1915] PLOWMAN—BOX ELDER 175 
twigs are not quite cylindrical, but more or less elliptical in section, 
the longer axis lying in the plane of the two leaves borne at the top 
of the internode. The surface of the young twigs is quite smooth 
and shining, except in those forms which develop a whitish bloom. 
Lenticels are not very numerous, but they are of rather large size, 
long and narrow in the early part of the season, and later becoming 
broad. 
Fig. 19 shows a section of a moderately vigorous young shoot 
of a male box elder. For comparison a section of a young stem of 
Acer saccharinum is shown in fig. 17. The characteristic 6-sided 
appearance of the pith in the box elder section is due to the sym- 
metrical arrangement of the 6 large leaf traces, 3 for each leaf. 
These leaf traces are of course most prominent near the top of the 
internode, but they are conspicuous even to the lower end. The 
section was taken from near the middle of an internode 5 inches 
long. While the leaf traces are prominent in all types of box elder, 
it is worthy of note that they are relatively much larger in those 
plants which bear leaves with the higher numbers of division. 
The medulla is usually about one-half the diameter of the stem. 
The pith cells are large, thick-walled, circular, oval, or hexagonal 
in outline, and usually quite empty, except in the outer amyliferous 
zone. Fig. 40 shows a portion of this zone, very greatly magnified. 
While the zone is very irregular in width, it is everywhere at least 3 
or 4 cells wide. In the true maples this starch zone is much 
narrower, even sometimes entirely absent. An average condition 
of the starch zone of Acer saccharinum is shown in the drawing 
fig. 4r. It will be observed, also, that the medullary ray of box 
elder is expanded at its inner end, thus affording a better connection 
with the starch zone than in the case of the maple. The box elder’s 
quick response to the first warmth of spring is doubtless made 
possible, in part at least, by its large starch-storing capacity and its 
highly efficient medullary rays. These rays are very numerous, 
mostly uniseriate, and seldom much farther apart than the width 
of a large trachea. The cambial zone, in the growing season, is 
made up of a considerable number of layers of actively dividing 
cells, thin-walled, and filled with protoplasm. The bark is com- 
paratively thin on young twigs, and is composed of small, thin- 
