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light. It is proljabk' that the fap is originally depofited in 

 the cells, in the ftate in which it is abforbed, that is, confiding 

 of water and carbonic acid gas, and that there, by the agency 

 of light, -.t undergoes a decompofition ; the oxygen contained 

 ill both the water and fixed air being difcharged by the pores 

 of the cuticle, the carbon of the carbonic acid, and the hydro- 

 gen of the water producing the oils ard the refins. A number 

 of confequences arife from this operation, not only to plants 

 thenifclvcs, but to the animal woild, which make it tiie 

 mo!l important proctfs carried on in the vegetable fyftem. 

 See Etiolation, Light, Oxygen, and Transpiration. 



Parenchyma. This part is compofed of cells like thofc 

 defcribed in the herbaceous tiffue ; indeed, the only differ- 

 ence which exifts between thefe two parts of the Cellular 

 fubilance, is in the colour of their contained fluid; the one 

 bt-ing ufually green, in confequcnce of its expofure to the 

 light; whiht the other, not being fituated fo fuperficially, is 

 generally found tranfparent. In other refpcfts, they agree 

 in (trufture, and appear to be formed of the continuation of 

 the fame membrane. 



The parenchyma of Mirbel corrcfponds with the tifTue 

 ccllulaire of Du Hamel, the utricles of Malpighi, and the 

 parenchymatous fabftance of Grew; whilfl: the herbaceous 

 tiffue is more flriftly the envelope cellulaire of Du Hamel. 



The parenchyma is not confined to the fuperficies of 

 vegetables ; it pafTes between the fibres "of the cortical and 

 ligneous layers, and forms thepith or medulla;thc pulpinefs of 

 leaves and petals depends upon its exiftence ; fruits, feed?, 

 and the cmbrio plant, are almoil entirely compofed of it ; 

 bulbous, and other fucculent roots, owe their bulk to it; no 

 other ilructure is obfervahle in the fungi and fuci: in {hort, 

 the cellular tiffue is the firll and fimpleft ftate of vegetable 

 organization, and ferves as the connetling medium between 

 all the parts of the plant. 



There is, ftrittly fpeaking, no circulation of the juices 

 contained in the cellular tiffue; fluids, however, being ad- 

 mitted into any ot the cells, eafily pafs into the neighbouring 

 ones, by means of the fmall pares of communication, already 

 defcribed. 



Tlie texture of the cellular fubflance is very fpecdily 

 broken down by maceration, or boiling in water; which cir- 

 cumilance (hould be recollefted in preparing the parts 

 of plants for examination ; otherwife the natural con- 

 r.ection?, which are produced by the cellular tiffue, may efcape 

 obfervation. 



The Cortlciil Layers and Liber. When the epidermis and 

 the cellular envelope have been removed, the remainder of 

 the bark appears to be made up of a number of reticulated 

 fibres, containing cellular fubftance in their interfpaces; this 

 appearance of tlie cortical fibres is plain to the naked eye, 

 efpecially if the cellular tiffue, which paffes amongft them, be 

 at all deftioyed by maceration, or other means; but if exa- 

 mined by the microfcope, thefe fibres become very diftinft ; 

 their arrangement is then perceived to be fingular, and diffi- 

 cult to defcribe. The fibres in their courfe, although longi- 

 tudinal with refpeft to the plant, are not parallel v.'ith each 

 other ; each mates a flight curve, and thus comes into con- 

 tact with the one adjoining, with which it ufually becomes 

 incorporated or united, and thus produces a plexus or net- 

 work, which was called by Du Hamel the cortical plexus; 

 fometimes thefe fibres merely touch each other, and tiien go 

 off again, to compofe another me(h in the plexus, ietjig. 6. 

