Page:History of botany (Sachs; Garnsey).djvu/323

 the extent to which the membrane is capable of distcntion; the blue colour especially depends on the absorption of a sufficient quantity of iodine. Greater interest, excited at first by a very important work by in 1844, was taken in the question of the chemical nature of the solid framework of the vegetable body, in which it was shown that the substance of all cell-membranes exhibits a similar chemical composition when freed from foreign elements. Payen considers that this material, cellulose, is present in a tolerably pure form in the membranes of young cells, but is rendered less pure in older ones by 'incrusting substances,' whose presence changes the physical and chemical characters of cell-membranes in various ways. These incrusting substances may be more or less removed by treating the membranes with acids, alkalies, alcohol, and ether, while other inorganic matters remain behind after combustion as an ash-skeleton. This theory, which has been more perfectly worked out in modern times, was soon afterwards met by Mulder with the assertion, that a large part of the layers composing the walls of cells consist from the first of other combinations and not of cellulose; he at the same time deduced from this view certain conclusions respecting the growth in thickness of cell-walls. He and Harting, relying on microscopic examination, maintained that the innermost tertiary layer in thickened membranes is the oldest, and that the other layers are deposited on the outside of this, and are not composed of cellulose. Von Mohl opposed this view decidedly and successfully in the Botanische Zeitung of 1847; he likewise in his work on the vegetable cell (p. 192), refuted the view of the varying substance of cell-membrane,