302 Examination of the Matured Framework [BOOK n. 



pit, while the inner border was the result of ordinary pit- 

 .formation. This view, which could be shown to be incorrect 

 by the history of development, arose in fact from inexact obser- 

 vation, a rare case with von Mohl ; the true course of events 

 in the formation of bordered pits was first described by 

 Schacht in 1860. 



It was mentioned above, that Meyen in his ' Neues System 

 der Physiologic' of 1837, i. p. 45, made cell-membranes con- 

 sist of spirally wound fibres; von Mohl had described in 1836 

 the structural relations of certain long fibrous cells of Vincaand 

 Nerium, which might be provisionally explained in this way ; 

 he was led by Meyen's ideas on the subject to a renewed and 

 minute examination of the more delicate structure of the cell- 

 membrane in 1837 ; he first of all cleared the ground round 

 the question, by distinguishing the cases in which real spiral 

 thickenings lie on the inner side of the membrane, from those 

 in which the membrane is smooth on the outside, but shows an 

 inner structure of fine spiral lines ; in these cases he assumed 

 a peculiar arrangement of the molecules of cellulose, and 

 endeavoured to illustrate the possibility of such a disposition by 

 the phenomena of cleavage in crystals (' Vermischte Schriften,' 

 p. 329) ; but he did not succeed in explaining these very delicate 

 conditions of structure, which we now call the striation of the 

 cell-membrane, so clearly as Nageli afterwards did in connection 

 with his molecular theory. 



3. The question of the substance and chemical nature of cell- 

 membranes was intimately connected with von Mohl's theory 

 of its growth in thickness ; he was engaged in 1840 in minutely 

 studying the reactions which various cell-membranes exhibit 

 with iodine solution under different conditions, a question on 

 which Schleiden and Meyen had recently disagreed ; von Mohl 

 arrived at the result, that iodine imparts very various colours to 

 vegetable cell-membrane, according to the quantity in which it 

 is absorbed ; a small amount produces a yellow or brown, a 

 larger a violet, a still larger a blue tint ; this depends partly on 



