30 BOTANICAL GAZETTE [JULY 
in the case of the innermost strata of the bast fibers of Nerium 
and rose, and in the lining of the canals of the rose and of the 
pits of the pine. In these cases we have the deposition first 
of a pectic layer, then of non-pectic or mixed layers, and then 
again of a pectic layer. The purplish tinge imparted to the bast 
thickening layers just mentioned by ruthenium red indicates that 
these layers consist of some pectic substance or substances, other 
than pectic acid, at least in the form in which it exists in the 
cambium walls and the middle lamellae of other tissues. 
Whether the period during which the cell generally deposits 
pectic acid, and thus forms the middle lamella, marks any spe- 
cial stage in its development which can be sharply distinguished 
from subsequent periods in its history is not certain. The evi- 
dence at present available seems to indicate that the pectic 
layer continues to increase in thickness about as long as the 
cambial cell is increasing in size. It is possible that the attain- 
ment by the cell of its adult size marks the limit of the growth 
of the middle lamella. In this case we might say that pectic 
acid is deposited so long as the metabolic processes of the cell 
result in a f/us which is expressed in cell growth, but that later, 
when a metabolic equilibrium has been established, or when the 
excess of food is stored instead of being used for growth, or 
when the protoplast degenerates, a predominance of other cell- 
wall materials is deposited. The evidence for such a view, 
however, is far from complete. It must be remembered, too, 
that in the cambium cells the distinction between middle lamella 
and inner non-pectic layers is very early present, and that the 
middle lamella continues to increase in thickness after it is sepa- 
rated from the protoplast by the non-pectic strata deposited 
later. The middle lamella is not to be considered as consisting 
merely of the wall layers early deposited by the protoplast, 
without undergoing any later modification. It is quite possible 
that the pectic acid which characterizes it may in part be sec- 
ondarily formed or deposited by infiltration through non-pectic 
layers which separate it from the protoplast. Some such 
hypothesis as this is necessary to account for its increase in 
