against the windows of my hot-house, some within the Plant house and others without it. Through holes made for this purpose in the panes of glass, I passed a branch of each of the shrubs, so that those in the inside had a branch without, and those on the outside one within; after this, I took care that the holes should be exactly closed and luted. This inverse experiment, I thought, if followed closely, could not fail affording sufficient points of comparison, to trace out the differences, by the observation of the effects.

"For the sap in all vegetables does probably recede in some measure from the tops of the branches, as the sun leaves them; because its rarefying power then ceasing, the greatly rarefied sap, and air mixed with it, will condense, and take up less room than they did, and the dew and rain will then be strongly imbibed by the leaves; whereby the body and branches of the vegetable which have been much exhausted by the great evaporation of the day, may at night imbibe sap and dew from the leaves; for by several experiments, plants were found to increase considerably in weight, in dewy and moist nights. And by other experiments on the vine, it was found that the trunk and branches of vines were always in an imbibing state, caused by the great perspiration of the leaves, except in the bleeding season; but when at night that perspiring power ceases, then the contrary imbibing power will prevail, and draw the sap and dew from the leaves, as well as moisture from the roots. "And we have a farther proof of this by fixing mercurial gages to the stems of several trees which do not bleed, whereby it is found that they are always in a strongly imbibing state, by drawing up the mercury several inches whence it is easy to conceive, how some of the particles of the gilded bud in the inoculated jessamine may be absorbed by it, and thereby communicate their gilding miasma to the sap of other branches; especially when, some months after the inoculation, the stock of the inoculated jessamine is cut off a little above the bud; whereby the stock, which was the counteracting part to the stem, being taken away, the stem attracts more vigorously from the bud.

"Another argument for the circulation of the sap is, that some sorts of the graffs will infect and canker the stocks they are grafted on: but by mercurial gages fixed to fresh cut stems of trees, it is evident that those stems were in a strongly imbibing state; and consequently the cankered stocks might very likely draw sap from the graff, as well as the graff alternately from the stock; just in the same manner as leaves and branches do from each other, in the vicissitudes of day and night. And this imbibing power of the stock is so great, where only some of the branches of a tree are grafted, that the remaining branches of the stock will, by their strong attraction, starve those graffs; for which reason it is usual to cut off the greatest part of the branches of the stock, leaving only a few small ones to draw up the sap.

"The instance of the ilex grafted upon the English oak, ɛeems to afford a very considerable argument against a circulation. For, if there were a free uniform circulation of the sap through the oak and ilex, why should the leaves of the oak fall in winter, and not those of the ilex?

"Another argument against an uniform circulation of the sap in trees, as in animals, may be drawn from an experiment, where it was found by the three mercurial gages fixed to the same vine, that while some of its branches changed their state of protruding sap into a state of imbibing, others continued protruding sap; one nine, and the other thirteen days longer."

To this reasoning of Dr Hales we shall subjoin an experiment made by Mr Mustel of the Academy of Sciences at Rouen, which seems decisive against the doctrine of circulation. His account of it is as follows." On

the 12th of January I placed several shrubs in pots

4

"The 20th of January, a week after this disposition, all the branches that were in the hot-house began to disclose their buds. In the beginning of February there appeared leaves; and towards the end of it, shoots of a considerable length, which presented the young flowers. A dwarf apple-tree, and several rose-trees, being submitted to the same experiment, showed the same appearance then as they commonly put on in May; in short, all the branches which were within the hot-house, and consequently kept in the warm air, were green at the end of February, and had their shoots in great forwardness. Very different were those parts of the same tree which were without and exposed to the cold. None of these gave the least sign of vegetation; and the frost, which was intense at that time, broke a rose-pot placed on the outside, and killed some of the branches of that very tree which, on the inside, was every day putting forth more and more shoots, leaves, and buds, so that it was in full vegetation on one side, whilst frozen on the other.

"The continuance of the frost occasioned no change in any of the internal branches. They all continued in a very brisk and verdant state, as if they did not belong to the tree which, on the outside, appeared in the state of the greatest suffering. On the 15th of March, notwithstanding the severity of the season, all was in full bloom. The apple-tree had its root, its stem, and part of its branches, in the bot-house. These branches were covered with leaves and flowers; but the branches of the same tree, which were carried on the outside, and exposed to the cold air, did not in the least partake of the activity of the rest, but were absolutely in the same state which all trees are in during winter. A rose-tree, in the same position, showed long shoots with leaves and buds; it had even shot a vigorous branch upon its stalk; whilst a branch which passed through to the outside had not begun to produce any thing, but was in the same state with other rose-trees left in the ground. This branch is four lines in diameter, and 18 inches high.

