CHAP. HI.] of Cell-membrane in Plants. 305 



difficult to understand and in some respects quite peculiar, 

 von Mohl believed that he saw in many cases in the higher 

 plants also between the sharply-defined membranes, which 

 bound the cell-spaces and which he regarded as the entire cell- 

 membranes, a substance in which the cells are imbedded, for 

 such is its appearance when it is largely developed ; when it 

 lies in small quantity only between cells in close apposition, it 

 looks like a thin layer or cement. After Meyen in his ' Neues 

 System,' pp. 162, 174, had declared against this view in 1837, 

 von Mohl too abandoned it more and more, and afterwards 

 limited the occurrence of intercellular substance to certain 

 cases, being convinced that much that he had before taken 

 for it consisted only of layers of secondary thickening, between 

 which he still saw the primary lamina of the cell-membrane. 

 The theory of intercellular substance was taken up and further 

 developed by other phytotomists, by Unger especially in the 

 ' Botanische Zeitung ' for 1847, p. 289, and afterwards chiefly by 

 Schacht ; Wigand came forward as an opponent of it in 1854 in 

 his ' Botanische Untersuchungen,' p. 65, and logically following 

 out von Mohl's theory of the cell-membrane, declared the thin 

 layers of intercellular substance as well as the cuticle, which had 

 been first correctly distinguished by von Mohl, to be laminae of 

 primary cell-membrane, the substance of which had undergone 

 profound chemical change. These ideas also of the intercellular 

 substance and the cuticle assumed an entirely different aspect 

 when Na'geli introduced his theory of intussusception. 



The limits imposed on this history render it necessary to be 

 content with these indications of von Mohl's share in the working 

 out of the theory of cells in its connection with the structure 

 of the solid framework of cell-membrane ; we shall return 

 again to his observations on the formation of individual cells. 



5. Forms of tissue and comparative anatomy. Phytotomy 



up to 1830 had been weak in its classification of tissues, 



in its ideas as to their arrangement, and consequently in 



its histological terminology; the inconvenience arising from 



x 



