Plant.

Fig. 10. A transverse section of a branch of afh, as it to the eye. Plate appears CCCXCIV

Plate

Fig. 11. The fame fection magnified. AA, the bark. BBB, an arched ring of fap-veffels next the fkin. CCC, the parenchyma of the bark with its cells, and another arched ring of fap-veffels. DD, a circular line of lymph-ducts immediately below the above arched ring. EÊ, the wood. F, the firft year's growth. G, the fecond. H, the third year's growth. III, the true wood. KK, the great air-veffels. LL, the leffer ones. MMM, the parenchymatous infertions of the bark represented by the white rays. NO, the pith, with its bladders or cells.

4. Of the Leaves.] The leaves of plants confift of the fame fubftance with that of the trunk. They are full of nerves or woody portions, running in all directions, and branching out into innumerable fmall threads, interwoven with the parenchyma like fine lace or gauze.

The fkin of the leaf, like that of an animal, is full of pores, which both serve for perspiration and for the abforption of dews, air, &c. Thefe pores or orifices differ both in fhape and magnitude in different plants, which is the cause of that variety of texture or grain peculiar to every plant.

The pulpy or parenchymatous part confifts of very minute fibres, wound up into fmall cells or bladders. Thefe cells are of various fizes in the fame leaf.

All leaves, of whatever figure, have a marginal fibre, by which all the reft are bounded. The particular fhape of this fibre determines the figure of the leaf.

The veffels of leaves have the appearance of inofculating; but, when examined by the microfcrope, they are found only to be interwoven or laid along each other.

What are called air-vessels, or those which carry no fap, are vifible even to the naked eye in fome leaves. When a leaf is flowly broke, they appear like small woolly fibres, connected to both ends of the broken piece.

Fig. 14. The appearance of the air-veffels to the eye, CCCXCV. in a vine-leaf drawn gently asunder.

Fig. 15. A fmall piece cut off that leaf. Fig. 16. The fame piece magnified, in which the veffels have the appearance of a screw.

Fig. 17. The appearance of these veffels as they exift in the leaf before they are ftretched out.

5. Of the Flower.] It is needlefs here to mention any thing of the texture, or of the veffels, &c. of flowers, as they are pretty similar to thofe of the leaf. It would be foreign to our prefent purpose to take any notice of the characters and diftinctions of flowers. These belong to the science of BOTANY, to which the reader is referred.

There is one curious fact, however, which muft not be omitted, viz. That every flower is perfectly formed in its parts many months before it appears outwardly; that is, the flowers which appear this year are not properly speaking the flowers of this year, but of the laft. For example, mezereon generally flowers in January; but these flowers were completely formed in the month of Auguft preceding. Of this fact any one may fatisfy himfelf by feparating the coats of a tulip-root about the beginning of September; and he will find that the two innermoft form a kind of cell, in the centre of which

stands the young flower, which is not to make its ap- Plant. pearance till the following April or May. Fig. 18. exhibits a view of the tulip-root when diffected in September, with the young flower towards the bottom. 6. Of the Fruit.] In defcribing the ftructure of fruits, a few examples fhall be taken from fuch as are most generally known.

A pear, befides the fkin, which is a production of the fkin of the bark, confifts of a double parenchyma or pulp, fap, and air-veffels, calculary and acetary.

The outer parenchyma is the fame fubftance continued from the bark, only its bladders are larger and more fucculent.

It is everywhere interfperfed with fmall globules or grains, and the bladders refpect these grains as a kind of centres, every grain being the centre of a number of bladders. The fap and air-veffels in this pulp are extremely small.

Next the core is the inner pulp or parenchyma, which confifts of bladders of the fame kind with the outer, only larger and more oblong, correfponding to those of the pulp, from which it feems to be derived. This inner pulp is much fourer than the other, and has none of the fmall grains interfperfed through it; and hence it has got the name of acetary.

Between the acetary and outer pulp, the globules or grains begin to grow larger, and gradually unite into a hard ftony body, efpecially towards the corculum or ftool of the fruit; and from this circumftance it has been called the calculary.

Thefe grains are not derived from any of the organical parts of the tree; but feem rather to be a kind of concretions precipitated from the fap, fimilar to the precipitation from wine, urine, and other liquors.

