PLA [ Plant form circulation of the fap through the oak and ilex, why fhould the leaves of the oak fall in winter, and not thofe of the ilex? "Another argument against an uniform circulation of the fap in trees, as in animals, may be drawn from an experiment, where it was found by the three mercurial gages fixed to the fame vine, that while fome of its branches changed their state of protruding fap into a ftate of imbibing, others continued protruding fap; one nine, and the other thirteen days longer." To this reasoning of Dr Hales we shall fubjoin an experiment made by Mr Muftel of the Academy of Sciences at Rouen, which feems decifive against the doctrine of circulation. His account of it is as follows." On the 12th of January I placed feveral fhrubs in pots against the windows of my hot-house, fome within the house and others without it. Through holes made for this purpose in the panes of glafs, I paffed a branch of each of the fhrubs, fo that those on the infide had a branch without, and thofe on the outfide one within; after this, I took care that the holes fhould be exactly clofed and luted. This inverse experiment, I thought, if followed clofely, could not fail affording fufficient points of comparifon, to trace out the differences, by the obfervation of the effects. "The 20th of January, a week after this difpofi- cold. any "The continuance of the froft occafioned no change 7 ] PLA the hot-houfe was covered with leaves and rofe-buds. l'Acad. Of the Perpendicularity of PLANTS.-This is a curious Memoires de Royal des phenomenon in natural history, which was firft obferved by M. Dodart, and published in an effay on the affectation Sciences, an of perpendicularity obferved in the ftems or ftalks of all 1708. es, as much as poffible. Though almost all plants rife plants, in the roots of many, and even in their brancha little crooked, yet the stems fhoot up perpendicularly, which by the declivity of the foil come out inclined, or and the roots fink down perpendicularly even thofe, those which are diverted out of the perpendicular by and recover their perpendicularity, by making a fecond any violent means, again redrefs and ftraighten themfelves and contrary bend or elbow without rectifying the first. We commonly look upon this affectation without any furprife; but the naturalift who knows what a plant is, Each feed we know contains in it a little plant, aland how it is formed, finds it a fubject of aftonishment. ready formed, and needing nothing but to be unfolded; the little plant has its root; and the pulp, which is ufually feparated into two lobes, is the foundation of the minate. If a feed in the earth therefore be difpofed so first food it draws by its root when it begins to geras that the root of the little plant be turned downwards, and the item upwards, and even perpendiculary up-wards, it is eafy to conceive that the little plant coming to unfold itself, its stalk and root need only follow the direction they have to grow perpendicularly. themselves or by man, fall in the ground at random ; But we know that the feeds of plants, whether fown of In all the reft, therefore, it is neand among the great variety of fituations with regard. to the talk of their plant, the perpendicular one up wards is but one. ceffary that the ftalk rectify itself, fo as to get out of To account for two fuch different actions, M. Dodart fuppofes that the fibres of the ftalks are of such a nature as to be contracted and fhortened by the heat of the fun, and lengthened out by the moisture of the earth; and, on the contrary, that the fibres of the roots are contracted by the moisture of the earth, and lengthened by the heat of the fun. When the plantule therefore is inverted, and the root at the top, the fibres which compofe one of the branches of the root are not alike expofed to the moisture of the earth, the lower part being more expofed than the upper. The lower again promoted by the lengthening of the upper, wheremuft of courfe contract the most; and this contraction is on Plant. juice that fucceed will follow the fame direction; and Plant. as all together form the stalk, that must of course be ver tical, unless fome particular circumftance intervene. on the fun acts with the greateft force. This branch of the root muft therefore recoil towards the earth, and, infinuating through the pores thereof, muft get underneath the bulb, &c. By inverting this reafoning we difcover how the stalk comes to get uppermoft. We fuppofe then that the earth attracts the root to itself, and that the fun contributes to its defcent; and, on the other hand, that the fun attracts the ftem, and the earth contributes to fend it towards the fame. With refpect to the ftraightening of the ftalks in the open air, our author imagines that it arifes from the impreffion