"The rose-tree on the outside was in the same state; but one of its branches drawn through to the inside of the hot-house was covered with leaves and rose-buds. It was not without astonishment that I saw this branch shoot as briskly as the rose-tree which was in the hothouse, whose roots and stalk, exposed as they were to the warm air, ought, it should seem, to have made it get forwarder than a branch belonging to a tree, whose roots, trunk, and all its other branches, were at the very time frost-nipped. Notwithstanding this, the branch did not seem affected by the state of its trunk; but the action of the heat upon it produced the same effect as if the whole tree had been in the hot house."

Memoires

de l'Acad

Of the Perpendicularity of PLANTS.-This is a curi- Royal des, ous phenomenon in natural history, which was first ob- Sciences, served by M. Dodart, and published in an essay on the an. 1708.

affectation

equally tend to straighten the part most exposed by the shortening they successively occasion in it; for moisture shortens by swelling and heat by dissipating. What that structure is which gives the fibres such different qualities, or whereon it depends, is a mystery as yet beyond our depth.

affectation of perpendicularity observed in the stems or stalks of all plants, in the roots of many, and even in their branches, as much as possible. Though almost all plants rise a little crooked, yet the stems shoot up perpendicularly, and the roots sink down perpendicularly: even those, which by the declivity of the soil come out inclined, or those which are diverted out of the perpendicnlar by any violent means, again redress and straighten themselves and recover their perpendicularity, by making a second and contrary bend or elbow without rectifying the first. We commonly look upon this affectation without any surprise; but the naturalist who knows what a plant is, and how it is formed, finds it a subject of astonishment.

Each seed we know contains in it a little plant, already formed, and needing nothing but to be unfolded; the little plant has its root; and the pulp which is usually separated into two lobes, is the foundation of the first food it draws by its root when it begins to germi nate. If a seed in the earth, therefore, be disposed so as that the root of the little plant be turned downwards, and the stem upwards, and even perpendicularly upwards, it is easy to conceive that the little plant coming to unfold itself, its stalk and root need only follow the direction they have to grow perpendicularly. But we know that the seeds of plants, whether sown of themselves or by man, fall in the ground at random; and among the great variety of situations with regard to the stalk of their plant, the perpendicular one upwards is but one. In all the rest, therefore, it is necessary that the stalk rectify itself, so as to get out of the ground: but what force effects this change, which is unquestionably a violent action? Does the stalk find a less load of earth above it, and therefore go naturally that way where it finds the least obstacle? Were this so, the little root, when it happens to be uppermost, must also follow that direction, and mount up.

To account for two such different actions, M. Dodart supposes that the fibres of the stalks are of such a nature as to be contracted and shortened by the heat of the sun, 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 sun. When the plantule therefore is inverted, and the root at the top, the fibres which compose one of the branches of the root are not alike exposed to the moisture of the earth, the lower part being more exposed than the upper. The lower must of course contract the most; and this contraction is again promoted by the lengthening of the upper, whereon the sun acts with the greatest force. This branch of the root must therefore recoil towards the earth, and, insinuating through the pores thereof, must get underneath the bulb, &c. By inverting this reasoning we discover how the stalk comes to get uppermost. We suppose then that the earth attracts the root to it self, and that the sun contributes to its descent; and, on the other hand, that the sun attracts the stem, and the earth contributes to send it towards the same. With respect to the straightening of the stalks in the open air, our author imagines that it arises from the impression of external causes, particularly the sun and rain. For the upper part of a stalk that is bent is more exposed to the rain, dew, and even the sun, &c. than the under; and these causes, in a certain structure of the fibres, both

M. de la Hire accounts for the perpendicularity of the stems or stalks of plants in this manner: he supposes that the root of plants draws a coarser and heavier juice, and the stem and branches a finer and more volatile one. Most naturalists indeed conceive the root to be the stomach of the plant, where the juices of the earth are subtilized so as to become able to rise through the stem to the extremity of the branches. This difference of juices. supposes larger pores in the roots than the stalk, &c.and, in a word, a different contexture. This difference must be found even in the little invisible plant inclosed in the seed in it, therefore, we may conceive a point of separation; such as, that all on one side, for example the root, shall be unfolded by the grosser juices, and all on the other side by the more subtle ones. Suppose 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 coarsest, and when they have opened and enlarged the pores so as to admit juices of a determinate weight, those juices pressing the root more and more will drive it downwards; and this will increase as the root is more extended or enlarged for the point of separation being conceived as the fixed point of a lever, they will act by the longer arm. The volatile juices at the same time having penetrated the stalk, will give it a direction from below upwards; and, by reason of the lever, will give it more and more every day. The little plant is thus turned on its fixed point of separation till it become perfectly erect.

