1901] THE MIDDLE LAMELLA 25 
sometimes found at the angles of cells before the appearance of 
empty intercellular spaces. Its ultimate fate is doubtless to be 
absorbed into the adjacent cell walls, so that it does not appear 
as a visible layer in the middle lamella of mature tissues except 
in the case of the areas represented by the red-staining bars of 
the pine tracheids (see Dippel’s figure 397), and except also, 
perhaps, on the surface of intercellular spaces. The intercellu- 
lar spaces of the pine wood, then, are to be interpreted as 
remnants of the radial clefts between the cambium cells, rather 
than as newly formed in the adult tissue. But this does not pre- 
clude the probability of changes in the form and extent of the 
Spaces due to the rounding up of the adjacent cells. It would 
be interesting to know whether this Zzwischensubstanz is of wide 
occurrence. I have not observed it elsewhere than in the pine, 
and Mangin does not specifically note its existence. 
A fact which is important as showing the plastic nature of 
the middle lamella at an early period of its history is its varia- 
tion in thickness in different portions of older walls. This is 
shown commonly by an enlargement at the angles anda decrease 
in proportional thickness at the sides of the cells. Such varia- 
tion is in marked contrast in the spermatophytes studied to the 
practical uniformity in thickness of the completely stained cam- 
bium walls. These are continuous with the middle lamella of 
the differentiated tissues, and, were it not for this change in 
form, would appear identical with the middle lamella. In some 
of the tissues examined, the substance of the middle lamella 
appears more dense at the enlarged angles, and seems to take up 
the characteristic stains more freely at those places. This 
appearance, however, may be accounted for by the optical effects 
produced by the greater thickness at the angles. But in other 
Cases, as in the fundamental parenchyma ot Pteris and the 
collenchyma of Nerium, these enlarged angles enclose a less 
deeply staining substance, or are becoming empty. These may 
be considered as stages in the development of the intercellular 
Spaces, which are located, like the areas of less deeply stained 
substance just mentioned, at the angles formed by the junction 
