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INTRODUCTION. 



Nutrition of Plants. 



The essential constituents of plants are oxygen, hydrogen, and carbon ; 

 nitrogen, though not forming a component part of their tissues, is always pre- 

 sent in growing organs, and is found in numerous -products of vegetation. 

 Various mineral substances also enter into the composition of different plants 

 and their secretions. All these constituents must be introduced into the plant 

 by absorption, without it be admitted that vegetables possess the power of 

 transforming one simple substance into another, which, although asserted, has 

 not been proved. The food of vegetables must always be taken in a fluid 

 form, as the spongioles of the roots, the principal avenues by which it is in- 

 troduced, can only absorb matters presented to them in a liquid state. The 

 hydrogen contained in plants is principally derived from the decomposition of 

 water, as this element does not exist in the soil in any appreciable quantity ; 

 but some is also derived from the atmosphere. Their oxygen is supplied 

 from the same sources. Their carbon enters them in the form of car- 

 bonic acid, either through the roots, or is absorbed in a gaseous state by the 

 leaves. Their nitrogen, which is always small in quantity, is derived from 

 the atmosphere ; a minute portion, perhaps, enters the plant in an uncom- 

 bined state, but the larger part is introduced in the form of ammonia. The 

 alkaline and earthy matters are furnished by the soil, from which each spe- 

 cies takes up various materials in different proportions, according to its nature 

 and wants. 



It has been thought that the spongioles of roots have no power of selection, 

 but absorb any thing presented to them in a liquid form, but this does not ap- 

 pear to be the case to the extent supposed ; and it seems probable that these 

 parts are endowed with a power of discrimination to a certain extent, and are 

 capable of selecting the substances best fitted to promote the growth of the 

 plant. It has been observed that roots excrete a peculiar matter, varying in 

 different species of plants. This has been supposed to be injurious, or at least 

 of no benefit to the plant, though it is nutritive to a vegetable of another kind. 

 These suppositions do not appear to be supported by facts, and the attempt to 

 explain the benefit of a rotation of crops by them, is erroneous. It has been 

 satisfactorily shown, that the cause of sterility in a soil is owing to. the ab- 

 sorption of all principles from it by the growing crop, and that if a supply of 

 them be artificially kept up, that the same species of plants may be advanta- 

 geously grown in the same spot, for an indefinite period. 



The fluid absorbed by the roots is carried into the stem, probably undergo- 

 ing some modification in its passage. It is now termed sap, and in its pro- 

 gress to the leaves, or its ascent, is still more altered by dissolving some of 

 the soluble matters with which it comes in contact, and by undergoing cer- 

 tain chemical changes. In a newly developed herbaceous stem, the sap 

 mainly ascends through the pith ; but in woody exogenous stems, it rises 

 through the alburnum, and in endogenous trunks through the whole wood. 

 When it reaches the leaves, it is widely diffused through them, and undergoes 

 its principal transformations. These are effected in the green cellular tissue, 

 or that containing the coloured particles, called chromule ; but all green parts 

 of vegetables act on the sap, as the outer bark of herbaceous plants, dec. 

 These changes do not take place when the direct action of light and air is with- 

 held. They are a concentration of the sap ; a decomposition of the carbonic 

 acid contained in it, and of that absorbed by the leaves ; the carbon and hy- 

 drogen being retained for the nourishment of the plant, whilst the oxygen is 

 set free. During vegetation, leaves exhale much water; but the amount va- 

 ries with the degree of warmth and dryness of the air, and the exposure to 