of external caufes, particularly the fun and rain. For the upper part of a stalk that is bent is more expofed to the rain, dew, and even the fun, &c. than the under; and these causes, in a certain ftructure of the fibres, both equally tend to straighten the part moft exposed by the fhortening they fucceffively occafion in it; for moisture fhortens by fwelling and heat by diffipating. What that structure is which gives the fibres fuch different qualities, or whereon it depends, is a mystery as yet beyond our depth. M. de la Hire accounts for the perpendicularity of the stems or stalks of plants in this manner: he fuppofes that the root of plants draws a coarfer and heavier juice, and the ftem and branches a finer and more volatile one. Moft naturalifts indeed conceive the root to be the ftomach of the plant, where the juices of the earth are fubtilized fo as to become able to rife through the ftem to the extremity of the branches. This difference of juices fuppofes larger pores in the roots than the ftalk, &c. and, in a word, a different contexture. This difference must be found even in the little invisible plant inclofed in the feed: in it, therefore, we may conceive a point of feparation; fuch as, that all on one fide, for example the root, fhall be unfolded by the groffer juices, and all on the other fide by the more fubtile ones. Suppofe the plantule, when its parts begin to unfold, to be entirely inverted, the root at the top, and the stalk below; the juices entering the root will be coarfeft, and when they have opened and enlarged the pores fo as to admit juices of a determinate weight, thofe juices preffing the root more and more will drive it downwards; and this will increafe as the root is more extended or enlarged: for the point of feparation being conceived as the fixed point of a lever, they will act by the longer arm. The volatile juices at the fame time having penetrated the ftalk, will give it a direction from below upwards; and, by reafon of the lever, will give it more and more every day. The little plant is thus turned on its fixed point of feparation till it become perfectly erect. When the plant is thus erected, the stalk should still rife perpendicularly, in order to give it the more firm biding, and enable it to withstand the effort of wind and weather. M. Parent thus accounts for this effect: If the nutritious juice which arrived at the extremity of a rifing ftalk evaporate, the weight of the air which encompaffes it on all fides will make it afcend vertically: but if, inftead of evaporating, it congeal, and remain fixed to that extremity whence it was ready to go off, the weight of the air will give it the fame direction; fo that the stalk will have acquired a fmall new part vertically laid over it, juft as the flame in a candle held in any way obliquely to the horizon ftill continues vertical by the preffure of the atmosphere. The new drops of The branches, which are at firft fuppofed to proceed laterally out of the stalk in the first embryo of the plant, though they fhould even come out in an horizontal direction, muft alfo raise themselves upwards by the conftant direction of the nutritious juice, which at firft fcarce meets any refiftance in a tender fupple branch; and afterwards, even though the branch grow more firm, it will act with the more advantage; fince the branch, being become longer, furnishes it with a longer arm or lever. The flender action of even a little drop becomes very confiderable by its continuity, and by the affiftance of fuch circumftances. Hence may we account for that regular fituation and direction of the branches, fince they all make nearly the fame conftant angle of 45° with the ftem, and with one another. M. Aftruc accounts for the perpendicularity of the ftems, and their redreffing themselves, thus: 1. He thinks the nutritious juice arifes from the circumference of the plant, and terminates in the pith: And, 2. That fluids, contained in tubes either parallel or oblique to the horizon, gravitate on the lower part of the tubes, and not at all on the upper. Hence it follows, that, in a plant placed either obliquely or parallel to the horizon, the nutritious juice will act more on the lower part of the canals than on the upper; and by this means they will infinuate more into the canals communicating therewith, and be collected more copiously therein: thus the parts on the lower fide will receive more accretion and be more nourished than those on the upper, the extremity of the plant will therefore be obliged to bend upwards. This principle brings the feed into its due fituation at firft. In a bean planted upside down, the plume and radicle may be feen with the naked eye fhooting at first directly for about an inch; after which they begin to bend, the one downward, and