24 ‘ BOTANICAL GAZETTE [JULY 
DISCUSSION, 
In almost every tissue studied in which there was evidence of 
the deposition of wall layers at different periods, the staining 
reactions of the middle lamella showed it to differ in chemical 
composition from the adjoining layers; my results confirm Man- 
gin’s in every respect upon this point. Its distinctive character 
was brought out most clearly by ruthenium red. The reasons for 
concluding that, in general, this peculiarly staining layer is com- 
posed of pectic acid and its derivatives, have already been given 
at length. It does not follow, however, that its chemical com- 
position is unchangeable. On the contrary, it is clear that, dur- 
ing cell growth and development, changes occur in the chemical 
constitution as well as in the form and mass of the middle 
lamella. 
In pointing out the existence of a Zwischensubstanz between 
the radial walls of the cambial cells, and in distinguishing this 
layer from the ‘intercellular substance,” either as characterized 
by Mangin or by himself, Dippel has contributed essentially to our 
clear conception of the conditions in the cambium of the pine; 
but his notion that the Zweschensubstanz is derived from the walls 
of the cambium mother cells has little evidence in its favor. I 
have never seen in this mass the fragments of these walls of 
which he speaks (2, p. 575). Besides, were Dippel’s view cor- 
rect, the tangential walls would be expected to contain at least 
as much of this substance as the radial walls; but, in fact, noth- 
ing of the sort is to be seen between the tangential walls, which, 
however, as shown in Dippel’s own figures, are no thicker than 
are the radial walls without the Zwischensubstanz. On this point 
of the origin of the Zwschensubstanz Dippel is not clear, for he 
distinctly says (p. 575) that it is derived from the degenerated 
cambium mother cell walls, and again, on the same page, he 
speaks of the latter as going to make up the intercellular sub- 
stance. It seems to me more probable that this Zwischensubstanz 
represents pectic acid which has exuded through the cambium 
cell walls into an intercellular cleft formed by the splitting of 
the radial walls, and that it is analogous to the lax substance 
