instead of doing so were moulded by the ideas current before
1845. It has been shown in the preceding chapter how von
Mohl gradually restricted the theory of intercellular substance
which he had proposed in 1836, and had come in 1850 to regard
this substance as only a cement which might in many cases be
perceived between the cell-walls. It should be added here,
that Schleiden in connection with his theory of cells considered
both the intercellular substance and the cuticle to be supplementary secretions from the cells, and made the former fill the
intercellular spaces, just as laticiferous and resiniferous passages
are filled with secretions from the adjacent cells (1845). Unger
too in 1855 ('Anatomic und Physiologic der Pflanzen') thought
the existence of a cement between the cells necessary to prevent their falling asunder. Schacht, who in his 'Pflanzenzelle'
of 1852 had followed Schleiden in explaining the intercellular
substance and the cuticle as secretions or excreta from the cells
of the plant, still kept on the whole to this view in 1858,
though he modified it in some important points. This theory
of Schleiden and Schacht was first opposed by Wigand in a
series of essays (1850-1861), in which in strict adherence to
von Mohl's theory of apposition he sought to prove, that the
layers which are visible in wood-cells as intermediate laminae in
the partition-walls, and which till then had been regarded as a
cement between contiguous cells, an intercellular substance,
were nothing else than the thin primary membranous laminae
formed in the process of cell-division, and subjected to subsequent chemical change, while the secondary layers of thickening
in von Mohl's sense lie on both sides of them. The cuticle
on the epidermis was explained in a corresponding manner.
Though Sanio in 1863 raised a variety of objections to Wigand's
view, he still adhered to it in principle, and found a strong confirmation of it in the fact, that he succeeded in producing the well-known cellulose-reaction in the intercellular substance of wood-cells when freed from foreign admixtures.
The researches of Wigand and Sanio were sufficient to over-