the other upward.' The fame is the cafe in a heap of barley to be made into malt, or in a quantity of acorns laid to sprout in a moift place, &c. Each grain of barley and each acorn has a different fituation; and yet every sprout tends directly upward, and every root downward, and the cur vity or bend they make is greater or lefs as their fituation approaches more or lefs to the direction wherein no curvature at all would be neceffary. But two fuch oppofite motions cannot poffibly arife without fuppofing fome difference between the two parts: the only one we know of is, that the plume is fed by a juice imported to it by tubes parallel to its fides, whereas the radical imbibes its nourishment at every pore in its furface. When the plume therefore is either parallel or inclined to the horizon, the nutritious juice, feeding the lower parts more than the upper, will determine its extremes to turn upward, for the reafons before given. On the contrary, when the radicle is in the like fituation, the nutritious juice penetrating through the upper part more copioufly than through the under, there will be a greater accretion of the former than of the latter; and the radicle will therefore be bent downwards, and this mu tual curvity of the plume and radicle must continue till fuch time as their fides are nourished alike, which cannot be till they are perpendicular. Of the Food of PLANTS.-This hath been fo fully difcuffed + Parti. fest. 1 quantity of phlogiftic matter contained in them, and the Plants. different action of the latent fire they contain: for all plants do not require an equal quantity of nourishment; and fuch as require but little, will be deftroyed by ha ving too much. The action of heat alfo is effentially neceffary to vegetation; and it is probable that very much of this principle is abforbed from the air by ve getables. But if the air by which plants are partly nourifhed contains too much of that principle, it is very probable that they may be deftroyed from this caufe as well as the other; and thus inflammable air, which contains a vast quantity of that active principle, may de ftroy fuch plants as grow in a dry feil, though it preferves those which grow in a wet one. See VEGETA Plants difcuffed under the article AGRICULTURE, that little remains to be faid upon the fubject in this place. The method of making dephlogifticated or vital air de novo, is now fo much improved, that numberless experiments may be made with it both on animals and vegetables. It appears, indeed, that these two parts of the creation are a kind of counterbalance to one another; and the noxious parts or excrements of the one prove falutary food to the other. Thus, from the animal body continually pafs off certain effluvia, which vitiate or phlogificate the air. Nothing can be more prejudicial to animal life than an accumulation of these effluvia: on the other hand, nothing is more favourable to vegetables than thofe excrementitious effluvia of animals; and accordingly they greedily abforb them from the earth, or from the air. With refpect to the excrementitious parts of living vegetables, the cafe is reverfed. The pureft air is the common effluvium which paffes off from vegetables; and this, however favourable to animal life, is by no means fo to vegetable; whence we have an additional proof of the doctrine concerning the food of plants delivered under the article AGRICULTURE. With regard to the effects of other kinds of air on vegetation, a difference of fome confequence took place between Dr Priestley and Dr Percival. The former, in the first volume of his Experiments and Obfervations on Air, had afferted that fixed air is fatal to vegetable as well as to animal life. This opinion, however, was oppofed by Dr Percival, and the contrary one adopted by Dr Hunter of York in the Georgical Effays, vol. v. The experiments related by these two gentlemen would indeed have been decifive, had they been made with fuffitient accuracy. That this was the cafe, however, Dr Priestley denies; and in the 3d volume of his Treatife on Air has fully detected the mistakes in DrPercival's Experiments; which proceeded in fact from his having ufed, not fixed air, but common air mixed with a fmall quantity of fixed air. His experiments, when repeated with the pureft fixed air, and in the moft careful manner, were always attended with the fame effect, namely, the killing of the plant. It had alfo been afferted by Drs Percival and Hunter, that water impregnated with fixed air was more favourable to vegetation than fimple water. This opinion was likewife examined by Dr Prieftley: however, his experiments were indecifive; but feem rather unfavourable to the ufe of fixed air than otherwise. Another very remarkable fact with regard to the food of plants has been difcovered by Dr Priestley; namely, that fome of them, fuch as the willow, comfrey, and duck-weed, are nourished by inflammable Prielley on air. The firft, he says, flourishes in this fpecies of air dir, vol. v.fo remarkably, that," it may be faid to feed upon it with great avidity. This procefs terminates in the change of what remains of the inflammable air into phlogisticated air, and fometimes into a fpecies of air as good as common air, or even better; fo that it muft be the inflammable principle in the air that the plant takes, converting it, no doubt, into its proper nourish Diffemination of PLANTS.