throw von Mohl's account of the intercellular substance and the
cuticle, but they had not proved that the intermediate laminae
are in fact the primary partition- walls on which von Mohl's secondary thickening-layers had been deposited, on both sides in the
case of the intercellular substance, on one side in that of the
cuticle. The structure of the partition-walls and the existence of
the cuticle could be explained in a totally different way from the
point of view now opened by Nägeli's theory of intussusception;
there was no need now to see either a secretion or a primary
cell-wall in the intermediate lamina of thickened cells or in the
cuticle, for it was possible that this lamination might be due to
subsequent chemical and physical differentiation of membranes
thickened by intussusception. As phytotomists are not yet
quite agreed as to the correctness of this view, we must be content with observing here that in the matter of the cuticle and the
intercellular substance lies one of the points, the determination
of which will involve the question of von Mohl's earlier theory
of apposition. It is not the purpose of this history to give the
more modern views that have asserted themselves since 1860,
especially where the question is still in debate.
It was a part of von Mohl's idea of the cell-tissue and one to which he had firmly adhered since 1828, that except in the cross walls of genuine wood-vessels and some very isolated cases the partition-walls in cellular tissue are never perforated; that both simple and bordered pits always remain closed by the very thin primary lamina of cellulose. But between 1850 and 1860 several cases were discovered which were at once exceptions to von Mohl's rule, and of great importance to physiology. Theodor Hartig, in his 'Naturgeschichte der forstlichen Kulturpflanzen Deutschlands' (1851), described peculiar rows of cells in the bast-system, in which the transverse and sometimes the longitudinal walls appear to be pierced like a sieve by numerous minute holes, and to these cells he gave the name of sieve-tubes. Von Mohl (1855), while in other points confirming and extending Hartig's discovery, declared against
