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Fig.21.

PEAR CUT LONGITUDINALLY.

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Plant.

Plate CCCCXXL fig. 11.

Fig. 12.

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The principal body of the bark is composed of pulp or parenchyma, and innumerable vessels much larger than those of the skin. The texture of the pulpy part, though the same substance with the parenchyma in roots, yet seldom appears in the form of rays running towards the pith; and when these rays do appear, they do not extend above half-way to the circumference. The vessels of the bark are very differently situated, and destined for various purposes in different plants. For example, in the bark of the pine, the innermost are lymph-ducts, and exceedingly small; the outermost are gum or resiniferous vessels, destined for the secretion of turpentine; and are so large as to be distinctly visible to the naked eye.

The wood lies between the bark and pith, and consists of two parts, viz. a parenchymatous and a ligneous. In all trees, the parenchymatous part of the wood, though much diversified as to size and consistence, is uniformly disposed in diametrical rays, or insertions running betwixt similar rays of the ligneous part.

The true wood is nothing but a congeries of old dried lymph-ducts. Between the bark and the wood a new ring of these ducts is formed every year, which gradually loses its softness as the cold season approaches, and towards the middle of winter is condensed into a solid ring of wood. These annual rings, which are distinctly visible in most trees when cut through, serve as natural marks to distinguish their age (fig. 11, 12.). The rings of one year are sometimes larger, sometimes less, than those of another, probably owing to the favourableness or unfavourableness of the season.

The pith, though of a different texture, is exactly of the same substance with the parenchyma of the bark, and the insertions of the wood. The quantity of pith is various in different plants. Instead of being increased every year like the wood, it is annually diminished, its vessels drying up, and assuming the appearance and structure of wood; insomuch that in old trees there is scarce such a thing as pith to be discerned.

A ring of sap-vessels is usually placed at the outer edge of the pith next the wood. In the pine, fig, and walnut, they are very large. The parenchyma of the pith is composed of small cells or bladders, of the same kind with those of the bark, only of a larger size. The general figure of these bladders is circular; though in some plants, as the thistle and borage, they are angular. Though the pith is originally one connected chain of bladders, yet as the plant grows old they shrivel, and open in different directions. In the walnut, after a certain age, it appears in the form of a regular transverse hollow division. In some plants it is altogether wanting; in others, as the sonchus, nettle, &c. there is only a transverse partition of it at every joint. Many other varieties might be mentioned; but these must be left to the observation of the reader.

Fig. 11. A transverse section of a branch of ash, as it appears to the eye.

Fig. 12. The same section magnified. AA, the bark. BBB, an arched ring of sap-vessels next the skin. CCC, the parenchyma of the bark with its cells, VOL. XVI. Part II.

and another arched ring of sap-vessels. DD, a circular line of lymph-ducts immediately below the above arched ring. EE, the wood. F, the first year's growth. G, the second. H, the third year's growth. III, the true wood. KK, the great air-vessels. LL, the lesser ones. MMM, the parenchymatous insertions of the bark represented by the white rays. NO, the pith, with its bladders or cells.

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

The skin of the leaf, like that of an animal, is full of pores, which both serve for perspiration and for the absorption of dews, air, &c. These pores or orifices differ both in shape 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 consists of very minute fibres, wound up into small cells or bladders. These cells are of various sizes in the same leaf.

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

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

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

Plant.

Plate CCCCXXI.

Fig. 13. The appearance of the air-vessels to the eye, fig. 13. in a vine leaf drawn gently asunder. Fig. 14.

Fig. 14. A small piece cut off that leaf. Fig. 15. The same piece magnified, in which the Fig. 15. vessels have the appearance of a screw.

Fig. 16. The appearance of these vessels as they ex- Fig. 16. ist in the leaf before they are stretched out.

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

There is one curious fact, however, which must 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 last. For example, mezereon generally flowers in January; but these flowers were completely formed in the month of August preceding. Of this fact any one may satisfy himself by separating the coats of a tulip-root about the beginning of September; and he will find that the two innermost form a kind of cell, in the centre of which stands the young flower, which is not to make its appearance till the following April or May. Fig. 17. ex- Fig. 17. hibits a view of the tulip-root when dissected in September, with the young flower towards the bottom. 6. Of the Fruit.] In describing the structure of fruits, 4 G

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Plant.

Fig. 18.

Fig. 19. Fig. 20.

Fig. 21.

a few examples shall be taken from such as are most generally known.

A pear, besides the skin, which is a production of the skin of the bark, consists of a double parenchyma or pulp, sap, and air-vessels, calculary and acetary.

The outer parenchyma is the same substance continued from the bark, only its bladders are larger and more succulent.'