-So great are the prolific powers of the vegetable kingdom, that a single plant almost of any kind, if left to felf, would, in a fhort time, over-run the whole word. Indeed, fuppofing the plant to have been only a single annual, with two feeds, it would, in 20 years, produce more than a million of its own fpecies; what numbers then must have been produced by a plant whofe feeds are fo numerous as many of those with which we are acquainted? See NATURAL Hiftory, fect. iii. p. 654, &c. In that part of our work we have given particular examples of the very prolific nature of plants, which we need not repeat here; and we have made fome observations on the means by which they are carried to diftant places. This is a very curious matter of fact, and as fuch we hall now give a fuller account of it. If nature had appointed no means for the scattering of thefe numerous feeds, but allowed them to fall down in the place where they grew, the young vegetables muft of neceffity have choaked one another as they grew up, and not a fingle plant could have arrived at perfection. But fo many ways are there appointed for the diffemination of plants, that we fee they not only do not hinder each others growth, but a fingle plant will in a fhort time spread through different countries. The most evident means for this purpose are, 1. The force of the air.-That the efficacy of this may be the greater, nature has raifed the feeds of vegetables upon ftalks, fo that the wind has thus an opportunity of acting upon them with the greater advan tage. The feed-capfules also open at the apex, left the ripe feeds fhould drop out without being widely difperfed by the wind. Others are furnifhed with wings, and a pappous down, by which, after they come to maturity, they are carried up into the air, and have been known to fly the diftance of 50 miles: 138 genera are found to have winged feeds. 2. In fome plants the feed-veffels open with violence when the feeds are ripe, and thus throw them to a confiderable distance; and we have an enumeration of 50 genera whofe feeds are thus difperfed. 3. Other feeds are furnished with hooks, by which, when ripe, they adhere to the coats of animals, and are carried by them to their lodging places. Linnæus reckons 50 genera armed in this manner. 4. Many feeds are difperfed by means of birds and other animals; who pick up the berries, and afterwards eject the feeds uninjured. Thus the fox diffeminates the privet, and man many fpecies of fruit. The plants found growing upon walls and houfes, on the tops of B high Plants Rain and fhowers carry feeds into the cracks of the Plants. high rocks, &c. are mostly brought there by birds; by and it is univerfally known, that by manuring a field with new dung, innumerable weeds will fpring up which did not exift there before: 193 fpecies are rec. koned up which may be diffeminated in this manner. 5. The growth of other feeds is promoted by animals in a different way. While fome are eaten, others are scattered and trodden into the ground by them. The fquirrel gnaws the cones of the pine, and many of the feeds fall out. When the loxica eats off their bark, almoft his only food, many of their feeds are committed to the earth, or mixed in the morafs with mofs, where he had retired. The glandularia, when the hides up her nuts, often forgets them, and they ftrike root. The fame is obfervable of the walnut; mice collect and bury great quantities of them, and being afterwards killed by different animals, the nuts ger minate. Amen. Academ. 6. We are astonished to find moffes, fungi, byffus, and mucor, growing everywhere; but it is for want of reflecting that their feeds are fo minute that they are almost invisible to the naked eye. They float in the air like atoms, and are dropped everywhere, but grow only in those places where there was no vegetation before; and hence we find the fame moffes in North America and in Europe. 