294 THE MICROSCOPICAL NEWS. 
the complete state, appears under all circumstances as a part of 
the singly refracted cell-wall proves that it is different in its molecu- 
lar structure from the primary walls, as it is from the condensation- 
layers, and contains no cell-substance even in its final condition, 
still the question remains to be decided (for the middle plate must 
be considered, in consequence of its position outside the primary 
walls, as a constituent part of the cell-wall, formed before the 
primary wall) of what kind is the first solid secretion-product of the 
living cell-body, vz., the first wall-formation, to which it (the middle 
plate) owes its origin? Here also observation with polarized light 
again affords an indubitable explanation, if we apply the same to 
the cell-wall during its development. 
A transverse section through the cambium-region (of a conifer 
for the best), when observed in the darkened field of view, shows 
the following state of affairs :— / 
Varying according to the season of slow or more rapid cell- 
multiplication, in which the wood was formed, the walls of a single 
cambium-mother-cell, or of several rows of cells, proceeding from 
the wood to the bast (the first generation of daughter-cells), appear 
darker than the ground of the field of view, whilst the same con- 
dition is recognised in the procession of those rows of cells lying 
next to the wood and bast, from those with still proportionally 
thinner walls to those with walls already more or less thickened, 
as aforesaid. Thus it is possible closely to follow the transit of 
the dark stripes through the bright net-work of the primary walls 
with the dark walls of the cambium-cells. Now these facts pyove 
that, prior to the primary wall being formed out of cell-substance, 
and manifesting itself at once by its double refraction, an envelope, 
singly refracted, and therefore not formed out of cell-substance, is 
secreted from the protoplasm for every daughter-cell originating 
in a cambium-mother-cell, which envelope remains during the 
conversion of the cambium daughter-cells into wood or bast-cells, 
and thus becomes the middle plate of the middle layer. The 
matter is therefore decided, and a firm ground obtained for the 
interpretation of the appearances which we obtain by re-agents 
upon the cell-substance, as well as by means of carmine and ani- 
line dyes, as I have described in another place; and what I there 
said I must now maintain as correct, on the ground of my exten- 
sive and careful research, v7z., that the middle layer is compounded 
of the two primary walls of adjoining cells, and of the cambium- 
layer common to each. 
Concerning the share to be assigned to the wall-layers in the 
formation of the pore-canals and the closure of the pores, various 
opinions notoriously prevail. With H. von Mohl, Schleiden, and 
others, some of the more recent botanists share the view that the 
closure is formed by the primary walls, and that the entire 
