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Plant. The skin of the bark is composed of very minute and another arched ring of sap.vessels. DD, a circu. Plant.
bladders, interspersed with longitudinal woody fibres, lar line of lymph-ducts immediately below the above and as in the nettle, thistle, and most herbs. The oustide arched ring
EE, the wood. F, the first year's
III, the true wood. KK, the great air vessels. LL,
which is the cause of that variety of texture or grain
These cells are of various sizes in the same leaf.
by which all the rest are bounded. The particular shape
Plate those of another, probably owing to the favourableness piece.
CCCCXX). or unfavourableness of the season.
Fig. 13. The appearance of the air-vessels to the eye,
thing of the texture, or of the vessels, &c. of flowers,
. varieties might be mentioned; but these must be left to beginning of September; and he will find that the two
innermost form a kind of cell, in the centre of which fig. II.
Fig. 1. A transverse section of a branch of ash, as stands the young flower, which is not to make its apit appears to the eye.
pearance till the following April or May. Fig. 17. ex- Fig. 17. Fig. 12. Fig. 12. The same section magnified. AA, the hibits a view of the tulip-root when dissected in Sep
bark. BBB, an arched ring of sap-vessels next the tember, with the young flower towards the bot!om.
a few examples shall be taken from such as are most ge this parenchyma; but the largest ones stand on the in- Platit perally known.
ner edge of the rind, and the outer edge of the pith, just A pear, besides the skin, which is a production of the at the two extremities of each lamella. skin of the bark, consists of a double parenchyma or The second kind of parenchyma is placed between pulp, sap, and air-vessels, calculary and acetary. the rind and the pith ; is divided into distinct bodies
The outer parenchyma is the same substance continu- by the lamellæ ; and each of these bodies forms a large ed from the bark, only its bladders are larger and more bag. succulent,
These bags contain a third parenchyma, which is It is everywhere interspersed with small globules or a cluster of smaller bags, distinct and unconnected with grains, and the bladders respect these grains as a kind of each other, having a small stalk by which they are fixed centres, every grain being the centre of a number of to the large bag. Within each of these small bags are bladders. The sap and air-vessels in this pulp are ex many hundreds of bladders, composed of extremely mitremely small.
nute fibres. These bladders contain the acid juice of the Next the core in the inner pulp or parenchyma, which lemon. consists of bladders of the same kind with the outer, only Fig. 22. a longitudinal section of a lemon. A A A, Fig. :: larger and more oblong, corresponding to those of the the rind with the vessels which contain the essential oil.
pulp, from which it seems to be derived. This inner B B, the substance corresponding to the pith, formed pulp is much sourer than the other, and has none of the by the union of the lamellæ or insertions. C C, its small grains interspersed through it; and hence it has continuation and connection with the rind, independent got the name of acetary.
of the insertions. Between the acetary and outer pulp, the globules or Fig. 23. a transverse section of the lemon. B B B, Fig. grains begin to grow larger, and gradually unite into a &c. the nine pulpy bags, or second parenchyma, placed hard stony body, especially towards the corculum or stool between the rind and the pith; and the cluster of small of the fruit; and from this circumstance it has been call- bags, which contain the acid juice, inclosed in the large ed the calculary.
ones. CC, the large vessels that surround the pith. These grains are not derived from any of the organi- D D, two of the large bags laid open, showing the cal parts of the tree; but seem rather to be a kind of seeds, and their connection with the lamellæ or menconcretions precipitated from the sap, similar to the pre branes which form the large bags. cipitation from wine, urine, and other liquors.
of the Perspiration of Plants, and the quantity of The core is a roundish cavity in the centre of the pear, moisture daily imbibed by then. These curious particuJined with a lard woody membrane, in which the seed lars have been determined with great accuracy by Dr is inclosed. At the bottom of the core there is a small Hales. The method he took to accomplish his purpose duct or canal, which runs up to the top of the pear ; was as follows. In the month of July, commonly the this canal allows the air to get into the core, for the warmest season of the year, he took a large sunflower purpose of drying and ripening the seeds.
three feet and a half high, which had been purposely plantFig. 18. a transverse section of a pear, as it appears ed in a flower-pot when young. He covered the pot with to the naked eye. A, the skin, and a ring of
sap-ves thin milled lead, leaving only a small bole to preserve ? sels. B, the outer parenchyma, or pulp, with its ves communication with the external air, and another by sels, and ligneous fibres interspersed. C, the inner pa which he might occasionally supply the plant with warenchyma, or acetary, with its vessels, which are larger ter. Into the former he inserted a glass tube nine inches tban the outer one. D, the core and seeds.
long, and another shorter tube into the hole by which he Fig. 19. a piece cut off, fig. 19.
poured in the water; and the latter was kept close stopFig. 20. is fig. 19. magnified. A A A, the small
A A A, the small ped with a cork, except when there was occasion to use grains, or globules, with the vessels radiated from them. it. The holes in the bottom of the pot were also stopped
Fig. 21. a longitudinal section of the pear, showing up with corks, and all the crevices shut with cement.a different view of the same parts with those of fig. 18. Things being thus prepared, the pot and plant were A the channel, or duct, which runs from the top of the weighed for 15 several days; after which the plant was pear to the bottom of the core.
