180 BOTANICAL GAZETTE [SEPTEMBER 
In radial and tangential views the most striking feature is the 
incoherence of the hard bast zones. Only rarely do the elements 
hold together in a diamond-mesh network, such as is so character- 
istic of the box elder. This is apparently due to the fact that most 
of the hard bast cells are comparatively short, blunt at the ends, 
and not interlocked to any great extent. In the outer part of the 
bark some of the fibers lie in the tangential plane almost at right 
angles to the axis of the stem, as the result of the great tangential 
tension in this region. There is extensive parenchymatous atrophy 
of the sclerotic elements in the outer bark. Only one phellem zone 
is commonly present. This is made up of 6-10 layers of small 
cells, quite thin-walled and strongly suberized. Crystallogenous 
cells are found in small numbers, principally in the middle and outer 
portions of the bark. Occasionally an older medullary ray call 5 is 
found containing tannin. 
In A. rubrum (fig. 16) the bark is much thinner than in box 
elder, smooth on young stems, and separating into thin regular 
plates on older trunks. Hard bast occurs in larger proportion 
than in A. saccharinum, and is distributed quite irregularly through- 
out the outer three-fourths of the thickness of the bark. The 
zonation is imperfect, and there is scarcely any tendency to form a 
diamond-mesh network (fig. 23). 
Only the larger medullary rays extend out into the bark. 
These are only slightly oblique as far as the first hard bast, beyond 
which they extend radially one-half to two-thirds of the way to the 
outer surface, becoming increasingly diffuse. Crystallogenous 
cells are quite numerous, while tanniniferous cells occur less fre- 
quently. The parenchyma cells are nearly all much flattened. 
The hard bast zones and masses are incoherent and the cells are 
drawn out to oblique and transverse positions in the outer bark. 
There is a single zone of phellem, 5—12 cells thick, the walls strongly 
lignified and suberized. 
The bark of A. saccharinum (fig. 15) is thin, and shows a definite 
zonation of hard bast in the inner two-thirds of its thickness. The 
zones are 2 or 3 cells thick, interrupted by the broad medullary rays 
which usually disappear about half-way out through the bark, 
beyond which point the sclerotic tissue is very irregularly scattered 
