1922] HYDE—GALL ON POPULUS 189 
course only in weaving around the medullary rays. The wood fibers 
are perforated with numerous, almost circular bordered pits. 
In cross-section the fibers are nearly square in outline, and have 
relatively thin walls. The walls are somewhat thicker in the sum- 
mer wood than in the spring wood. The fibers, according to 
REcorD,? have a maximum length of 1.90 mm., a minimum length 
of o.50 mm., and an average length of 1.15 mm. 
Bark.—The bark as herewith considered includes all that por- 
tion of the stem outside the cambium layer, and is composed of 
epidermis, cortex, and phloem. 
The epidermis of young twigs is smooth and colorless. Beneath 
this is found a layer of parenchyma, from four to seven cells thick, 
encircling the stem. As the stem becomes older the outer tissues 
disappear and are replaced by suberized tissue produced through 
the activity of the phellogen. In the case of normal twigs this cork 
tissue never becomes very thick. The fibers are thick-walled ele- » 
ments, with sharp pointed, unbranched ends. They are arranged 
in groups which are in turn arranged in bands that extend around 
the stem, concentric with the cambium ring. 
The phloem is composed of sieve tubes, companion cells, and 
phloem parenchyma. In many of the cells comparatively large 
spherical crystals are found. 
The pith rays extend into the phloem. They are uniseriate, but 
broaden considerably as they progress into the cortex. This broad- 
ening is due to the fact that the cells are thickened tangentially, and 
not to any increase in number of rows of cells. 
Piru.—The central pith of the stem is composed of cylindrical 
cells with thin walls. The pith area is about 0.8 mm. in diameter, 
and varies in color from a light gray to a reddish brown. Many of 
the cells contain starch and crystals. The crystals are spherical in 
shape, and never more than one is found within a cell. The outline 
of the pith area in cross-section is usually distinctly five-angled, but 
all gradations between this and a circular outline occur. 
CamBium.—The cambium of this tree is similar to that of other 
dicotyledons. It consists of a layer of thin-walled, delicate, tabular 
2 Recor, S. J., Economic woods of the United States. New York: Wiley and 
Sons. 1919. 