When the plant is thus erected, the stalk should still rise 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 rising stalk evaporate, the weight of the air which encompasses it on all sides will make it ascend vertically: but if, instead 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 same direction; so that the stalk will have acquired a small new part vertically laid over it, just as the flame in a candle held in any way obliquely to the horizon still continues vertical by the pressure of the atmosphere. The new drops of juice that succeed will follow the same direction; and as all together form the stalk, that must of course be vertical, unless some particular circumstance intervene.

The branches, which are at first supposed to proceed laterally out of the stalk in the first embryo of the plant, though they should even come out in an horizontal direction, must also raise themselves upwards by the constant direction of the nutritious juice, which at first scarcely meets any resistance in a tender supple branch ; and afterwards, even though the branch grow more firm, it will act with the more advantage; since the branch, being become longer, furnishes it with a longer arm or lever. The slender action of even a little drop becomes very considerable by its continuity, and by the assistance of such circumstances. Hence we may ac count for that regular situation and direction of the branches,

M. Astruc accounts for the perpendicularity of the stems, and their redressing themselves, thus: 1. He thinks the nutritious juice arises 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 insinuate more into the canals communicating there with, and be collected more copiously therein thus the parts on the lower side 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 seed into its due situation at first. In a bean planted upside down, the plume and radicle may be seen with the naked eye shooting at first directly for about an inch; after which they begin to bend, the one downward, and the other upward. The same is the case in a heap of barley to be made into malt, or in a quantity of acorns laid to sprout in a moist place, &c. Each grain of barley and each acorn has a different situation; and yet every sprout tends directly upward, and every root downward, and the curvity or bend they make is greater or less as their situation approaches more or less to the direction wherein no curvature at all would be necessary. But two such opposite motions cannot possibly arise without supposing some 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 sides, whereas the radicle imbibes its nourishment at every pore in its surface. 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 reasons before given. On the contrary, when the radicle is in the like situation, the nutritious juice penetrating through the upper part more copiously than through the uuder, there will be a greater accretion of the former than of the latter; and the radicle will therefore be bent downwards, and this mutual curvity of the plume and radicle must continue till such time as their sides are nourished alike, which cannot be till they are perpendicular.

Of the Food of PLANTS.-This hath been so fully discussed under the article AGRICULTURE, that little remains to be said upon the subject in this place. The method of making dephlogisticated or vital air de novo, is now so 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 salutary food to the other. Thus, from the animal body continually pass off certain effluvia, which vitiate or phlogisticate the air. Nothing can be more prejudicial to ani

mal life than an accumulation of these effluvia: on the other hand, nothing is more favourable to vegetables than those excrementitious effluvia of animals; and accordingly they greedily absorb them from the earth, or from the air. With respect to the excrementitious parts

With regard to the effects of other kinds of air on vegetation, a difference of some consequence took place between Dr Priestley and Dr Percival. The former, in the first volume of his Experiments and Observations on Air, had asserted that fixed air is fatal to vegetable as well as to animal life. This opinion, however, was opposed by Dr Percival, and the contrary one adopted by Dr Hunter of York in the Georgical Essays, vol. v, The experiments related by these two gentlemen would indeed have been decisive, had they been made with suf ficient accuracy. That this was the case, however, Dr Priestley denies; and in the 3d volume of his Treatise on Air has fully detected the mistakes in Dr Percival's Experiments; which proceeded in fact from his having used, not fixed air, but common air mixed with a small quantity of fixed air. His experiments, when repeated with the purest fixed air, and in the most careful manner, were always attended with the same effect, namely, the killing of the plant.

It had also been asserted by Drs Percival and Hunter, that water impregnated with fixed air was more favourable to vegetation than simple water. This opinion was likewise examined by Dr Priestley; however, his experiments were indecisive; but seem rather unfavourable to the use of fixed air than otherwise.

Another very remarkable fact with regard to the food of plants has been discovered by Dr Priestley; namely, that some of them, such as the willow, comfrey, and duckweed, are nourished by inflammable air. The first, he says, flourishes in this species of air so remarkably, that," it may be said to feed upon it with great avidity. Presty This process terminates in the change of what remains on Ar of the inflammable air into phlogisticated air, and sometimes into a species of air as good as common air, or even better; so that it must be the inflammable princi ple in the air that the plant takes, converting it, no doubt, into its proper nourishment."

