1915] : PLOW MAN—BOX ELDER 185 
pits. Tannin is found in these larger cells. There are but few 
wood parenchyma cells, and these do not contain crystals. Tra- 
cheae in the older parts of the wood contain much tannin and 
some tyloses. The medullary rays do not differ essentially from 
those of A. saccharinum, except that the cells are much shorter at 
the close of the season’s growth, and elsewhere an occasional cell 
is shorter, thick-walled, and filled with tannin. 
In A. saccharum the end walls of tracheal cells are about 45° 
oblique, but with no definite direction of dip. The smaller tracheae 
show scalariform and imperfect spiral markings. Tracheids are of 
three kinds, quite similar to those of box elder. Crystallogenous 
wood parenchyma cells are numerous, especially along the sides of 
the medullary rays. The ray marginal cells are very irregular in. 
shape, and show a marked tendency to overlap each other. 
In A. rubrum the end walls of the tracheal cells have a dip of 
30-60", chiefly in the tangential direction. The smaller vessels — 
show scalariform but not spiral markings. There are occasional 
tannin plugs and diaphragms in the tracheae of the older wood. 
Tracheids are of two types. The autumn tracheids are flattened 
to a greater extent than in other species, measuring 10-14 u by 
only 3~4 win cross-section. The wall is 1.5 yu thick except near the 
edges of the flat cells, where it is 2 zor more. Many older tracheids 
contain crystals and tannin, and small simple pits are common in 
the walls. The medullary ray cells are often hexagonal in tan- 
gential view, and the walls are unusually thick. The rays are 
frequently much broader than in other species. The marginal cells 
are larger and quite irregular. They are much shorter at close of | 
the season’s growth, and often erect on the edge of the ray. The 
most striking feature of this species is found in the much greater 
number and larger size of pits in all kinds of elements than in other 
species of maple. 
Further details in regard to size of elements in the wood of the 
various species studied may be found in the table of measurements 
on p. 177. 
Anatomy of the leaf 
It has already been stated that the leaf of box elder is thicker 
and of softer texture than are the leaves of the true maples. The 
principal features of the minute structure are indicated in fig. 32. 
