The Food Of Plants

Tho principal purpose of the root is to absorb the food of the plant trom the earth. In wlmt does this food consist '( In substances of five kinds ; water, air or gases, vegetable extracta, salta and e irths. Every one is aware how necessary water is to the existence of plants ; without it they droop and fade, and are again revived by its application. The bulbous roots, as the hyacinths and others, grow in water only, and inany small plants may be raised from the seed in wet cotton, sponge, and other substancps of the so:t, and soine flourish upon the bare rjck. Indeed water is so important and produces such considerable "ffeots, that many havesupposed that it was the only food of pía nts and that from only water all its parts were created. There are some experimenta which at first sight would appear decisive on thifr poinf, but as they are all of similar characler we will meution but one. Van Helmont. planteil a willow weig-hing fifty pounds in an earthen pot tilled with erth, which had been dried in an oven. He watcred it with rain or distilled wa!er (inly. At the en 1 of flveyeaa it was weighed with all lts leaves on and round to have gained one bundred and nineteen and a quarter pounds, while the earth had lost only two ounces. Henee Van Helinont concluded that the willow had dyrived its nourishnifut from water alone It has been since conoluded however, that there was a fallacy in the experiment, and that water is not the only food of plants. For in the first place, plants cultivated in this way do fiually decline and die betore their natural period, and secondly the willow might have absorbed soiue vegetable extract from the surrouiiding soil throngh the pores of the pot. Besides there is a small quantity of earth contained in all rain water; lastly, all plnnts contain a principie called carbon which is not contained in water, and cannot be derived Froui it. Water, then, though the principal is uot the only focd OÏ' plants. Ihey consume also the gases of which atuiosDberio air is composed. Atuiosphi'iic air is nut asimple but acompound substance, coniposed of two principa] parts, oxygen Hnd nitrogen - and a small proportion of another, caroonic acid gas or n'xed .tir. The two last only appear tj afFord nourishment to vegetables, though the forinor, namely nitrogen gas, seems to be necessury both to animáis and vegetables in soine viay t'iat is not viy well uuderstood. The next kind ot fi)od is vegetable extract ; all have olserved that the upper layer of the earth is a black rnotild. This mould is eomposed of decayed animal aud vegetable s:il)stances. Nuw if a qaantity of this be steeped in boiliug water, and the water strained vff and evaporated. it leaves a thicK mass which is called iu ohemical laiisuag n extract. Plants absorh also various kinds of slts, as comniou salt, Raltpeter, et'!.. Tríese salts are known to exist in the enrth, and are probably dissolved in thwater, which 's taken up by the vegetablo. Lastly, plants consume various kinds ot' enrths, as chalk, fliut, etc. The latter indeed exists in large proportion in süiiie plHnts, as the common reed, or care pole, the rattan, and Dutch bush, wliose outer skin contains a good dtal of fl-nt. Different vegetables requirió different propui-tions of these articles of food, and will not flourish well exeept in situations whioh supply theuj. Thus the willow leves a wet soil, and the grape a dry one. etc. The experienoe of man has taught him to supply the defioieney or exhaustiou of' soils, by various additions whioh are known by the general name of dressings or manures These usually coiitain a quantity of vegetable extract, which seetns to be one of the most important anieles. As plant also require different kinds of food, another agricultural art is to raise various crops in succession upon the sami' spot, so that one shall obtain what the fot nier had left. The soil in this wny, howver, is finally exhausted and rvquires eitlier artificial dressing, or elsto be abanduned for a time to uature't innuerces, fiom which it at length derives th matevials for renewed fertility These various articles are absorbed frorn the soil bj the roots, and soraetimes with great rupidity. Dr. Hules laid hanthe root of a pear tree half au inch in (liiiimtei, nnd ceinented t into a tube, wliich vh8 fillt-d with water, and had is otlier cxtrehiity immersed in quicksilver The absorption was so rapid that the quickbilver roe eight inches in 6Íx iniuutes. H. w does the mot nbsorb these fiuids'r By nieiins of its fibrils. Evciy root, wbether Bpindle-shajM d or what ever ite fiirurc, is uniighed with numorou delicate filiéis, which extetid in every dir-clion Hinl ure tumishfd with pores, whiuh suck up the necessHry fluirts ; froin thest pores procetd tulies whiuh conduct the food into the trunk and branches.- Mant Plovghman.