THE USE OF POLARIZED LIGHT IN VEGETABLE HISTOLOGY. 293 
prejudice, to give another interpretation to appearances upon which 
facts are founded, or to exaggerate the significance of facts estab- 
lished by inexact observation. 
That primary walls, each belonging to two different cells, enter 
into the formation of the middle layer, we recognise at once in 
cases where, in the natural object, the middle plate becomes 
loosened through any circumstances (as in the wood of some old 
conifers) ; and this is further demonstrated by the results of mace- 
ration. But has it not been attempted to explain the separation 
in the former case as the effect of tension-relations; and, in the case 
of maceration, has not the decay of the tissues been explained by 
the loosening of the entire middle layer, composed, as it is, of the 
three plates? 
Furthermore, it can be proved beyond doubt that the middle 
plate, in its chemical behaviour, differs from the cell-walls ; indeed 
the usual re-agents upon the cell-substance prove this, as well as 
tinting, only, however, after suitable preparation, as I have ex- 
plained in another place. But has it not been tried to depreciate 
these facts by a few re-actions, often executed by very juvenile 
hands, mostly only after tormentings of the preparations concerned 
in different ways, by means of the most various expedients, the 
effect of which on the molecular relations of the cell-walls is beyond 
control, and by means of which, in one case, a transitory, feeble, 
and in another a dubious blue colouring has been claimed to have 
been revealed, and thus to disprove the assertion that the middle 
plate could not have formed a strong woody part of the original 
wall, consisting of cell-substance ? 
On the contrary, observation with \ polarized light, in both cases, 
leads to unassailable results, provided, however, that the specimens 
have been successfully prepared. In the former case, such obser- 
vation shows, in all stages of the influence of the macerating 
medium, that only the middle plate (first made known through 
polarization) is subject to loosening ; whilst in other cases it proves 
that the lignification of the substance of the cell-walls neither 
lessens nor suspends the double refraction, seeing that the same 
(the middle plate) through all stages of preliminary treatment 
of the section, not subjected to re-agents, to the complete removal 
of the woody substance in the primary walls, retains the same 
degree of strength as in the condensation-layers. At the same 
time it proves with absolute certainty that the middle plate, recog- 
nised as singly refracted at the first observation, remains so until 
it arrives at the point which precedes the complete separation, and 
when it is no further dyed yellow by iodo-chloride of zinc, and when 
the cell-substance, if it existed, must thus be recognised, if only 
feebly, by double refraction. 
If then the fact that the middle plate of the “middle layer,” in 
