Apr., 1921] 
MCNAIR — RHUS DIVERSILOBA 
183 
studying Rhus Toxicodendron L., and Inui (10), studying R. Toxicodendron 
var. radicans, were unable to discover resin ducts in the pith. Jadin (11) 
cited 18 species of the genus which are provided with permanent pith resin 
ducts, and 9 species which do not have them. 
At the periphery of the pith the small outer cells acquire a thick wall 
and become sclerenchymatous. These thick-walled cells may assist the 
inner large-celled and the outer small-celled pith to maintain a circular 
outline. A semi-circular row of bast fibers lies external to the primary 
phloem and serves mechanically to protect the phloem with its resin ducts 
from external injury. 
In the phloem of the second year, new resin ducts appear. These lie 
neither in radial nor in tangential rows, but are so arranged as to be very 
nearly equidistant. The first appear in the secondary phloem between 
two primary resin ducts, and more are formed in a corresponding manner. 
It must not be forgotten, however, that the formation of resin ducts does 
not occur in a regular manner. 
New bast fibers do not appear to be formed in the pericycle. The 
epidermis has been almost wholly lost in the second year and is replaced 
by cork. 
The histology of the pith, wood, and bark of the older stems will be 
treated individually. 
The pith cells are polygonal and lie close together; they are generally 
wider than high, so that their vertical measurement is the smallest. In the 
specimens examined, the pith cells contained for the most part no particular 
substance; starch was found sparingly, and tannin sacs appeared as narrow, 
elongated cells. Tannin sacs, according to Engler (5), appear abundantly 
in the pith of the Anacardiaceae and in all species of Rhus which he investi- 
gated. Pith tannin sacs are not necessarily characteristic of toxic species of 
Rhus, as Mobius (22) was unable to find them in R. vernicifera L. 
The bulk of the wood consists of simple pitted wood fibers. In trans- 
verse section they are bordered at right angles, and are assembled in rows. 
The narrower and thicker-walled cells of the fall wood contain starch; the 
wider and thinner-walled ones of spring wood appear empty. 
The pits of the tracheal vessels are exclusively simple with circular or 
elliptical outlines. The walls are relatively thick. The structure of the 
vessel wall, where it is in contact with wood parenchyma, is characteristic. 
In these places simple pits of large size are found chiefly on the vessel wall, 
and, side by side with them, either transitional or true bordered pits, but 
no separate bordered pits were noticed. The elliptical pits are transverse 
to the longitudinal axis of the vessel and parallel to one another, so that 
they remind one of scalariform perforations. 
The medullary rays are, as a rule, uniseriate; sometimes, however, they 
are biseriate. In tangential longitudinal section they are from three to 
eighteen cells high; radially their cells are joined together as are the stones 