What the followers of Stabl call phlogisticated air and inflammable air, are so closely allied to each other, that it is no wonder they should serve promiscuously for the food of plants. The reason why both are not agreeable to all kinds of plants, most probably is the different quantity of phlogistic matter contained in them, and the different action of the latent fire they contain; for all plants do not require an equal quantity of nourishment: and such as require but little, will be destroyed by having too much. The action of heat also is essentially ne cessary to vegetation; and it is probable that very much of this principle is absorbed from the air by vegetables. But if the air by which plants are partly nourished contains too much of that principle, it is very probable that they may be destroyed from this cause as well as the other; and thus inflammable air, which contains a vast quantity of that active principle, may destroy such plants as grow in a dry soil, though it preserves those which grow in a wet one. See VEGETATION.

Dissemination of PLANTS.-So great are the prolific powers of the vegetable kingdom, that a single plant almost of any kind, if left to itself, would, in a short time,

Overrun

vul. v. p.

overrun the whole world. Indeed, supposing the plant to have been only a single annual, with two seeds, it would, in 20 years, produce more than a million of its own species; what numbers then must have been produced by a plant whose seeds are so numerous as many of those with which we are acquainted? 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 some observation on the means by which they are carried to distant places. This is a very curious matter of fact, and as such we shall now give a fuller account of it.

If nature had appointed no means for the scattering of these numerous seeds, but allowed them to fall down in the place where they grew, the young vegetables must of necessity have choked one another as they grew up, and not a single plant could have arrived at perfection. But so many ways are there appointed for the dissemination of plants, that we see they not only do not hinder each others growth, but a single plant will in a short 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 raised the seeds of vegetables upon stalks, so that the wind has thus an opportunity of acting upon them with the greater advantage. The seed-capsules also open at the apex, lest the ripe seeds should drop out without being widely dispersed by the wind. Others are furnished 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 distance of 50 miles: 138 genera are found to have winged seeds.

2. In some plants the seed-vessels open with violence when the seeds are ripe, and thus throw them to a considerable distance; and we have an enumeration of 50 genera whose seeds are thus dispersed.

3. Other seeds 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 seeds are dispersed by means of birds and other animals; who pick up the berries, and afterwards eject the seeds uninjured. Thus the fox disseminates the privet, and man many species of fruit. The plants found growing upon walls and houses, on the tops of high rocks, &c. are mostly brought there by birds; and it is universally known, that by manuring a field with new dung, innumerable weeds will spring up which did not exist there before: 193 species are reckoned up which may be disseminated in this manner.

5. The growth of other seeds is promoted by animals in a different way. While some are eaten, others are scattered and trodden into the ground by them. The squirrel gnaws the cones of the pine, and many of the seeds fall out. When the loxia eats off their bark, almost his only food, many of their seeds are committed to the earth, or mixed in the morass with moss, where he had retired. The glandularia, when she hides up her nuts, often forgets them, and they strike root. The same is observable of the walnut; mice collect and bury great quantities of them, and being afterwards killed by different animals, the nuts germinate.

6. We are astonished to find mosses, fungi, byssus, and mucor, growing everywhere; but it is for want of

2

reflecting that their seeds are so 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 same mosses in North America and in Europe.

Plant.

7. Seeds are also dispersed by the ocean, and by rivers, "In Lapland (says Linnæus), we see the most evident Amoen. proofs how far rivers contribute to deposit the seeds of Acad. plants. I have seen Alpine plants growing upon their shores frequently 36 miles distant from the Alps; for their seed falling into the rivers, and being carried along and left by the stream, take root there.-We may gather likewise from many circumstances how much the sea furthers this business.-In Roslagia, the island of Græsca, Oeland, Gothland, and the shores of Scania, there are many foreign and German plants not yet naturalized in Sweden. The centaury is a German plant, whose seeds being carried by the wind into the sea, the waves landed this foreigner upon the coasts of Sweden. I was astonished to see the veronica maritima, a German plant, growing at Tornea, which hitherto had been found only in Græsca: the sea was the vehicle by which this plant was transported thither from Germany; or possibly it was brought from Germany to Græsca, and from thence to Tornea. Many have imagined, but erroneously, that seed corrupts in water, and loses its principle of vegetation. Water at the bottom of the sea is seldom warm enough to destroy seeds; we have seen water cover the surface of a field for a whole winter, while the seed which it contained remained unhurt, unless at the beginning of spring the waters were let down so low by drains, that the warmth of the sun-beams reached to the bottom. Then the seeds germinate, but presently become putrescent; so that for the rest of the year the earth remains naked and barren. Rain and showers carry seeds into the cracks of the earth, streams, and rivers; which last, conveying them to a distance from their native places, plant them in a foreign soil."

