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are in this mr.nner left furrour.ded by the cracks, are larger 

 or have more extent, the nearer they approach to the 

 wood, in confequence of the moll external epidermis hav- 

 ing firll yielded to the growth of the tree. 



No fubjec't has occafioiied greater coiitvoverfyamongft vege- 

 table anaton^ilb, than the mode in which the cnticle of plants 

 is formed. It was the opinion of Malpighi and Grew, that the 

 epidermis was produced by the laft vefick-s of the cellular enve- 

 lope, in confequence of their expofure to the air ; but it the 

 cuticle was formed by the deficcation of the cellular tiffi'.e, it 

 would not admit of that extenfion which takes place in all 

 circumftances to a certain degree, and which is fo remarkable 

 in the cuticle covering leaves, flowers, and fruits, and all 

 parts of which the growth is rapid. 



Several other circumftances might be mentioned to fliew 

 that the epidermis can be produced by the di7ing of the 

 cellular fubftance; thus when itis wounded or deftroyed, and 

 the part perfeclly fecluded from the aaion of the air, a 

 new epidermis is foon produced without any exfoliation. 

 The cuticle is in fome inftances formed, and in others 

 continues to grow, under circumftances entirely beyond 

 the agency of heat and evaporation, as may be obferved in 

 the fatal plant, and its appendages, and the internal fur- 

 faces of buds, &c. ; but although the epidermis does not 

 appear to be the cellular tiffue fimply dried by expofure, it 

 is fufficicntly plain tluit it is the continuation of the fame 

 membrane which forms the cellular envelope. 



According to the lateft obfervations made on this fubjeft by 

 Mirbel, who is one of the moll ingenious vegetable analomifts 

 oftheprefenttime, the lines which give the reticulatedappear- 

 ance to the epidermis, correfpond in figure with the cells of 

 the parenchyma, and are really the termination of the fepta of 

 thefe cells in the cuticle. (Seey%. 2. Philc I. Vegetable Ana- 

 tomy.) The tubular tiffue, which in fome cafes is perceived 

 upon the fupertkies of plants, enters alfo into the compofi- 

 tion of the epidermis, as reprefented in Jig. 3. Plate I. Ve- 

 getable Anatomy. Thefe fmall tubes are however, upon clofe 

 infpeftion, found to be compofed of cells very much elong- 

 ated, from whence it would appear, that the cellular fub- 

 ftance is alone fitted for producing the epidermis of vege- 

 tables. The manner in which the cellular tiffue is converted 

 into cuticle, is probably beyond the reach of inveftigation ; 

 but that it is not the mere rtfult of expofure to the air, is 

 fufficiently plain from the fa£ts already noticed. Like many 

 other of the changes and operations of organic matter, we 

 are una'ole to difcover its immediate clncient caufe, and in 

 fuch cafes, we mull be content with obfervlng the phenome- 

 non, and Hating it as the effccl of a law of the fyftem. 



The growth of the cuticle is fubjeft to confiderable vari- 

 ety, according to the plant to which it may appertain, or 

 the different parts of the fame vegetable: thus, on leaves, 

 flowers, fruits, &c. we do not meet with the fuccefiive 

 layers of dead epidermis that exift upon trunks and 

 branches. Some vegetables again have greater accumula- 

 tions of dead cuticle than others ; fome get rid of thefe by 

 repeated exfoliations ; the plantane calls its cuticle every 

 year ; the epidermis of herbaceous plants and thofe which 

 are not perennial, is always moll delicate in its ilrufture. 



The epidermis prefents no peculiarities in the monocoty- 

 ledons, or thofe plants with one feminal leaf. 



The ufes of the external or cuticular portion of the bark 

 have been much contefted, although many of thefe are ex- 

 ceedingly obvious : it is evident that it ferves as a de- 

 fence to the whole furface of the vegetable ; and accord- 

 ingly we find its compofition and flrength dependent upon 

 the functions which each part of the plant performs, and 

 the injuries to which it is liable ; on the roots it is tough 

 and flexible ; on the trunk rough, thick; and unyielding ; 



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on the leaves, flowers, and fuch parts as are only to pofFcfs 

 temporary cxiflence, and which at the fame time exercife im- 

 portant funftions, the cuticle is thin, delicate, and foft. 