 Plate I. of Vegetable Anatomy ; aaa point out the reticulation 

 produced by the fibres, and bhb the mefhes, or fpaces left 

 between them. The meflies are not vacant in the recent ve- 

 getable, but filled with cellular tiffue, which admits of the 

 tranfwrfe motion of the fluids in plants. Du Hamel ftates, 

 2 



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that upon examining thefe fibres by a high magnifying 

 power, each appeared to be a fafciculus, the fibres of which 

 could be again refolved into fibres, and thefe again could be 

 divided into others, until they became too minute for obferv- 

 ation ; he, however, as well as other authors upon the 

 fubject, fuppofe the cortical fibres to be veffcls. See 

 Vessels. 



The cortical layers, as the term implies, are not fingle, 

 but confift of a number of concentric laxir.se, placed upon 

 each other in fuc!i a m.anner that the mclhcs of one plexus 

 are fituated oppofite to thofe of another. F':g. 7. Plate I. 

 exhibits thii circumftance as it has been reprefented by 

 Du Hamel. The cellular tiffue paffes through all thelc 

 melhes, and thus produces a kind of intcitexture, which Mal- 

 pighi compared to cloth, calling the longitudinal fibres the 

 warp, and the tranfverfe th.e weft. 



The melhes of the feveral net-works are fmaller, the more 

 internally they are fituated ; the gradation in this refpccl is 

 regulnr from the external layer to the wood, as may be per- 

 ceived in/^f. 8, 9, lo. in PA?/.- 1, of Vegetable Jlnalotiiy. In 

 tlie mod internal plexus,j%. 10, the longitudinal fafciculi are 

 nearly parallel, and fo clofe to each other thai the inter- 

 fpaces are almoft obliterated. 



The cortical layers, or net works, are found to increafe in 

 number according to the age of the part which fuftains them. 

 Thus Du Hnmel reckoned only five or fix plexufes upon the 

 upper branch of the linden-tree, and feventeen at the bafeof 

 the trunk of the fame tree. 



The fame difpofition of fibres does not exift in all plants; 

 in thelagetto, or the lace bark tree, for inftance, the cortical 

 plexus exiiibits a texture like gauze or lace. See_^T. n. 

 Plate I. of Vegetable Anatomy. 



Much conlufion may be obfer\'ed in the dcfcriptions 

 which authors have given of that part of the bark called 

 liber. The name would appear to have taken its origin from 

 the likentfs which the cortical plexufes, when partially fepa- 

 rated, bear to the leaves of a book; and, conformablv to this 

 idea. Grew and others have confidered all the cortical layers 

 as belonging to tlie liber; whilll, on the other hand, Mal- 

 pighi has given this name to the innermoft layer only. The 

 liber is, however, generally allowed to be the moft important 

 part of the bark, and is that fubftance from wliich the cor- 

 tical layers are formed. When the bark is ftripped off a 

 tree in a ftate of full vegetation, in a very (hort time a 

 gelatinous fubftance is obferved to exude upon the furface 

 of the wood; this fubftance acquires organization, and is 

 converted into a new bark. It was termed cambium by 

 Du Hamel ; the manner in which it is produced, and its 

 comoofition, are both unknown, but its high uti'ity in the 

 vegetable economy is proved by fome beautiful exoeriments. 

 This formative or organizing fubftance is conftantly renewed 

 during the period of vegetation, and immediately pr duces 

 the liber, which is infenfibly converted into the layers of 

 bark, and the alburnum, or uliite imperfect wood, which is 

 next the bark ; and hence the acccffions of bulk in perennial 

 vegetables, wliich are made every year, and indicate the age 

 of the tree. That the liber is the immediate fource of both 

 the wood and the bark, or the central point or fountain of or- 

 ganization, is proved by two very elegant experiments made 

 by Du Hamel. He feparated a portion of the bark of a 

 plum-tree, and made fure that it poHeffed the inner cortical 

 layers or fiber ; he then removed a fimilar p'ortien of bark 

 from a peach-tree, and replaced it with the piece taken from 

 the plum-tree. The grait perfectly fucceeded ; and upon a 

 future examination he found, that not only ihe engrafted 

 bark continued to grow, but that a correfpondiug portion 

 of wood was produced, which was veiy dillinguilliablc from 



the 



