304 Examination of the Matured Framework [BOOKII. 



which Schleiden had founded on some obscure chemical 

 grounds. 



It would carry us much too far to enter into the details of 

 this scientific dispute ; Payen's view of the chemical nature of 

 the vegetable cell-wall, which von Mohl adopted and elaborated, 

 has maintained itself to the present day, and is generally con- 

 sidered to be the true one ; on the other hand, the foundations 

 of von Mohl's theory of growth in thickness were shaken in 1858 

 by Nageli's observations, and we may say that on the whole it 

 has been for ever superseded. It has been nevertheless of 

 great service in the development of our views on cell-structure 

 in plants ; keeping closely to the facts directly observed, it 

 served to bring almost all the conditions of the sculpture of 

 cell-walls under one point of view, and to refer their formation 

 to one general and very simple scheme ; every such theory 

 helps to advance science, because it facilitates mutual under- 

 standing ; in this case, when Nageli proposed his more pro- 

 found theory of intussusception, the understanding of it was 

 essentially assisted by a previous exact knowledge of von Mohl's 

 theory in its principles and results. In conclusion it may be 

 mentioned here that von Mohl afterwards in his investigation 

 into the occurrence of silica in cell-membranes made a large and 

 important addition to the knowledge of their more delicate 

 structure, and of the way in which incrusting substances are 

 deposited in them ('Botanische Zeitung,' 1861). 



4. The views of phytotomists on the so-called intercellular 

 substance during the twenty years from 1836 to 1856 were closely 

 connected with the older theories of cell-formation, but were 

 opposed to the modern doctrine of the cell founded by Nageli 

 in 1 846. Von Mohl himself had introduced this idea for the first 

 time into the science in 1836 in one of his earlier and inferior 

 essays, ' Erlauterung meiner Ansicht von der Structur der Pflan- 

 zensubstanz,' rather in opposition to than in connection with his 

 own theory of the growth and structure of cell-walls. Setting 

 out from modes of formation of cell-membranes in some Algae, 