The core is a roundish cavity in the centre of the pear, lined with a hard woody membrane, in which the feed is inclosed. At the bottom of the core there is a fmall duct or canal, which runs up to the top of the pear; this canal allows the air to get into the core, for the purpofe of drying and ripening the feeds.

Plate

Fig. 19. a tranfverfe fection of a pear, as it appears to the naked eye. A, the skin, and a ring of fap-vef-CCCXCVI fels. B, the outer parenchyma, or pulp, with its veffels, and ligneous fibres interspersed. C, the inner parenchyma, or acetary, with its veffels, which are larger than the outer one. D, the core and feeds.

Fig. 20. a piece cut off fig. 19.

Fig. 21. is fig. 20. magnified. A A A, the smalf grains or globules, with the veffels radiated from them.

Fig. 22. a longitudinal fection of the pear, fhowing a different view of the fame parts with thofe of fig. 19. A the channel, or duct, which runs from the top of the pear to the bottom of the core.

In a lemon, the parenchyma appears in three different forms. The parenchyma of the rind is of a coarfe texture, being composed of thick fibres, woven into large bladders. Thofe nearest the furface contain the effen tial oil of the fruit, which bursts into a flame when the fkin is fqueezed over a candle. From this outmost parenchyma nine or ten infertions or lamellæ are produced, which run between as many portions of the pulp, and unite into one body in the centre of the fruit, which correfponds to the pith in trunks or roots. At the bottom and top of the lemon, this pith evidently joins with the rind, without the intervention of any lamellæ. This

circumstance

Plant. circumftance fhows, that the pith and bark are actually connected in the trunk and roots of plants, though it is difficult to demonftrate the connection, on account of the closeness of their texture, and the minutenefs of their fibres. Many veffels are difperfed through the whole of this parenchyma; but the largest ones ftand on the inner edge of the rind, and the outer edge of the pith, just at the two extremities of each lamella. The fecond kind of parenchyma is placed between the rind and the pith; is divided into diftinct bodies by the lamella; and each of thefe bodies forms a large bag.

Plate

Thefe bags contain a third parenchyma, which is a cluster of smaller bags, diftinct and unconnected with each other, having a fmall ftalk by which they are fixed to the large bag. Within each of these fmall bags are many hundreds of bladders, compofed of extreme ly minute fibres. Thefe bladders contain the acid juice

of the lemon. Fig. 12. a longitudinal fection of a lemon. A AA, CCCXCV. the rind with the veffels which contain the effential oil. B B, the fubftance correfponding to the pith, formed by the union of the lamella or infertions. CC, its continuation and connection with the rind, independent of the infertions.

Fig. 13. a tranverfe fection of the lemon. B B B, &c. the nine pulpy bags, or fecond parenchyma, placed between the rind and the pith; and the cluster of fmall bags, which contain the acid juice, inclofed in the large ones. CC, the large veffels that furround the pith. D D, two of the large bags laid open, fhowing the feeds, and their connection with the lamelle or membranes which form the large bags.

Of the Perfpiration of PLANTS, and the quantity of moiffure daily imbibed by them.-Thefe curious particulars have been determined with great accuracy by Dr Hales. The method he took to accomplish his purpofe was as follows. In the month of July, commonly the warmest feason of the year, he took a large fun-flower three feet and an half high, which had been purpofely planted in a flower-pot when young. He covered the pot with thin milled lead, leaving only a fmall hole to preferve a communication with the external air, and another by which he might occafionally fupply the plant with water. Into the former he inferted a glafs tube nine inches long, and another shorter tube into the hole by which he poured in the water; and the latter was kept elofe ftopped with a cork, except when there was occafion to use it. The holes in the bottom of the pot were alfo flopped up with corks, and all the crevices fhut with cement. Things being thus prepared, the pot and plant were weighed for 15 feveral days; after which the plant was cut off close to the leaden plate, and the Aump well covered with cement. By weighing, he found that there perspired through the unglazed porous pot two ounces every 12 hours; which being allowed for in the daily weighing of the plant and pot, the greatest perspiration, in a warm day, was found to be one pound 14 ounces; the middle rate of perfpiration,, one pound four ounces; the perfpiration of a dry warm night, without any fenfible dew, was about three ounces; but when there was any fenfible though small dew, the perfpiration was nothing; and when there was a large dew, or fome little rain in the night,

