T E T 



TE T 



which is in itfelf, with the exception of one genus, C/eome, 

 a natural clafs, comprehending all the cruciform flowers. 

 Its effential charafter confills in having fix llamcns, four of 

 which rife above the reft. This is indeed fo naturally dif- 

 tinft a tribe of plants, that it is hard to trace any particular 

 affinity between them and any others. The following is 

 the charafter of the flowers. 



Cal. Perianth oblong, of four ovate-oblong, concave, 

 obtufe, converging leaves, projcfting downwards at their 

 bafc, the oppofite ones moil; fimilar to each other, deci- 

 duous. The projeftlon generally obfervable at the bafe is 

 for the lodgment of honey, the calyx here being a ncdiary, 

 fo far at leall as containing the honey. Cor. termed cruci- 

 form, of four equal petals; their claws inverfely awl-ihapcd, 

 or tapering downwards, flattened, ercdt, rather longer than 

 the calyx ; limb flat, or nearly fo, the border of each petal 

 being dilated outwards, obtufe, its fides fcarcely touching 

 its neighbour. The petals are inferted into the fame circle 

 in which the fl:amens are placed. Stam. Filaments iix, awl- 

 fhaped, eredt, the two oppofite ones the length of the 

 calyx, the refl: rather lojiger, but not equal to the corolla ; 

 anthers fomewhat oblong, pointed, thickefl at the bafe, 

 ereft, their points recurved. Neiftariferous glands, vari- 

 oufly circumftanccd in the different genera, are feated at 

 the bafe of the ilamens, efpecially between the fliorter ones 

 and the piftil, thofe ftamens, to avoid prefilng upon fuch 

 glands, being moft;ly curved, by which they become fliorter 

 than the four others. Pijl. Germen fuperior, daily in- 

 creafing in height ; ftyle either the length of the longer 

 ilamens, or wanting ; lligma obtufe. Perk. Pod of two 

 valves, and frequently as many cells, burfting from the bafe 

 to the fummit, the partition, if prefent, more or lefs ex- 

 tended beyond the valves, the prominent part being the ori- 

 ginal flyle. Seeds roundifli, drooping or pendulous, ranged 

 alternately, in a double row, along the partition, in which 

 they make flight depreffions. Receptacle linear, furrounding 

 the edge of the partition, and embraced by the mai-gins of 

 the valves. 



Linnoeus obferves, that all fyftematic botanifts, even the 

 moft eminent, have unanimoufly confidered this as a truly 

 natural clafs of plants. Some of them have neverthelefs 

 admitted into it, here and there, a genus or two in oppofi- 

 tion to nature, which fault he jufl:ly conceives himfelf to 

 have avoided, except with regard to Cleome. 



The plants of this clafs have univerfally been termed an- 

 tifcorbutic. Their flavour is generally acrid, though 

 watery. Few vegetables yield lefs of an effential oil ; yet 

 this fubftance is to be obtained from them by cohobation, 

 or repeated diflillation, and its qualities are peculiarly acrid 

 and foetid, fomewhat like volatile alkali. This oil gives the 

 offenfive fcent to water in which cabbage has been boiled, 

 and it caufes thefe vegetables to difagree with fome 

 ftomachs, though they are generally reputed wholefome. 



The clafs in queftion is divided into two orders, i. Silicu- 

 Infa, in which the pod is roundifh, and for the moft. part 

 hardly longer than its ft;yle. 2. Stliquofa, with a very long 

 pod, to which the fcarcely perceptible ftyle bears no pro- 

 portion. 



The genera of the firft order are diftributcd into two 

 fections, one having the pod, here termed a fiUcula, or 

 pouch, entire, and the other furniflied with a notched, or 

 emarginate, pouch. The genera here aie eharaiStcriled by 

 the different fiiapes of their pouch, or its valves, and the 

 comparative length of the ftyle. 



The order of S'lViqmfa is divided into fuch as have a clofe 

 calyx, whofe leaves converge longitudinally, and fuch as 

 have a gaping, or fpreading calyx. The genera here arc 



partly defined by the form of the pod and its valves, and 

 their mode of burfting, and partly by the fituation of the 

 nedtarifcrous glands. 



