Page:Cyclopaedia, Chambers - Supplement, Volume 2.djvu/665

 T R A

T R A

are placed is different from that which furrounds it, being much harder and more firm than that.

Moft of thefe fafciculi or foramina finally become divided into two or three portions, and the interftitial matter in that cafe is always the fame with that which furrounds them. If any number of thefe fafciculi are examined, the order and difpofi- tiou of the foramina, and even their numbers, are found the fame in all ; they are fo regularly continued along the redt, that air, and light fluids, may be fucked into the mouth thro' pieces of the root from two to nine inches long, and this even when they are contorted and crooked in their growth : the cafe is the fame alfo, whether the piece be cut from the root or from the ftalk of the plant, or from both; for when fuch a piece is cut, half of which is root and half ftalk, the con- tinuation of thefe foramina is found to be the fame, and the air pafTes, whether it be blown in at one end or the other : this is plainly i'ttn on immerfing one end in water, and blow- ing in at the other; the bubbles of air arififtg readily and in great quantities. Comment. Petropolit. Vol. 4. p. 182. In the trunk or ftalk of the melon, there are always regularly twelve of thefe fafciculi with their regular foramina ; and it is evident to the eye that thefe foramina are perfectly empty, if a fmall fegment of the root or ftalk be placed between the eye and a ftrong light; The number of fafciculi in the ftalk is the fame, whether it be cut near the root, or at ever fo great a diftance from it * the fame in the fmaller branches, and even in the pedicle which fuftains the fruit. The number of the cellules in the fruit alfo anfwers to that of thefe fafci- culi in the ftalk, there being always the fame number when the fruit is perfectly and regularly formed, tho' fometirries from accidents there are only ten or eleven to he counted. In the pedicle of the fruit there are indeed more than twelve to be obferved, but thefe fupernumeraries are only- ramifica- tions of the orignal twelve; on the contrary in the pedicles of the leaves there are only nine of thefe fafciculi to be counted; five of thefe are placed on the convex fide of the ftalkj and are greatly ftronger than the others ; and there are two middle- fized, and two others very fmall, in the other part. Thefe have their origin from the nine fafciculi of the ftalk which are neareft the pedicle ; the other three run on without any divarication or without fending off any branches, and form the pedicle of the next leaf; fo that new leaves are produced by thofc alternate fafciculi. The three which run on unin- terrupted, form the three great middle ribs of the leaf into which the pedicle expands, and the two lateral fmaller ribs arc formed each of three other fafciculi taken in from the ftalk ; arid in fome of thefe the three fafciculi may be eafily difcerned on cutting them tranfverfely ; in others only two fhew themfelves on examination ; and fome part of one of thefe fafciculi may be always traced running out into every ramification of thefe ribs in the leaf, fo far as the beft glafles are able to carry the fcarch. Thefe fafciculi^ as they are con- tinued both thro' the ftalk and pedicles and even thro' the ribs of the leaves, carry the external appearance of a white lig- neous fibre ; and as they are protruded to great lengths and run into very flender ramifications, their foramina become Iefs and lefs apparent, and at lengfh efcape the obfervation with the very beft mifcrofcopes.

If at any time the ftulk near the infertion of a leaf, and the leaf itfelf near that part; has happened to rot ; it is eafy to draw out thefe fafciculi regularly in the proper number ; and thefe in their larger part fhew all their foramina very beauti- fully, while in the fmaller they become lefs and lefs diftinct. The main ftalk or trunk of the plant haS a cavity or hol- low in the middle^ which is not continued to the root, nor to the pedicles of the leaves ; and near the origin of the young brandies, in that part of the ftalk to which the inner part of the leaf corresponds, there is a greenifh diaphragm which occupies the middle of tiie ftalk^ into which the fibres of the ftalk, after making their ramification for forming the fafciculi of the pedicle, are inferted laterally ; after which, pe- netrating the ftalk, and coming out at that part where the origin of the leaf is to bej they form a fort of thin mem- brane, which covers the tender firft fhoot of the leaf and the young branch. This membrane finally fhews its twelve faf- ciculi; and thus the true number is everywhere continued, un- lefsin fome few places where two of them have happened to co- here on their running too clofe together; but in this cafe they foon divide again, and fhew their true number. Comment. Pe- tropol. Vol. 4; p. 184.

