1915] PLOWMAN—BOX ELDER 181 
and progressively atrophied. Most of the parenchyma cells are 
irregularly flattened. Both tanniniferous and crystallogenous 
cells are collenchymatous. The single phellem zone is 5—10 layers 
of cells thick, suberized, but only slightly lignified. Sieve plates 
are prominent in tangential view of the youngest phloem. 
A. platanoides (fig. 14) shows a much smaller proportion of 
hard bast than do most of the other maples. There are only 2-4 
narrow zones, and these are widely interrupted by the broad, 
irregular medullary rays, which extend about two-thirds of the 
way to the outer surface of the bark. The rays are not much 
deflected from a radial course, and the deflection is not at all uni- 
form. The parenchyma is less flattened than in other forms. The 
bast fiber network is least coherent in this species. Crystallo- 
genous and tanniniferous cells are very numerous. Parenchyma- 
tous atrophy is pronounced in the outer part. There is a single 
zone of phellem, very irregular in thickness. The cells are highly 
suberized. The collenchyma zone just inside of the phellem is con- 
spicuous and of uniform width around the stem. 
These brief studies of box elder and of four species of maples, 
together with the measurements of elements tabulated on p. 177, 
show that the bark of the maples is more dense and better able 
to resist unfavorable conditions and the attack of enemies, 
but less rapid in growth, less elastic, and hence less perfectly 
adapted to the needs of a quick-growing tree than is the bark of 
the box elder. 
In figs. 26, 27, and 28 are shown sections in three planes of the 
wood of A. saccharinum, while the three succeeding figures show 
corresponding sections of box elder wood. The maple wood is evi- 
ently more compact, with somewhat smaller elements arranged 
with greater regularity than in the case of box elder, thus readily 
accounting for the fact that the maple wood splits more easily than 
the box elder wood. The groups of tracheae are larger in the box 
elder, showing as many as four or five elements in a radially dis- 
posed row, while the maple rarely shows more than three in a group. 
The medullary rays are not so straight in the box elder, hence the 
radial section does not show such large plates of “silver grain” as 
in the maple. 
