STRUCTURE.] SCLEROGEN THICKENING PROCESSES. 11 



by Professor Mohl. Turpin has remarked that the thicken- 

 ing of the membranous sides of cells by means of a hard 

 sedimentary matter, called by him Sclerogen, is what causes 

 the grittiness of the pear, and the boniness of the stone of the 

 peach and plum, in all which the osseous parts were originally 

 membranous. It is, however, by no means in old or woody 

 parts alone that a thickening of the membrane takes place : 

 it may be observed distinctly in the cells of the corolla of 

 Convolvulus tricolor, and in fact occurs in all parts con- 

 taining fluid matter exposed to vital action. 



Mohl and others are of opinion that all addition to the 

 thickness of vegetable ^membrane takes place on its inner 

 face ; but the universal presence of a cuticle overlying the 

 outer series of cells- in every organ exposed to air, and the 

 granulations found on the outside of old hairs (see Elements 

 of Botany, fig. 67), render it difficult to deny the increment 

 of membrane in thickness on both sides. This view has more 

 especially been taken by Harting and Mulder, who conclude 

 from chemical evidence, that in the development of cell- 

 membrane, all the layers which in a full-grown cell have a 

 peculiar chemical reaction not occurring in the young mem- 

 brane, have been formed subsequently to that membrane, 

 which consists entirely of cellulose ; and that since such layers 

 occur on the outside of full-grown cells (the innermost 

 layer of which is composed of cellulose, and therefore corre- 

 sponds to the membrane of the young cell), the cell-membrane 

 must have increased in thickness in consequence of a subse- 

 quent deposition, from within outwards, of layers having a 

 different chemical constitution. Mohl has discussed this 

 opinion with his usual sagacity. " Let us examine," he says, 

 " whether these conclusions may not be too hasty. It does 

 not admit of the slightest doubt, that the chemical compounds 

 which are coloured yellow by iodine and sulphuric acid, and 

 which characterise the outer and intermediate layers of most 

 full-grown cells, are of later origin than the cellulose which 

 forms the membrane of the young cell . From this fact, how- 

 ever, it is a great leap to the assumption, that those layers 

 which are composed of a substance differing from cellulose, 

 are in reference to their situation also newly-formed layers, 



