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302 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 observation,—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 consist 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 Nägeli 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