cut off close to the leaden plate, and the stump well coIn a lemon, the parenchyma appears in three different vered with cement. By weighing, he found that there forms. The parenchyma of the rind is of a coarse tex perspired through the unglazed porous pot two ounces ture, being composed of thick fibres, woven into large every 12 hours; which being allowed for in the daily bladders. Those nearest the surface contain the essen-weighing of the plant and pot, the greatest perspiration, tial oil of the fruit, which bursts into a flame when the in a warm day, was found to be one pound 14 ounces ; skin is squeezed over a candle. From this outmost pa the middle rate of perspiration, one pound four ounces ; renchyma nine or ten insertions or lamellæ are produ- the perspiration of a dry warm night, without any senced, which run between as many portions of the pulp, sible dew, was about three ounces; but when there was and unite into one body in the centre of the fruit, which
sensible though small dew, the perspiration was nocorresponds to the pith' in trunks or roots. At the bot- thing; and when there was a large dew, or some little ton and top of the lemon, this pith evidently joins with rain in the night, the plant and pot was increased in the rind, without the intervention of any lamellæ. This weight two or three ounces. circumstance shows, that the pith and burk are actually In order to know what quantity was perspired from a connected in the trunk and roots of plants, though it is square inch of surface, our author cut off all the leares difficult to demonstrate the connection, on account of the of the plant, and laid them in five several parcels, accloseness of their texture, and the minuteness of their cording to their several sizes; and then measured the surfibres. Many vessels are dispersed through the whole of face of a leaf of each parcel, by laying over it a la se
Fig. 19. Fig. 20.
Plant. lattice made with threads, in which each of the little the work itself; however, his reasoning against the cir- Plan.
squares was ; of an inch í by numbering of which, he culation of the sap will be sufficiently intelligible with
off through the leaves : but when the surface of the tree 'The quantity perspired by different plants, however, is greatly diminished by the loss of its leaves, tben also is by no means equal. A vine-leaf perspires only yor the perspiration and motion of the sap is proportionably of an inch in 12 hours ; a cabbage perspires yo of an diminished, as is plain from many of the foregoing exinch in the same time; an apple-tree roin 12 hours; periments : so that the ascending velocity of the sap is. &nd a lemon tig in 12 hours.
principally accelerated by the plentiful perspiration of Of the circulation of the Sap in Plants.-Concerning the leaves, thereby making room for the fine capillary this there have been great disputes; some maintaining, vessels to exert their vastly attracting power, which that the vegetable sap has a circulation analogous to the perspiration is effected by the brisk rarefying vibrations: blood of animals; while others affirm, that it only as of warmth ; a power that does not seem to be any ways cends in the day-time, and descends again in the night. well adapted to make the sap descend from the tops of In favour of the doctrine of circulation it has been urged, vegetables by different vessels to the root. that upon making a transverse incision into the trunk of “If thesap circulated, it must needs have been seen dea tree, the juice which runs out proceeds in greater scending from the upper part of large gashes cut in branquantity from the upper than the lower part; and the ches set in water, and with columns of water pressing on swelling in the upper lip is also much greater than in their bottoms in long glass tubes. In both which cases the lower. It appears, however, that when two similar it is certain that great quantities of water passed through incisions are made, one near the top and the other near the stem, so that it must needs have been seen descending, the root, the latter expends much more sap than the if the return of the sap downwards were by trusion or pulformer. Hence it is.concluded, that the juice ascends sion, whereby the blood iu animals is returned through the by one set of vessels and descends by another. But, in veins tot he heart; and that pulsion, if there were any, must order to show this clearly, it would be necessary first to necessarily be exerted with prodigious force, to be able to prove that there is in plants, as in animals, some kind drive the sap through the finer capillaries. So that, if of centre from which the circulation begins, and to there be a return of the sap downwards, it must be by atwhich it returns; but no such centre bas been discovered traction, and that a very powerful one, as we may see by by any naturalist; neither is there the least provision ap- many of these experiments. But it is hard to conceive parently made by nature whereby the sap might be pre. what and where that power is which can be equivalent vented from descending in the very same vessels through to that provision nature has made for the ascent of the sap which it ascends. In the lacteal vessels of animals, in consequence of the great perspiration of the leaves. which we may suppose to be analogous to the roots of “ The instances of the jessamine-tree, and of the pasvegetables, there are valves which effectually preventsion tree, have been looked upon as strong proofs of the the chyle when once absorbed from returning into the circulation of the sap, because their branches, which intestines; but no such thing is observed in the vessels were far below the inoculated bud, were gilded : hut we of vegetables ; whence it must be very probable, that have many visible proofs in the vine, and other bleeding when the propelling force ceases, the juice descends by trees, of the sap's receding back, and pushing forwards. the very same vessels through which it ascended. This alternately, at different times of the day and night. And matter, however, has been cleared up almost as well as there is great reason to think that the sap of all other
the nature of the subject will admit of by the experiments trees, bas such an alternate, receding, and progressive Vegetable of Dr Halest. These experiments are so numerous, motion occasioned by the alternacies of day and night, Statics, vol.
that for a particular account of them we must refer to warm and cool, moist and dry.
1. p. 142
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