8. Lastly, some seeds assist their projection to a distance in a very surprising manner. The crupina, a species of centaury, has its seeds covered over with erect bristles, by whose assistance it creeps and moves about in such 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 sleeve or neck-band, travelling over the whole body. If the bearded oat, after harvest, be left with other grain in the barn, it extricates itself from the glume; nor does it stop in its progress till it gets to the walls of the building. Hence, says 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 separate from them; for every oat has a spiral arista or beard annexed to it, which is contracted in wet, and extended in dry weather. When the spiral is contracted, it drags the oat along with. it the arista being bearded with minute hairs pointing downward, the grain necessarily follows it; but when it expands again, the oat does not go back to its former place, the roughness of the beard the contrary way preventing its return. If you take the seeds of equisetum, or fern, these 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 separated and dispersed one from another; so that a person ignorant of

this

this property would pronounce these seeds to be so many mites or small insects.

We cannot finish this article without remarking, that many ingenious men believe that plants have a power of perception. Of this opinion we shall now give an account from the second volume of the Manchester Transactions, where we find some speculations on the perceptive power of vegetables by Dr Percival, who attempts to show, by the several analogies of organization, life, instinct, spontaneity, and self motion, that plants, like animals, are endued with the powers both of perception and enjoyment. The attempt is ingenious, and is ingeniously supported, 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 physiology, almost all the terms employed in the description of plants. But we cannot from thence conclude, that their organization, though it bears an analogy to that of animals, is the sign of a living principle, if to this principle we annex the idea of perception; yet so fully is our author convinced of the truth of it, that he does not think it extravagant to suppose, that, in some future period, perceptivity may be discovered to extend even beyond the limits now assigned to vegetable life. Corallines, madrepores, millepores, and sponges, were formerly considered as fossil bodies: but the experiments of Count Marsigli evinced, that they are endued with life, and led him to class them with the maritime plants. And the observations of Ellis, Jussieu, and Peysonel, have since raised them to the rank of animals. The detection of error in long established opinions concerning one branch of natural knowledge, justifies the suspicion of its existence in others, which are nearly allied to it. And it will appear from the prosecution of our inquiry into the instincts, spontaneity, and self-moving power of vegetables, that the suspicion is not without foundation.

He then goes on to draw a comparison between the instincts of animals and those of vegetables; the calf, as soon as it comes into the world, applies to the teats of the cow; and the duckling, though hatched under a hen, runs to the water.

"Instincts analogous to these (says our author), ope- ant. rate with equal energy on the vegetable tribe. A seed contains a germ, or plant in miniature, and a radicle, or little root, intended by nature to supply it with nourishment. If the seed be sown in an inverted position, still each part pursues its proper direction. The plumula turns upward, and the radicle strikes downward into the ground. A hop-plant, turning round a pole, follows the course of the sun, from south to west, and soon dies, when forced into an opposite line of motion: but remove the obstacle, and the plant will quickly return to its ordinary position. The branches of a honeysuckle shoot out longitudinally, till they become unable to bear their own weight; and then strengthen themselves, by changing their form into a spiral: when they meet with other living branches, of the same kind, they coalesce, for mutual support, and one spiral turns to the right and the other to the left; thus seeking, by an instinctive impulse, some body on which to climb, and increasing the probability of finding one by the diversity of their course: for if the auxiliary branch be dead, the other uniformly winds itself round from the right to the left.

"These examples of the instinctive economy of vegetables have been purposely taken from subjects familiar to our daily observation. But the plants of warmer climates, were we sufficiently acquainted with them, would probably furnish better illustrations of this acknowledged power of animality: and I shall briefly recite the history of a very curious exotic, which has been delivered to us from good authority; and confirmed by the observations of several European botanists."

The doctor then goes on to give a description of the dionæa muscipula (B), for which see vol. vi. p. 32.; and concludes, that if he has furnished any presumptive proof of the instinctive power of vegetables, it will necessarily follow that they are endued with some degree of spontaneity. More fully to evince this, however, the doctor points out a few of those phenomena in the vegetable kingdom which seem to indicate spontaneity.— "Several years ago (says he), whilst engaged in a course of experiments to ascertain the influence of fixed air on vegetation, the following fact repeatedly occurred to me. A sprig of mint, suspended by the root, with the head downwards,