether by repetition or rotation of the same, or of different crops. In this 

 case, it should be the object of the phytologist, as well as of the practical cultivator, 

 to ascertain by what means fertility is to be restored to an exhausted soil, or commu- 

 nicated to a new one. In the breaking up of new soils, if the ground has been wet or 



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