1915] PLOW MAN—BOX ELDER 183 
autumn tracheids is frequently divided into two parts by the inter- 
polation of a few thick-walled summer tracheids. All tracheids 
show a few small circular pits in their walls at points of contact 
with medullary rays, while elsewhere their pits are very rare. 
Faint oblique striae are occasionally found in the walls of the 
thicker-walled sorts. 
Wood parenchyma cells in the mature parts of box elder are 
very few, small, short, and thin-walled. They are usually found 
bordering upon the larger tracheae. Very rarely they become 
crystallogenous. The medullary rays are 1-3-seriate, about 
100-120 w apart, and slightly wavy as seen in cross-section of the 
wood. In radial and tangential sections the medullary rays show 
a breadth of about 200-500 yp, and a thickness of about 20. The 
ray cells are of two fairly distinct kinds (figs. 34, 36). (1) The 
ray body cells are nearly cylindrical, 7-10 in diameter, with 
walls 2-2.5 u thick. Their length is from 25 » to 40 mu, except in 
the region of the autumn growth, where the length hardly exceeds 
the diameter, and where the walls are slightly thicker. The end 
walls are usually only slightly oblique, with very many minute 
simple pits. The pits communicating with the tracheae are large 
and numerous, while those connecting with tracheids are small and 
few. (2) The ray marginal cells form usually a single, sometimes 
a double, row on each edge of the ray. These are a little larger 
than the ray body cells, with somewhat thinner walls. They are 
triangular in section, and when seen in radial sections they show 
marked irregularity in form along the free border (fig. 36). The 
pits are larger and more numerous than in the body cells. Much 
protoplasm and large nuclei are usually to be seen in the ray body 
cells, while the marginal cells are usually almost empty. 
Where injuries have been inflicted, the wood of box elder shows 
traumatic tissue made up of thin-walled, unlignified cells contain- 
ing a very little tannin. 
As would be expected from the comparison of figs. 27 and 30, 
the density of box elder wood is considerably less than that of the 
maples. The average for box elder wood is 27 pounds per cubic 
foot, while the maples range from 32 to 43 pounds per cubic foot. 
In color the wood of box elder is a pale cream or white. Its rather 
