188 BOTANICAL GAZETTE [OCTOBER 
to use the airpump to remove entirely the air from the blocks. 
Sections 10 yw in thickness were easily obtained by these methods. 
In staining sections it was found that Delafield’s haematoxylin in 
combination with safranin gave satisfactory results. 
Anatomy of healthy stem 
XyLEM.—The normal wood is of the diffuse, porous type (fig. 1), 
corresponding in this respect to other species of Populus. The ves- 
sels are numerous, visible to the naked eye, and are slightly larger 
in the spring wood than in the summer wood. They form a con- 
spicuous ring of large pores within the earlier wood where it joins 
with the late wood of the previous season. In most growth rings 
an irregular, oblique, tangential-radial arrangement of pores can be 
seen, which, however, do not cross the junction of the spring wood 
and summer wood of the preceding season’s growth. The vessels 
are of the bordered pitted type in the xylem of secondary growth. 
In the xylem that arises from the growing point, however, a great 
many vessels with spirally thickened walls are formed. 
MEDULLARY RAY.—The rays of this species, as in all the species 
of Populus which the writer has examined, are scarcely visible with 
the ordinary hand lens. Only uniseriate rays have been found to 
occur in normal wood, the cells of which are richly supplied with 
simple pits (fig. 3). The ray is made up of uniform parenchyma 
cells elongated horizontally. Viewed radially, a ray appears as 4 
muriform structure composed of several rows of tabular cells. The 
walls of the cells of the medullary ray are pitted, and the walls 
therefore have a lattice-like appearance when seen in radial section. 
The ray cells in most cases contain a considerable amount of starch. 
In tangential section, the cross-sections of the rays are shown to 
good advantage. The uniseriate character is very evident. The 
rays vary from three to seventeen cells in height, and are distinctly 
spindle-shaped, the end cells tapering to a decided point. The cells 
between the end cells vary in shape from cubical to prismatic (fig. 3). 
In the radial aspect one is best able to study the wood fibers that 
compose the bulk of the xylem. These wood fibers are slender, non- 
septate, spindle-shaped, sharp-pointed cells with narrow cavities. 
They extend nearly parallel to one another, and diverge from their 