the plant and pot was increased in weight two or three Plant. ounces. In order to know what quantity was perfpired from a fquare inch of furface, our author cut off all the leaves of the plant, and laid them in five feveral parcels, according to their feveral fizes; and then meafured the furface of a leaf of each parcel, by laying over it a large lattice made with threads, in which each of the little fquares were of an inch; by numbering of which, he had the furface of the leaves in square inches; which, multiplied by the number of leaves in the correfponding parcels, gave the area of all the leaves. By this method he found the furface of the whole plant above ground to be 5616 fquare inches, or 39 fquare feet. He dug up another fun-flower of nearly the fame fize, which had eight main roots, reaching 15 inches deep and fidewife, from the ftem. It had befides a very thick bush of lateral roots from the eight main roots, extending every way in a hemifphere about nine inches from the ftem and main roots. In order to eftimate the length of all the roots, he took one of the main roots with its laterals, and measured and weighed them; and then weighed the other feven with their laterals; by which means he found the fum of all their lengths to be 1448 feet. Suppofing then the periphery of these roots at a medium to be 0.31 of an inch, then their furface will be 2276 square inches, or 15.8 fquare feet; that is, equal to 0.4 of the furface of the plant above ground. From calculations drawn from thefe obfervations, it appears, that à fquare inch of the upper furface of this plant perfpires

part of an inch in a day and a night; and that a fquare inch of the furface underground imbibed of an inch in the fame time.

The quantity perfpired by different plants, however,. is by no means equal. A vine-leaf perfpires only TT of an inch in 12 hours; a cabbage perfpires of an inch in the fame time; an apple-tree in 12 hours; and a lemon in 12 hours.

Of the circulation of the Sap in PLANTS.- Concerning this there have been great difputes;. fome maintaining, that the vegetable fap has a circulation analogous to the blood of animals; while others affirm, that it only afcends in the day-time, and defcends again in the night. In favour of the doctrine of circulation it has been urged, that upon making a tranfverse incifion into the trunk of a tree, the juice which runs out proceeds in greater quantity from the upper than the lower part; and the fwelling in the upper lip is alfo much greater than in the lower. It appears, however, that when two fimilar incifions are made, one near the top and the other near the root, the latter expends much more fapthan the former. Hence it is concluded, that the juiceafcends by one fet of veffels and defcends by another. But, in order to fhow this clearly, it would be neceffary first to prove that there is in plants, as in animals,. fome kind of eentre from which the circulation begins,. and to which it returns; but no fuch centre has been difcovered by any naturalift; neither is there the least provifion apparently made by nature whereby the fap might be prevented from defcending. in the very fame veffels through which it ascends. In the lacteal veffels of animals, which we may fuppofe to be analogous to the roots of vegetables, there are valves which effectual

ly

'Plant.

ly prevent the chyle when once abforbed from returning into the inteftines; but no fuch thing is obferved in the veffels of vegetables: whence it must be very probable, that when the propelling force ceafes, the juice defcends by the very fame veffels through which it ascended.This matter, however, has been cleared up almost as well as the nature of the subject will admit of by the Vegetable experiments of Dr Hales ‡. Thefe experiments are fo Statics, vol. numerous, that for a particular account of them we muft i. p. 142. refer to the work itfelf; however, his reafoning against the circulation of the fap will be fufficiently intelligible without them. "We fee (fays he), in many of the foregoing experiments, what quantities of moisture trees daily imbibe and perfpire: now the celerity of the fap must be very great, if that quantity of moisture muft, most of it, afcend to the top of the tree, then defcend, and afcend again, before it is carried off by perspiration.

"The defect of a circulation in vegetables feems in fome measure to be fupplied by the much greater quantity of liquor, which the vegetable takes in, than the animal, whereby its motion is accelerated; for we find the fun-flower, bulk for bulk, imbibes and perfpires 17 times more fresh liquor than a man, every 24 hours. "Befides, Nature's great aim in vegetables being only that the vegetable life be carried on and maintained, there was no occafion to give its fap the rapid motion which was neceflary for the blood of animals.

