CHAP, in.] of Cell-membrane in Plants. 303 



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 PAYEN ' 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, 



1 Anselm Payen > 1795-1871) was born at Paris and was Professor of In- 

 dustrial Chemistry in the Ecole des Arts et Metiers in that city. His most 

 important botanical works were his ' Memuire stir I'amidon, 1 etc., Paris 

 (18391, an d his ' Memoire sur le developpement des Vegetaux,' published in 

 the Memoirs of the Academy of Paris. 



