26 BOTANICAL GAZETTE (JULY 
of three or four cells, are enclosed within the apparently split 
middle lamella, and are commonly angular in form. 
Since there i§ no trace, except in the pine, of a Zwzschensub- 
stanz whose absorption would leave intercellular spaces, such 
spaces in the tissues of other plants can be accounted for only 
by the rounding up and drawing apart at their corners of adjoin- 
ing cells. This induces a splitting of the middle lamella. The cleft 
so formed may be temporarily filled by a lax, fluid or semi-fluid 
substance, a solution, perhaps, of pectic acid or one of its trans- 
formation products, but this in time is absorbed into the walls, 
leaving an empty space. The surfaces of the wall exposed to 
the so-formed intercellular space may become further modified 
chemically, and even so softened as to flow enough to form a 
rounded instead of an angular space; but there is no evidence 
that such chemical changes ever occur except where the wall is 
exposed in an intercellular space, and it is more likely that the 
plasticity here displayed by the pectic acid has characterized it 
from the time of its deposition, but has not been shown so 
plainly because of its confinement within comparatively rigid 
limits. 
On the basis of the later investigations respecting the rela- 
tion of the cell wall to the cell plate, we have seen that the 
middle layer appearing after the splitting of the cell plate is to 
be considered as formed by deposition from the split halves of 
the original plate; the middle lamella of mature tissues would 
include, then, in, addition to possible later deposits, both the 
layers deposited on the inner surfaces of the daughter plasma 
membranes. This being the case, we might expect to find in the 
history of the development of the middle lamella evidences that 
it consists of two layers. This is just what is found in the case 
of the intercellular spaces, which are very evidently caused by a 
split through the center of the middle lamella. If the middle 
lamella were not of a double nature, we should hardly expect it 
always to split through the middle, but, in view of its marked 
difference from the adjoining layers, we should expect that 
sometimes the whole layer would be pulled to one side or the 
