1915] PLOWMAN—BOX ELDER 179 
are I5 uX35-40u in size, becoming crystallogenous in the older 
part; the smaller 8-10 uX 30-45 mu in size, and are more numerous, 
in the ratio of about 3 to 1 of the larger. The walls of the larger 
are not lignified at any stage, while the walls of the smaller cells 
are slightly lignified at maturity. Sieve tubes are numerous in 
the younger bark, with large sieve plates especially conspicuous in 
tangential sections. The outer, dead bark consists of a mass of 
crushed, irregular, thin-walled cells, alternating with several zones 
of thick-walled phellem. Each zone of phellem is made up of 
3-8 layers of brick-shaped cells, 7-12 w thick radially, and 20-35 pu 
square on the tangential surface. These cells are very thick-walled 
(2-4 m), lignified, and only the outer ones in each zone are con- 
spicuously suberized. Progressive parenchymatous degeneration 
of the hard bast and stone cells occurs in the outer portion of the 
live bark, so that very few of these elements are to be found in the 
dead bark of the box elder. 
In the radial section of box elder bark (fig. 25) the bast fibers 
appear apparently in discontinuous masses, scattered throughout 
the live portion of the bark. As a matter of fact these strands are 
connected longitudinally for great distances, but, owing to the fact 
that they are tangentially oblique, the section shows only short 
portions of each strand. This type of structure evidently gives 
great strength to the bark, while at the same time it secures a 
maximum degree of elasticity even where the bark is quite thick, 
thus readily permitting the rapid expansion so characteristic of the 
growing box elder stem. 
In the case of A. saccharinum, the bark is thinner than in 
box elder, and quite smooth even in older trunks, scaling off from 
larger trunks in thin, even plates (fig. 3). Under the microscope, 
the cross-section of a 10-year twig shows the bark to be made up of 
parenchyma in alternating narrow zones of radially compressed 
cells and round cells, with 2 or 3 nearly continuous zones of hard 
bast lying close together near the middle of the bark, and also many 
small scattered islands of sclerotic cells, both inside and outside 
of the bast fiber zones. 
The medullary rays are commonly bent aside from the radial 
line 20-30°, to the first hard bast zone, where they are again nearly 
radial, but beyond which they quickly disappear. 