7. Seeds are alfo difperfed by the ocean, and by ri vers. "In Lapland (fays Linnæus), we see the most evident proofs how far rivers contribute to depofite the feeds of plants. I have feen Alpine plants growing upon their fhores frequently 36 miles diftant from the Alps; for their feeds falling into the rivers, and being carried along and left by the stream, take root there. We may gather likewife from many circumstances how much the fea furthers this bufinefs. -In Roflagia, the ifland of Grafwa, Oeland, Gothland, and the thores of Scania, there are many foreign and German plants not yet naturalized in Sweden. The centaury is a German plant, whofe feeds being carried by the wind into the fea, the waves landed this foreigner upon the coafts of Sweden. I was aftonished to fee the veronica maritima, a German plant, growing at Tornea, which hitherto had been found only in Grafoa; the fea was the vehicle by which this plant was tranfported thither from Germany; or poffibly it was brought from Germany to Græfca, and from thence to Tornea. Many have imagined, but erroneously, that feed corrupts in water, and lofes its principle of vegetation. Water at the bottom of the fea is feldom warm enough to deftroy feeds; we have feen water cover the furface of a field for a whole win ter, while the feed which it contained remained unhurt, unlefs at the beginning of fpring the waters were let down fo low by drains, that the warmth of the funbeams reached to the bottom. Then the feeds germinate, but prefently become putrefcent; fo that for the refl of the year the earth remains naked and barren, 8. Laftly, fome feeds affift their projection to a di ftance in a very furprifing manner. The crupina, a fpecies of centaury, has its feeds covered over with erect briftles, by whofe affiftance it creeps and moves about in fuch a manner, that it is by no means to be kept in the hand. If you confine one of them between the stocking and the foot, it creeps out either at the fleeve or neck-band, travelling over the whole body. If the bearded oat, after harveft, be left with other grain in the barn, it extricates itself from the glume; nor does it ftop in its progrefs till it gets to the walls of the build. ing. Hence, fays Linnæus, the Dalecarlian, after he has cut and carried it into the barn, in a few days finds all the glumes empty, and the oats feparate from them; for every oat has a spiral arifta or beard annexed to it, which is contracted in wet, and extended in dry weather, When the fpiral is contracted, it drags the oat along with it: the arifta being bearded with minute hairs pointing downward, the grain neceffarily follows it; but when it expands again, the oat does not go back to its former place, the roughnefs of the beard the contrary way preventing its return. If you take the feeds of equifetum, or fern, thefe being laid upon paper, and viewed in a microscope, will be seen to leap over any obstacle as if they had feet; by which they are feparated and difperfed one from another; fo that a perfon ignorant of this property would pronounce these feeds to be fo many mites or fmall infects. We cannot finish this article without remarking, that many ingenious men (4) believe that plants have a power of perception. Of this opinion we fhall now give an account from the fecond volume of the Manchester Tranfactions, where we find fome fpeculations on the perceptive power of vegetables by Dr. Percival, who attempts to fhow, by the feveral analogies of organization, life, inftinct, fpontaneity, and felf-motion, that plants, like animals, are endued with the powers both of perception and enjoyment. The attempt is ingenious, and is ingenioully fupported, but in our opinion fails to convince. That there is an analogy between animals and vegetables is certain; but we cannot from thence conclude that they either perceive or enjoy. Botanists have, it is true, derived from anatomy and phyfiology, almost all the terms employed in the defcription of plants. But we cannot from thence conclude, that their organization, tho' it bears an analogy to that of animals, is the fign of a living principle, if to this principle we annex the idea of perception; yet fo fully is our author convinced of the truth of it, that he does not think it extravagant to fuppofe, that, in fome future period, perceptivity may be difcovered to extend even beyond the limits now a figned to vegetable life. Corallines, madrepores, millepores, and fpunges, were formerly confidered as foffil bodies: (A) The ingenious Dr Bell held this opinion, as appears from the close of his Thefis de Phyfiologia Plantarum, which was published at Edinburgh, June 1777, and a translation of which by Dr Currie we find in the fecond volume of the Manchester Tranfactions, where our readers will alfo find memoirs of its author. Dr Currie in forms us, that Dr Hope, the late excellent profeffor of botany in Edinburgh, in his courfe of lectures, used to fpeak of Dr Bell with the highest elteem; but did not approve of the idea which he entertained refpecting the feeling or perception of plants. |