292 THE MICROSCOPICAL NEWS. 
discuss some facts, to the confirmation of which it affords the most 
valuable aid. 
If we examine in common light a very thin transverse section, 
cut quite perpendicularly to its long axis, through a tissue with 
thickened cell-walls, ¢. g., through the wood of a leafy or coniferous 
tree, we see, as a rule, the well-known net-work (which I designate 
the net-work of the primary walls, whilst Hofmeister, Sachs, and 
others have conferred upon it the name of “‘middle-layer”) in a 
manner which causes us to conceive the same as a structural form 
common to contiguous cells, in which, beyond the well-known 
gusset in the angles, where three or four cells are joined together, 
no further differentiation is perceptible. -(Fig. 1.) 
We now examine the same transverse section by means of 
polarized light, and in sooth in the dark field of view (with the 
crossed Nichols) appears a substantial alteration of the appearance 
ey: 
of the so-called middle layer. The whole of the previously homo- 
geneous net-work appears, whilst the primary partitions shine in 
clear white light, (about what happens to the other parts of the 
wall nothing need here be said), with a fine black line drawn 
through them from the gussets, thus forming three stripes, of which 
one of those illuminated belongs to one of the neighbouring cells, 
the one not illuminated appears common to both. (Fig. 2.) Ob- 
servation with polarized light thus tells us very decisively that the 
middle layer is not single, but consists of three plates, the middle 
one of which is singly refracted, whilst those on each side are 
doubly refracted, the one thus indicating a totally different mole- 
cular structure, and even chemical constitution, from the other 
two. 
It is true that there are other means of recognising the constitu- 
tion of the middle layer ; but how easily is one disposed, by a little 
