THE POOD OF VEGETABLES. 



Ill 



Soil is composed of certain mixtures of the 

 following substances. The earths, silex, alumina, 

 lime, magnesia. The alkalies, potass, soda, and 

 ammonia, oxide of iron, and small portions of 

 other metallic oxides, and a considerable propor- 

 tion of aqueous moisture, and several gases, as 

 oxygen, hydrogen, carbonic acid, besides the 

 soil also contains vegetables and animal matters 

 either partially or wholly decomposed. 



The atmosphere, again, consists of oxygen, 

 nitrogen, and carbonic acid, with a varying por- 

 tion of aqueous vapour. 



All these ingredients, however, are not taken 

 up by vegetables ; part only are selected, and in 

 certain proportions. The elementary ingredi- 

 ents of all plants consists of carbon, oxygen, and 

 hydrogen, in various proportions, and under 

 various modifications. Now, most soils would 

 yield these ingredients, and yet it is well known 

 that certain plants only thrive in particular 

 soils. Nor does it follow that these ingredi- 

 ents enter the plant in an uncombined state, 

 because they do not actually exist in this free 

 and uncombined state in all soils. A power of 

 selection then is made by the plant, and this it 

 is enabled to do by the vital powers of assimila- 

 tion, by which it is endowed. We shall con- 

 sider the food of plants under the six heads of 

 water, gases, vegetable extracts, salts, earth, and 

 manures. 



Water. A certain degree of fluidity is ne- 

 cessary for all organised bodies before the prin- 

 ciple of germination can be excited in a seed. It 

 must first be moistened with water, and after 

 the process of evolution has been completed, a 

 regular supply of fluid is necessary to fill the 

 cells and tubes of the growing plant. Plants 

 will not continue to vegetate unless their roots 

 are supplied with water ; and if they are kept 

 long without it the leaves will droop and be- 

 come flaccid, and assume a withered appearance. 

 Now this is evidently owing to the loss of fluid ; 

 for if the roots are again well supplied with 

 water the weight of the plant is increased, and 

 its freshness restored. But many plants will 

 grow, and thrive, and efifect the development of 

 all their parts, if the root is merely immersed in 

 water, though not fixed in the soil. Lilies, 

 hyacinths, and a variety of plants with bulbous 

 roots, may be so reared, and are often to be met 

 with so vegetating ; and many plants will also 

 vegetate though wholly immersed. Most of the 

 marine plants are of this description. It can 

 scarcely be doubted, therefore, that water serves 

 for the purpose of a vegetable aliment. 



But if plants cannot be made to vegetate 

 without water, and if they wiU vegetate, some, 

 when partly immersed without the assistance of 

 soil ; and some even when totally immersed so 

 as that no other food seems to have access to 

 them, does it not follow that water is the sole 



food of plants, the soil being merely the basis on 

 which they rest, and the receptacle of their 

 food? This opinion has had many advocates, 

 and the arguments and experiments adduced in 

 support of it were at one time thought to have 

 completely established its truth. It was indeed, 

 the prevailing opinion of the seventeenth cen- 

 tury, and was embraced by several philosophers 

 even of the eighteenth century; but its ablest 

 and most zealous advocates were Van Ilelmont, 

 Boyle, Du Hamel, and Bonnet, who contended 

 that water, by virtue of the vital energy of the 

 plant, was sufBcient to form all the difi^erent 

 substances contained in vegetables. 



Van Helmont planted a wiUow weighing fifty 

 pounds in an earthen vessel containing a kno^Ti 

 quantity of earth which had been previously 

 dried in an oven. He moistened it with distilled 

 water, or with rain water, and took care to pre- 

 vent any accession of other earth. At the end 

 of five years the plant was taken up and weighed. 

 Its weight, together with that of all its leaves, 

 was 16&J pounds, and the weight of the earth, 

 only two ounces less than at first, giving an 

 accession of 119;^ pounds, which is to be ac- 

 counted for only from the water with which the 

 earth was moistened. Hence, it was concluded 

 that water is the sole food of plants ; the two 

 ounces of earth lost being regarded as bearing 

 too small a proportion to the increased weight 

 of the willow to deserve any notice in the cal- 

 culation. 



Boyle dried a quantity of earth in an oven, 

 which after having weighed he put into an 

 earthen pot. He then sowed some gourd seed 

 in the earth, and watered it with spring or rain 

 water. A plant was ultimately produced that 

 weighed three pounds, and in a subsequent ex- 

 periment, a plant that weighed four pounds; 

 and yet the weight of the earth, when dried and 

 weighed again, was not perceptibly diminished. 

 This seemed to give weight to the foregoing 

 conclusion. 



Du Hamel placed some bulbous roots merely 

 in moss or wet sponges, and they vegetated ; and 

 beans and peas when so treated even flourished 

 and produced fruit. Bonnet in repeating the 

 experiments of Du Hamel had the same result, 

 and in trying its operation upon vines, found 

 that they produced excellent grapes. Nothing 

 further seemed necessary to determine the point 

 at issue, and it was accordingly believed that 

 water is the sole food of plants, and that the 

 other substances which they may contain are 

 formed merely from the water, by virtue of the 

 vital energy of the plant. 



But though these experiments have the ap- 

 pearance of being somewhat decisive, yet there 

 are others by the same experimenters which are 

 not quite so favourable to the opinion they were 

 intended to support. Du Hamel reared in the 