"In animals, it is the heart which fets the blood in motion, and makes it continually circulate; but in vegetables we can discover no other cause of the fap's motion but the ftrong attraction of the capillary fap veffels, affifted by the brisk undulations and vibrations caufed by the fun's warmth, whereby the fap is carried up to the top of the tallest trees, and is there perfpired off through the leaves: but when the furface of the tree is greatly diminished by the lofs of its leaves, then alfo the perfpiration and motion of the fap is proportionably diminished, as is plain from many of the foregoing experiments: fo that the afcending velocity of the fap is principally accelerated by the plentiful perfpiration of the leaves, thereby making room for the fine capillary veffels to exert their vaftly attracting power, which perfpiration is effected by the brifk rare fying vibrations of warmth; a power that does not feem to be any ways well adapted to make the fap defcend from the tops of vegetables by different veffels to the root.

"If the fap circulated, it must needs have been feen defcending from the upper part of large gafhes cut in branches fet in water, and with columns of water preffing on their bottoms in long glafs tubes. In both which cafes, it is certain that great quantities of water paffed through the ftem, fo that it muft needs have been feen defcending, if the return of the fap downwards were by trufion or pulfion, whereby the blood in animals is returned through the veins to the heart; and that pulfion, if there were any, muft neceffarily be exerted with prodigious force, to be able to drive the fap through the finer capillaries. So that, if there be a return of the fap downwards, it must be by attraction, and that a very powerful one, as we may fee by many of thefe experiments. But it is hard to conceive what and where that power is which can be equivalent to that provision nature has made for the af

cent of the fap in confequence of the great perfpira- Plant. tion of the leaves. "The inftances of the jeffamine-tree, and of the paffion-tree, have been looked upon as strong proofs of the circulation of the fap, because their branches, which were far below the inoculated bud, were gilded: but we have many visible proofs in the vine, and other bleeding trees, of the sap's receding back, and pushing forwards alternately, at different times of the day and night. And there is great reason to think that the fap of all other trees has fuch an alternate, receding, and progreffive motion, occafioned by the alternacies of day and night, warm and cool, moist and dry.

"For the fap in all vegetables does probably recede in fome measure from the tops of the branches, as the fun leaves them; because its rarefying power then ceafing, the greatly rarefied fap, and air mixed with it, will condenfe, and take up lefs room than they did, and the dew and rain will then be ftrongly imbibed by the leaves; whereby the body and branches of the vegetable which have been much exhaufted by the great evaporation of the day, may at night imbibe fap and dew from the leaves; for by feveral experiments, plants were found to increase confiderably in weight, in dewy and moift 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 perspi ration of the leaves, except in the bleeding season; but when at night that perfpiring power ceafes, then the contrary imbibing power will prevail, and draw the fap 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 ftems of feveral trees which do not bleed, whereby it is found that they are always in a ftrongly imbibing ftate, by drawing up the mercury feveral inches: whence it is eafy to conceive, how fome of the particles of the gilded bud in the inoculated jeffamine may be abforbed by it, and thereby communicate their gilding miafma to the fap of other branches; efpecially when, fome months after the inoculation, the flock of the inoculated jeffamine is cut off a little above the bud; whereby the ftock, which was the counteracting part to the ftem, being taken away, the ftem attracts more vigorously from the bud.

"Another argument for the circulation of the fap is, that fome forts of the graffs will infect and canker the ftocks they are grafted on: but by mercurial gages fixed to fresh-cut ftems of trees, it is evident that those ftems were in a strongly imbibing ftate; and confequently the cankered flocks might very likely draw fap from the graff, as well as the graff alternately from the ftock; juft in the fame manner as leaves and branches do from each other, in the viciffitudes of day and night. And this imbibing power of the stock is fo great, where only fome of the branches of a tree are grafted, that the remaining branches of the ftock will, by their strong attraction, ftarve those graffs; for which reafon it is ufual to cut off the greatest part of the branches of the ftock, leaving only a few small ones to draw up the fap.

"The inftance of the ilex grafted upon the English oak, feems to afford a very confiderable argument against a circulation. For, if there were a free uni