From the whole it may be concluded, that if the Trachea of plants are continuous canals, containing only air, and com- pofed of a firm matter in their fides, thefe fafciculi before defcribed, as found in the root, ftaiks, feV. of the melon^ are true Trachea ; for it is plain that they are empty canals, or containing no other matter than air ; neither need it be doubted that there are fuch in all plants, becaufe in fome even the beft microfcopes cannot difcover them : for we find in this very plant that thofe Trachea, which in a part of their length are very confpicuous, become fmall and not to be diftinguifhed as hollow, in their finer and fmaller extremities ; and in fuch plants as are fuppofed to have none of them, they may either be lb minute as not to have a difcernahle hollow, or elfe their cavity, or at leaft the new made orifice of it, may be occluded Suppl. Vol, II.

by the other vefTels of the plant throwing out their juict-}" into it, on being wounded. We fee that in the melon thefe Trachea are carried from the root to every part of the plant with great regularity, and that they are included in what we commonly call the Woody fibres of plants, and have their fides formed of the matter of thofe fibres J whether thefe fi- bres have any juices circulating thro' other fmaller canals, or are only deftined to fupport the Trachea^ or air vefl'els, is a queftion not eafily determined : they are much dryer than any other fibres of the plant, and fcem to contain no juices, ex- cept thofe deftined for their own proper nutrition. They have by fome been fuppoled to ferve for the carrying back to the root fuch juices as were not employed in the nourifhing the plant; but all experiments feem to difcountenance this con- jecture, and it fcems to have been built upon the obfervin°- this part of the vegetable without feeing its true u(c, or find- ing out the numerous Trachea which it fuftained. The ca- vity in the middle of the ftalk may be attributed to the ex- panfion of the green fibres, and of the utriculi which form fo great a part of the plant ; and the motion of the juices in thefe may poflibly be owing to the motion of the air in thefe Trachea, regularly dilating and contracting them. But the great eafe and plainnefs with which they are followed in this fearch is owing to their largenefs, and the tendcrnefs of the ftalk.

It is eafy to fee in what manner the ligneous parts of plants contribute to vegetation, fince in thofe parts only the Trachea are fituated : and hence alfo the reafon appears very obvious, why grafts do not fucceed unlefs the ligneous part of the ftock be touched in the operation ; all the Trachea of the ftock being in this part, and it being impoflible that the Tra- chea of the graft fhould have any communication with them, unlefs brought into contact by opening this ligneous part of the tree. Act. Petropol. Vol. 4. p. 187. Nothing fhews the Trachea of plants in fo beautiful a manner as a tranfverfe fegment of a young fhoot of the vine. They may be in this difcovered in a good light even by the naked eye ; but with the help of but a fmall magnifying glafs they appear very beautiful and diftinct. In the generality of other trees, thefe Trachea are fo finally that even the largeft mag- nifiers in our microfcopes cannot well diftinguifti them : fo that Fontenelle and many other great men have doubted their exiftence*. but even where thefe are fmalleft of all, as in the pctiola or middle ribs of the leaves of plants* they may, tho' wholly imperceptible to the fight, be proved to exift, by experiments. If a fmall cylindric glafs be filled with water, and have all its air exhaufted from it by the air-pump, and the middle rib of a leaf be then cleared from the other parts, and cut off at both ends, and plunged at one end into this water; while yet in the exhaufted ftate, the bottom of the petiolum being fupported from touching the bottom of the glafs, the fituation and number of thcTrachea in it will be eafily diftinguifhed by a firing of bubbles, which, arifing from each of them, will make a row of beads, as it were fi- fing in a continued chain to the furface. Act. Erudit. Ann. 1722. p. 24.

TRACHELAGRA, a term ufed by fome medical writers to ex- prefs the gout in the neck.'

TRACHELIUM; in Mr. Tournefort's fyftem of botany, the name of a diftinct genus of plants the characters of which are thefe : The flower confifts of one leaf, and is fhaped like, a funnel, and divided into feveral fegments at the ed^e. The cup of the flower is finally converted into a membranaceous fruit* which is ufuallyof a trigonal form, and is divided into three cells,' which ufually contain a number of very final! feeds.

The fpecies of Trachclium enumerated by Mr. Tourhefort are thefe: 1. The blue-flowered umbelliferous Ti-achelium. 2. The jagged-leaved violet-coloured umbelliferous TracheUum. 3. The hairy TrachelUan with flowers growing in clufters from the alas of the leaves. 4. The rock-Trachcliitm with flowers collected into heads. 5; The violet-coloured African Tracbelium, with flowers growing fcattercdly all upon the ftaiks. 6. The Ame- rican fow-thjftle-leaved Trachelium y with very long white flowers. Tournef. Inft. p.' 130.

TRACHELOMASTOIimUS, in anatomy, a name given by Albinus to a mufelc^ called by Winflow and others the corn- plexus minor; or jnajloidatts lateralis.

TRACHEIjOS, a word ufed by fome anatomical authors to ex- prefs the heck.

TRACH1DNA, in ichthyology, a name given by Jovius and fome others to the draco marinus of the old authors, called by us the iveever.

It is a fpecies of the trachinus^ defcribed by Artedi under the name of the trachinus without beards and with the lower jaw longer than the upper. See the next article.

TRACHINUS, in the Linnasan fyftem of zoology, the name of a genus of fifties, of the general order of the acanthop- terygii : the characters of thefe are, that the opercula of the gills are pointed, and the eyes are placed near one another in the topof the head. Linnai Syftema Nature, p. 53. The characters of this'genus, according to Artedi, are thefe: There is one or more prickles at the upper angle of the co- verings of the gills, and the head has fe-me rough tubercles R r r r on