It is everywhere interspersed with small globules or grains, and the bladders respect these grains as a kind of centres, every grain being the centre of a number of bladders. The sap and air-vessels in this pulp are extremely small.

Next the core in the inner pulp or parenchyma, which consists of bladders of the same kind with the outer, only larger and more oblong, corresponding to those of the pulp, from which it seems to be derived. This inner pulp is much sourer than the other, and has none of the small grains interspersed 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 stony body, especially towards the corculum or stool of the fruit; and from this circumstance it has been called the calculary.

These grains are not derived from any of the organical parts of the tree; but seem rather to be a kind of concretions precipitated from the sap, similar 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 seed is inclosed. At the bottom of the core there is a small duct or canal, which runs up to the top of the pear; this canal allows the air to get into the core, for the purpose of drying and ripening the seeds.

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Fig. 18. a transverse section of a pear, as it to the naked eye. A, the skin, and a ring of sap-vessels. B, the outer parenchyma, or pulp, with its vessels, and ligneous fibres interspersed. C, the inner parenchyma, or acetary, with its vessels, which are larger than the outer one. D, the core and seeds.

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

Fig. 20. is fig. 19. magnified. AA A, the small grains, or globules, with the vessels radiated from them. Fig. 21. a longitudinal section of the pear, showing a different view of the same parts with those of fig. 18. 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 coarse texture, being composed of thick fibres, woven into large bladders. Those nearest the surface contain the essential oil of the fruit, which bursts into a flame when the skin is squeezed over a candle. From this outmost parenchyma nine or ten insertions 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 corresponds 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 shows, that the pith and bark are actually connected in the trunk and roots of plants, though it is difficult to demonstrate the connection, on account of the closeness of their texture, and the minuteness of their fibres. Many vessels are dispersed through the whole of

this parenchyma; but the largest ones stand on the in- Plant ner edge of the rind, and the outer edge of the pith, just' at the two extremities of each lamella.

The second kind of parenchyma is placed between the rind and the pith; is divided into distinct bodies by the lamella; and each of these bodies forms a large bag.

These bags contain a third parenchyma, which is a cluster of smaller bags, distinct and unconnected with each other, having a small stalk by which they are fixed to the large bag. Within each of these small bags are many hundreds of bladders, composed of extremely minute fibres. These bladders contain the acid juice of the lemon.

Fig. 22. a longitudinal section of a lemon. A A A, Fig. 22. the rind with the vessels which contain the essential oil. B B, the substance corresponding to the pith, formed by the union of the lamellæ or insertions. CC, its continuation and connection with the rind, independent of the insertions.

Fig. 23. a transverse section of the lemon. BBB, Fig. &c. the nine pulpy bags, or second parenchyma, placed between the rind and the pith; and the cluster of small bags, which contain the acid juice, inclosed in the large ones. CC, the large vessels that surround the pith. DD, two of the large bags laid open, showing the seeds, and their connection with the lamella or membranes which form the large bags.

Of the Perspiration of PLANTS, and the quantity of moisture daily imbibed by them.-These curious particulars have been determined with great accuracy by Dr Hales. The method he took to accomplish his purpose was as follows. In the month of July, commonly the warmest season of the year, he took a large sun-flower three feet and a half high, which had been purposely planted in a flower-pot when young. He covered the pot with thin milled lead, leaving only a small hole to preserve a communication with the external air, and another by which he might occasionally supply the plant with wa ter. Into the former he inserted a glass tube nine inches long, and another shorter tube into the hole by which he poured in the water; and the latter was kept close stopped with a cork, except when there was occasion to use it. The holes in the bottom of the pot were also stopped up with corks, and all the crevices shut with cement.Things being thus prepared, the pot and plant were weighed for 15 several days; after which the plant was cut off close to the leaden plate, and the stump 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 perspiration, one pound four ounces; the perspiration of a dry warm night, without any sensible dew, was about three ounces; but when there was any sensible though small dew, the perspiration was nothing; and when there was a large dew, or some little rain in the night, the plant and pot was increased in weight two or three ounces.

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In order to know what quantity was perspired from a square inch of surface, our author cut off all the leaves of the plant, and laid them in five several parcels, cording to their several sizes; and then measured the surface of a leaf of each parcel, by laying over it a large

lattice

the work itself; however, his reasoning against the cir- Plant.
culation of the sap will be sufficiently intelligible with-
out them. "We see (says he), in many of the forego-
ing experiments, what quantities of moisture trees daily
imbibe and perspire: now the celerity of the sap must
be very great, if that quantity of moisture must, most
of it, ascend to the top of the tree, then descend, and
ascend again, before it is carried off by perspiration.

