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304 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 considered 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growth in thickness were shaken in 1858 by Nägeli's observations, and we may say that on the whole it has been for ever superseded.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 understanding; in this case, when Nägeli proposed his more profound 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 Nägeli in 1846. Von Mohl himself had introduced this idea for the first time into the science in 1836 in one of his earlier and inferior essays, 'Erläuterung meiner Ansicht von der Structur der Pflanzensubstanz,' 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,