28 BOTANICAL GAZETTE [JULY 
considerably in thickness in different tissues, according as a greater 
or less amount of material rich in pectic substance has been 
deposited inthe cell. That it is not the only part of the cell 
wall, however, that contains pectic compounds is shown from the 
taking up by the other layers of the wall, though less freely than 
by the middle lamella, of pectic acid stains; it is shown also in 
the pectic layers lining the pits of the pine and the canals in 
the bast fibers of the rose, and in the innermost thickening layers 
of the bast of Nerium and rose. 
Although the middle lamella usually retains its pectic nature, 
it undergoes a change by which, in the course of cell develop- 
ment, it loses the power of adapting itself to the varying form 
of the adjoining cells and becomes fixed and inflexible. The evi. 
dence seems satisfactory that this change, as Mangin suggests, 
is one from pectic acid to insoluble pectates, chiefly the calcium 
salt. Suchachange is indicated by a less deep staining in older 
tissues, unless they are first treated with acid alcohol. A further 
transformation appears to take place in the cork cells of Tilia, 
by which the power of distinctive staining is entirely lost, and the 
middle lamella cannot be distinguished from the other layers of 
the suberized wall. The pectic acid here seems to have been 
partly replaced by suberin. The coloration of the cork walls is 
purplish, indicating a possible similarity in chemical nature to 
’ the later layers of the bast elements in Nerium and rose. In the 
sclerenchyma and stone cells of Pteris, also, though a middle 
lamella is plainly present, its staining reactions do not distinguish 
it from the rest of the wall; yet it can be traced as a continua- 
tion of the thin, characteristically-staining walls of the funda- 
mental parenchyma. It is possible that the middle lamella in this 
case was from the start non-pectic ; but it seems more probable 
that, as secondary thickening proceeded, the pectic acid was 
replaced or masked by material similar to that deposited to form 
the other layers. Further light could be thrown upon this ques- 
tion by tracing the development of the wall from the meristem to 
the mature tissues. The difference in depth of stain between radial 
and tangential walls in Tilia may indicate that the tangential 
