122 ON THE PLANT-CELL. 



upon the epidermis-cells is not proved by Hartig in a single actual in- 

 stance ; and proof is also wholly wanting, that the delicate " cuticula," 

 rendered evident on the embryo by sulphuric acid, is identical with the 

 presumed "ptychode" of the primary cell lying immediately upon the 

 epidermis-cells, and much less with the outermost secreted layer (the 

 "eustathe") of the epidermis-cells. I consequently consider the view 

 which I have developed, and which is not, as Hartig says, the general 

 one, but peculiar to myself, to be, as yet, the better founded and more 

 correct of the two. 



The two cells of the stomate do not differ, as has been previously re- 

 marked, in their contents and vital properties from those of the subjacent 

 parenchyma. The width of opening of the fissure left between them 

 varies at different times, and in different parts in the same plant ; and 

 it is thence evident that the degree of admission of atmospheric air to 

 the parenchyma is variously modified. Our knowledge with respect to 

 these cells is, as yet, very imperfect, and we do not even know whether 

 the contraction of the fissure be effected by the turgescence or by the 

 collapse of the cells. The latter appears to me to be the more probable 

 supposition, because when the evaporation is too rapid, in which case 

 these cells are evidently the first affected, it would by this means be 

 checked. 



The appendicular organs, again, consist of cells, which, like the paren- 

 chyma, have necessarily relinquished less of their individuality, owing 

 to which innumerable peculiar processes continue to be exhibited in 

 them, the production of special substances, some of which 

 are secreted, particularly viscous, saccharine, resinous matters, 

 and essential oils. The conditions exhibited in these organs 

 are endlessly multifarious, and what is necessary regarding 

 them has already been partly referred to. 



To one phenomenon, however, I must here direct attention. 

 The stinging hairs of the Boraginacece (Borago officinalis, 

 fig. 107.) and Urticacece become filled, when old, from the point 

 towards the base, with an assimilated material deposited in 

 laminae, and differing from the wall. In the Urticacece (in the 

 BoraginacecB I have not as yet been able to observe any thing 

 similar), this deposit, when it has extented to the dilated base 

 of the hair (fig. 108. c), constitutes a globular mass projecting 

 into the base of the hair, the peduncle of which is sometimes 

 longer and sometimes shorter (Ficus, Broussonetia), and 

 which is occasionally beset with minute crystals of carbonate 

 of lime (fig. 109.). In Cannabis a minute point only of 

 these hairs projects above the epidermis ; in Urtica cana- 

 densis there is merely a large globular cell, level with the sur- 

 face of the epidermis ; in Parietaria judaica, Humulus, Forskcehlia tenacis- 

 sima, a similar cell (a) is placed beneath the epidermis (&). I believe 

 the latter might be regarded as stinging hairs, normally undeveloped in 

 accordance with a specific law. * 



* Meyen (Miiller's Archiv, Jahrg. 1839, p. 257.) discovered these concretions in Ficus. 

 Payen (Froriep's Notizen, Vol. xvi. No. 335.) found them in several plants, and there- 

 upon has woven a prolix soi-disant theory a la mode Frangaise, which, to a physiologist 

 with more precise notions, carries its own refutation with it. 



K7 Upper part of a hair of Borago afficinalis, at first thickened by a laminated de- 

 posit, and afterwards gradually filled up from above downwards by a solid material. 



107 



