Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/72

 46 A G R I C U L T U R E. fels, whatever it i&, that firft puts the Tap in motion, be- plant have frequently various fmells, taftes, <bc. although fore any tranfpiration has commenced. the nourilhment derived from the root muft be the fame. Heat, moifture, and air, are the three chief circum- This is an evidence, that the different ftiudUire of parrs ftances that promote the afcent of the fap. Hence no- in the fame plants is capable of producing very fenfible thing is more favourable to vegetation than warm Weather changes in the nature and quality of the fap. accompanied with rain: on the other hand, cold dry Repeated experiments fhow, .that many plants of very weather is its greateft enemy. In a wet, cold feafon, oppofite qualities, and even trees, have been nourifhed every thing rots; and in hot dry weather every thing is and brougjht tq maturity by the pureft water alone. parched. But the circumftances moft favourable to ve- It is obferved, op the other hand, that different plants getation are cloudy, hot weather, inclinable to thunder, require different foils.. This is certainly true: But what Succeeded by plentiful rains. then ? Does not this difference in foil rather depend upon the greater or leffer quantity, than any peculiar quain the food ? Thyme grows heft in a dry foil; but Sect. VIII. Of the Food of Plants. lity it will grow equally well in earth carried from a marfh It is thought to be an important queftion in agriculture, to the top of a mountain. whether the feveral kinds of plants require the fame, or The roots of plants are fitted to abforb every fluid that comes within their reach. They have been found by different nourifhment. Upon a fuperficial view of this queftion, it would appear experiment to imbibe fluids that actually poifon them. very improbable, that the fame matter could nourifh fuch From this circumftance it may be fairly concluded, that a variety of plants, differing fo effentially in fmell, tafte, they have not, like animals, the fagacity of chufing the figure, <&c. Much, however, may depend upon the in- food that is moft proper for nourifhing them, and rejectternal ftrudlure and arrangement of the veffels. One ing that which is either hoxious or lefs nourifiiing. thing is certain, that if the veffels in any plant be un- Mr Dickion, author of an excellent treatife on agricommonly fmall, parts will be reje&ed by that plant culture, publi/hed in 1765, has endeavoured to fix the which would be abforbed by one whofe veffels are larger. particular ingredients that enter into the compofition of Nay, changes may be made in the crude homogeneous the food of vegetables. He contends, that neither earth, nourifhment, by a fmall difference, in the figure or action water, air, oil, nor fait, can be called the food of plants ; ' but he thinks that it confifts of a combination of all thefe of the veffels. It is given out as a fadt, by writers on this fubjedt, fubftances. His arguments in fupport of this theory are that one plant will ftarve another, by robbing it of its chiefly drawn from the chemical analyfis, which Ihows, nourifhment. This does not feem to affedt either fide of •that all thefe fubftances may be extorted from vegetables the queftion; for it may ftarve its neighbour, either by by the force of fire; and from a confideration that a due extending its roots, and requiring a greater quantity of admixture of .thefe .fubftances (or fuch things as connourifhment than the other; or it may abforb the pecu- tain them) is favourable, and even neceffary, to vegetaliar food which is neceffary for the growth of the other tion. plant. In either cafe, the plant is deprived of a proper His laft argument is good : But whoever attempts to difcover the properties of plants, or the ingredients of quantity of nourifhment. It is likewife propofed as a difficulty, Why a poifonous their food, from a chemical analyfis, will probably never plant and its antidote will grow in the fame foil, and very do much fervice to the fcience of agriculture. Fire and near each others This argument is of the fame nature a retort is capable of torturing either animals or vegeWith the former. It may be owing either to thefe plants tables into forms and qualities which never exifted either imbibing different juices from the earth, or to peculiari- in theft bodies, or in their food. ties in the ftrudlure and adtion of their vefTeJs. Thefe, We fhall conclude this fedtion with obferving, that the and many other ambiguous fadls, (have been advanced on farmer, in nourifhing his plants, fhould be direfted entireboth fides of this queftion, which we fhall not fpend time ly by experience. If he knows, that putrid animal iii enumerating. and vegetable fubftances, that lime, foot, marie, foe. The argument drawn from grafted plants, feems more when applied with judgment, aflift the growth of his diredl and decifive. A ftalk of a lemon, grafted on a plants, and augment his crop, it is of little confequence branch of an orange-tree, grew, ripened its fruit, and whether he be acquainted with their chemical analyfis, preferved the figure and all the other qualities belong- or the particular mode of their operation. We do not ing to a lemon. This plainly indicates, that the or- mean that he fhould continue obftinately in the old beatganisation of the lemon had given a different modifica- en track, as it is called ; but rathe/r that he fhould try tion to the juices of the orange, through the interven- whether he can by any means improve upon the old tion of which it received its nourifhment. method, and that his pra&ice fhould be diredted accordIt is alfo certain, that the different parts of the fame ing to the fuccefs of theft trials. PART