"The defect of a circulation in vegetables seems in some measure to be supplied by the much greater quantity of liquor, which the vegetable takes in, than the. animal, whereby its motion is accelerated; for we find the sun-flower, bulk for bulk, imbibes and perspires 17 times more fresh liquor than a man, every 24 hours.

Plant. lattice made with threads, in which each of the little squares was of an inch; by numbering of which, he had the surface of the leaves in square inches; which, multiplied by the number of leaves in the corresponding parcels, gave the area of all the leaves. By this method he found the surface of the whole plant above ground to be 5616 square inches, or 39 square feet. He dug up another sun-flower of nearly the same size, which had eight main roots, reaching 15 inches deep and sidewise, from the stem. It had besides a very thick bush of lateral roots from the eight main roots, extending every way in a hemisphere about nine inches from the stem and main roots. In order to estimate the length of all the roots, he took one of the main roots with its laterals, and measured and weighed them; and then weighed "Besides, Nature's great aim in vegetables being the other seven with their laterals; by which means he only that the vegetable life be carried on and maintainfound the sum of all their lengths to be 1448 feet. ed, there was no occasion to give its sap the rapid moSupposing then the periphery of these roots at a medium tion, which was necessary for the blood of animals. to be 0.131 of an inch, then their surface will be 2276 "In animals, it is the heart which sets the blood in square inches, or 15.8 square feet; that is, equal to 0.4 motion, and makes it continually circulate; but in veof the surface of the plant above ground. From calcula- getables we can discover no other cause of the sap's tions drawn from these observations, it appears, that a motion but the strong attraction of the capillary sapsquare inch of the upper surface of this plant perspires vessels, assisted by the brisk undulations and vibrations Y part of an inch in a day and a night; and that a caused by the sun's warmth, whereby the sap is carried square inch of the surface underground imbibed of up to the top of the tallest trees, and is there perspired an inch in the same time. off through the leaves: but when the surface of the tree is greatly diminished by the loss of its leaves, then alsothe perspiration and motion of the sap is proportionably diminished, as is plain from many of the foregoing experiments: so that the ascending velocity of the sap is principally accelerated by the plentiful perspiration of the leaves, thereby making room for the fine capillary vessels to exert their vastly attracting power, which perspiration is effected by the brisk rarefying vibrations of warmth; a power that does not seem to be any ways well adapted to make the sap descend from the tops of vegetables by different vessels to the root.

+Vegetable Statics, vol. p. 142.

The quantity perspired by different plants, however,
is by no means equal. A vine-leaf perspires only
of an inch in 12 hours; a cabbage perspires of an
juch in the same 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 disputes; some maintaining,
that the vegetable sap has a circulation analogous to the
blood of animals; while others affirm, that it only as-
cends in the day-time, and descends again in the night.
In favour of the doctrine of circulation it has been urged,
that upon making a transverse incision into the trunk of
a tree, the juice which runs out proceeds in greater
quantity from the upper than the lower part; and the
swelling in the upper lip is also much greater than in
the lower. It appears, however, that when two similar
incisions are made, one near the top and the other near
the root, the latter expends much more sap than the
former. Hence it is concluded, that the juice ascends
by one set of vessels and descends by another. But, in
order to show this clearly, it would be necessary first to
prove that there is in plants, as in animals, some kind
of centre from which the circulation begins, and to
which it returns; but no such centre has been discovered
by any naturalist; neither is there the least provision ap-
parently made by nature whereby the sap might be pre-
vented from descending in the very same vessels through
which it ascends. In the lacteal vessels of animals,
which we may suppose to be analogous to the roots of
vegetables, there are valves which effectually prevent
the chyle when once absorbed from returning into the
intestines; but no such thing is observed in the vessels
of vegetables; whence it must be very probable, that
when the propelling force ceases, the juice descends by
the very same vessels 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 experiments
of Dr Hales+. These experiments are so numerous,
that for a particular account of them we must refer to

"If the sapcirculated, it must needs have been seen descending from the upper part of large gashes cut in branches set in water, and with columns of water pressing on their bottoms in long glass tubes. In both which cases it is certain that great quantities of water passed through the stem, so that it must needs have been seen descending, if the return of the sap downwards were by trusion or pulsion, whereby the blood in animals is returned through the veins tot he heart; and that pulsion, if there were any, must necessarily be exerted with prodigious force, to be able to drive the sap through the finer capillaries. So that, if there be a return of the sap downwards, it must be by attraction, and that a very powerful one, as we may see by many of these 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 ascent of the sap in consequence of the great perspiration of the leaves.

"The instances of the jessamine-tree, and of the passion tree, have been looked upon as strong proofs of the circulation of the sap, 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 sap of all other trees, has such an alternate, receding, and progressive motion occasioned by the alternacies of day and night, warm and cool, moist and dry. 4 G 2

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