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Igor | THE MIDDLE LAMELLA 27 
other as the cells round up and draw apart at their corners. As 
noted above, in sections split in cutting, there were also frequent 
cases where the middle lamella split rather than tore away from 
the other layers. The splitting in all these cases seems to indi- 
cate a weakness of cohesion in the plane between the two layers 
first deposited by the plasma membranes; and confirmation is 
thus given of the view advanced by Strasburger and confirmed 
by Timberlake that the cell plate splits before the new cell wall 
is laid down, the latter thus having a double nature from the 
start. We should not overlook the possibility that the very first 
layer which appears between the daughter plasma membranes is 
so thin as not to form a noticeable fraction of the thickness of 
the mature lamella; and that the splitting at a later stage in the 
middle lamella’s history is due to a decomposition of this first 
thin layer, or that the split is really on one side or the other 
of it. I have, however, seen no evidence in support of either of 
these hypotheses. 
It cannot be supposed that the middle lamella consists only 
of the material first deposited from the young plasma mem- 
branes. Its thickness in adult cells, and its varying thickness in 
different tissues of the same plant, ¢.g., the large vessels and 
the sclerenchyma of Pteris, at once negative such an assump- 
tion. The older view, that it was the cell plate directly meta- 
morphosed into cell wall material, is just as effectively negatived - 
by the same considerations, which should have had more weight 
with the earlier observers. The middle lamella is rarely difficult 
of differentiation in adult thick-walled tissues simply by its 
greater density and less apparent stratification. Moreover, its 
growth in thickness can often be traced, for it stains continuously 
from the youngest wall (the thin tangential red lines in the cam- 
bium of the rose) to the middle lamella of mature tissues. We 
must conclude that the middle lamella consists of the layers 
first deposited by the plasma membranes plus a certain amount 
of material subsequently deposited in contact with these layers, 
which is generally rich in pectic compounds as compared with 
still later deposited strata. The middle lamella may vary 