 The epidermis ferves to guard the plant againll the effefts 

 of meteors ; it likevvifc affiils to moderate the operation of 

 heat and cold, ar.d thereby contributes to the maintenance 

 of the equality and independence of vegetable temperature ; 

 it regulates the aftion of light upon the cellular tilluc, and 

 thus co-operates in the fixation of that fubtle matter ; but 

 the moft important, perhaps, of all its ufcs is the giving 

 paflage both to the .^uids abforbed for the nutrition of the 

 plant, and thofe expelled by tranfpiration, &c. I'efides 

 thefe known offices of the cuticle, C/thers are afcribed to it. 

 Many authors imagined that it rellrained the growth, of the 

 whole tree ; this however is dilprovcd by the fimple experi- 

 ment of removing a portion of the cuticle, when it lias been 

 found that no bur or fweliing took place ; thofe trees alfo 

 which are moft diftinguifhed for the cracks of the cuticle, 

 are not obferved to grow fafter than others. For a further 

 account of the fun6lions of the cuticle of vegetables, fee 

 Epidermis, Leaves, Etiolation, Transpiration, 

 Inhalation, and Pores. 



The cellular envelope. This was the name given by Du 

 Hamel to the cellular fubftance immediately under the cu- 

 ticle, in confequence of its extending over every part of 

 the plant; by Grew it was called parenchyma ; and Mubel, 

 whofe abilities have been already mentioned, has with pro- 

 priety made a diftinftion between the cellular tiffue imme- 

 diately next the cuticle, and that which is continued into 

 the cortical layers ; the firll he terms iha herbaceous till'ite, the 

 other X.\\c parenchywa. 



Tlie herbaceous tiffuen a compolition of cells of an hexagonal 

 figure, fo applied to each other that each of the fides alTift* 

 in form.ing the parietes of the adjoining cell, precifely like 

 the conftruftion of a honey-comb. The membranes com- 

 pofing thefe cells are extremely fine and tranfparent. See 

 Jig. 4. Plate I. of Vegetable Anatomy, in which the cellular 

 ftrufture is highly magnified, and alio fome foramina or pores, 

 which eftablifh a communication between the feveral apart- 

 ments. Thefe pores are not above the 3codth part ot a line 

 in diameter. In fome inftances, the cells are elongated, 

 efpecially in the parenchyma of the monocotyledons, which il 

 exhibited in Jig. 5. of Plate I. ; and it is remarkable, that in 

 proportion as the veficles become elongated the pores of 

 communication are m.ore frequent and regular. In fome 

 cafes, where the cells are vei7 much elongated, they are 

 arranged in rows fucceeding each other by intervals, which 

 are perfeelly regular. (See Glands, and Pores.) The 

 cellular tiflue has been defcnbed, by fome writers, as 

 compofed of a number of fibres, interwoven hke the texture 

 of felt. Hill fays it only differs from the epidermis in having 

 its parts more diflinfl ; and IVIalpighi believed the cellular 

 fubftance to be made up of diftinft veficles, collefted together, 

 which he called utricles, but, as we have already faid, this is 

 not the cafe; the whole being one continuous membrane, 

 every part of which enters into the conftruftion of two cells. 

 Grew compared the cellular fubftance to the bubbles ob- 

 ferved upon the furface of fermenting liquors, which is a 

 very happy fimilitude, inafmuch as it conveys a very perfeA 

 idea of its appearance when only exam.ined with a iingle 

 lens; but when the higheft magnifying powers are employed, 

 the hexagonal figure of the cells becomes evident. The 

 herbaceous tiffue is the immediate caufe of the colour of the 

 epidermis, and its own colour, again, depends upon that of 

 the fluid contained in the cells, wliich is ufually green, but is 

 fometimes brown, red, yellow, &c. This juice is. of a re- 

 finous nature, which circumftance would appear, as well as 

 the colour, to be the effect of its continual exnofure to the 



light. 



