182 BOTANICAL GAZETTE [SEPTEMBER 
Fig. 39 shows a bit of the cross-section of box elder wood highly 
magnified. The tracheae are elliptical or oval in section, with 
the longer axis radially disposed. ‘They range in size from 30 yp to 
65 u, with an average size of 40X55 mu. The individual cells 
of which the tracheae are composed are 150-200 yp long, with their 
end walls obliquely disposed at an angle of about 45°, the dip in 
almost every case being radial, so that the end wall seems to be 
quite transverse as seen in tangential section (fig. 31). The perfora- 
tion through the end wall is about one-half the size of the end plate, 
and elliptical in shape. The tracheal walls are marked off in a 
regular hexagonal pattern, each area of which is about 5 uw across, 
with a simple pit 0.3 wX1.5 wu transversely disposed at the center. 
Where tracheae lie in contact with medullary rays, the pits are 
circular and 1.5-2.5 w in diameter. Some smaller tracheae show 
occasional traces of scalariform and even spiral thickening of the 
walls. In the acute angle with the oblique end wall, there is some- 
times a considerable area of the tracheal wall in which the thickness 
is uniform and unbroken by pits. In these regions quantities of 
tannin may be stored. 
The tracheids of box elder are of three fairly distinct sorts. 
(1) The thin-walled summer tracheids are 12-15 w in diameter, 
with walls only o.5-o.8 w thick. These occur in largest propor- 
tion near the beginning of the season’s growth, but they are also 
to be found in small numbers even bordering upon the zone of the 
thick-walled autumn cells. (2) The thick-walled summer tracheids 
are 8-10 p in diameter, and their walls are 1.5-2.5 wu thick. These 
occur in small groups at the beginning of the annual ring, the groups 
becoming larger and more numerous as the season’s growth pro- 
gresses. The two kinds of summer tracheids are commonly grouped 
in such a way as to give a distinctly marbled appearance to the 
cross-section. All of these cells are angular and very irregular in 
shape. The majority of them are 400-600 yp in length, and they are 
firmly interlocked at the ends. (3) The autumn tracheids consti- 
tute a dense zone 3-6 cells thick at the close of the season’s growth. 
These cells are much flattened, measuring 10-15 uw tangentially and 
5-7 w radially. The walls are 2 » or more in thickness and more 
strongly lignified than other parts of the wood. This zone of 