Thefe principles of generic diftinftion, laid down by 

 Linnxus, h«ve not proved fo fatisfaftory in praAice as 

 could have been wiflied, infomuch that, not only Haller, 

 but fome lefs controvcrfial botanills, have differed from the 

 learned Swede in their ideas of fevoral of the cruciform 

 ■ genera. The bcft attempt to reform them has lately been 

 made by Mr. R. Urown, in the fecond edition of Mr. 

 Aiton's Hortus Kemenfis. Tliis able obferver has recurred 

 for afiillance to the cotyledons, taking into confideration 

 their being either folded or flat, accumbent or incumbent. 

 The latter difference we beheve to have been firft noticed 

 by Gsertner, in his charafters of Eryftmum. By accumbent 

 is meant that the edges of the unexpanded cotyledons are 

 applied, in a parallel manner, to the infant radicle ; and by 

 incumbent, that the flat fide, or back, is prefented to that 

 part. The latter is feen in Eryfimum, and is the moft un- 

 ufual pofition. The number of feeds in each cell of the 

 pouch is alfo adverted to by Mr. Brown, as well as feveral 

 other incidental marks ; by the affillance of all v.hich the 

 whole tribe appears much more fatisfa£toriIy arranged than 

 heretofore, though we do not profefs to agree, in every point, 

 with our leariicd friend. His Mathiola, coiififting of the 

 hoary fpecies of Cheiranthus, fuch as incanus, finuatus, tn- 

 cufpidalus. See. feems lefs happily feparated from the ori- 

 ginal genus, than his Malcomta, compofed of C. mnritimus, 

 Hefper'is africana. Sec. In the latter cafe, the fimple acute 

 ftigma, incumbent cotyledons, and the habit of tiie plants, 

 afford a fufliciently clear diftiiiftidn. In the forjner, what- 

 ever difference there may be in habit, the charafters feem 

 to us not fufliciently evident or important. It is alfo 

 proper to remark, that whatever alliftance fuch a difference 

 as that above defcribed in the cotyledons may afford, to- 

 wards forming a philofophical idea of a genus, its great ob- 

 fcurity renders it unfit for pradical and daily ufe. On 

 this fubjeft we need not enlarge upon what Linnseus has fo 

 happily enforced, and generally pracfifed. 



TETRAEDRON, or Tetraiiedro.v, formed of TfTfor, 

 four, and ^a^y, Jide, in Geometry, one of the five regular 

 or Platonic bodies of folids, comprehended under four equi- 

 lateral and equal triangles. 



The tetraedron may be conceived as a triangular pyramid 

 of four equal faces. Such is that i-eprefented in Plate XV. 

 Geomet.Jig. 4. 



It is demonftrated by mathematicians, that the fquare of 

 the fide of a tetraedron is to the fquare of the diameter of 

 a fphere, in which it may be infcribed, in a fubfefquialteral 

 ratio : whence it follows, that the fide of a tetraedron is to 

 the diameter of a fphere it is infcribed in, as , 2 to the ^'3 : 

 confequently they are incoramenfurable. See Regular 

 Body. 



TETRAETERIS, TETjasTn^ic, in the Athenian Chrono- 

 logy, a cycle of four years ; for which fee Potter, Archaol. 

 Graec. lib. ii. cap. 26. tom. i. p. 459. 



TETRAGASTRIS, w Botany, from T'Tfji and ya<rl,if, 

 thejlomach or belly, becaufe of the four protuberant lobes 

 of the feed-veffel. — Gasrtn. v. 2. 130. t. 109. f. 5. — Clafs 

 and order, as well as Nat. Ord. unknown. 



Nothing is known of this genus but its fruit, which 

 Gxrtner obtained from the colleftion at the botanic garden 

 of Amftenlam. He defcribes it as a dcpreffed berry, of 

 four lobes and io\u cells, with folitary feeds. 



Its form is nearly globofe, a little depreffed, convex and 

 pointed at the fummit, marked with four longitudinal fur- 

 rows, fcparating the prominent, culhion-like lobes, into 



which 



