Igor | THE MIDDLE LAMELLA 31 
thickness during the later stages of cambial growth. Of inter- 
est upon this point is the fact noted by Mangin that, although 
in the meristem of some of the plant tissues studied by him no 
middle pectic layer could be detected by staining, yet, upon a 
treatment known in other cases to dissolve pectic acid, the cells 
are dissociated. My study of very young rose shoots, though 
not carried back far enough to dispute Mangin’s results, yet 
shows that, at a point where the tissues have become very 
slightly differentiated from the meristem, the cambium walls 
show strongly by their affinity for ruthenium red their pectic 
nature. 
CONCLUSIONS. 
The facts that have been cited seem to me to show conclu- 
sively that the middle lamella is not merely the partition wall 
first laid down, either as a single or a double layer, by the 
plasma membranes. Nor is it, on the other hand, merely an 
intercellular substance of cement, a means for-binding the cells 
together, as Mangin holds. Individual cells are not separated 
from one another, either in their formation or their later develop- 
ment, and no reason appears why an intercellular cement should 
be secreted. There is, however, abundant reason why, in woody 
tissues especially, there should be during cell growth a plastic 
region in the cell wall, which should be in a measure adaptable 
to the changing size and form of the protoplast itself, and to the 
firm, resistent layers whose form must correspond to that of the 
protoplast at the time of their deposition. The middle lamella 
is, therefore, a wall layer with a complicated history, undergoing 
after its first appearance changes in form, increase, and probably 
at times decrease in mass, and changes in chemical composition; 
its history, too, is not identical in different tissues. There can 
be no doubt that, in the higher plants studied, the young cam- 
bium walls are included in, and form the basis of, the middle 
lamella of the older tissues. These may at first include, at least 
in the pine, an amorphous Zwischensubstanz, which occupies a 
cleft formed by the splitting of the young radial walls. Quite 
early in the history of the cambium cells, a non-pectic layer 
