MEMOIRS. FROM THE DEPARTMENT OF BOTANY OF COLUMBIA COLLEGE. 175 
80-100 w in length; their shape is a little more regular than that of the elements of the 
preceding tissue and often approaches a cylindric form. 
The stereome cylinder is strong and continuous, varying from three to five cells deep 
and from 80-150 win thickness. The stereids are prismatic, sometimes tapering, many- 
sided and, as a rule, large, some reaching 50 « in diameter; 250-350 w covers their 
length, and the septa are oblique. The large ones between the bundles are often incon- 
spicuous both on account of the width of the cells and the extent of their cavities, while 
in others the cavity is almost obliterated. The stereome is mostly followed by the xylem, 
but small fascicles of phloem do occur in their normal positions. These fascicles measure 
from 40-100 uw in diameter and often contain only a dozen elements. In favorable long 
sections the sieve-tubes are distinct, varying from 150-250 w in length. As the xylem 
appears in cross section, one would expect to find the tracheae strongly marked; the 
woody fibers are remarkably large, thick-walled and short; in cross section they appear 
as square or oblong cells (long axes tangential) ranging from 10-40 w in diameter; the 
walls and cavities are usually of about the same thickness; in long section some of the 
tracheids appear as vermiform fibres ranging from 150-250 w in length but strongly 
marked with border-pits. No large tracheides were observed in the stem sectioned, but 
vessels were plentiful; they are of irregular prismatic shapes and placed in rather an irre- 
gular manner; their diameter ranges from 20-60 uw, and we find large and small spiral, 
large annular and scalariform vessels. _Wood-parenchyma is almost abundant towards 
the inner sides of the bundles, and is usually quite distinctly differentiated from the 
pith. A row of cells similar to those in subgenera PrrstcartA and Tovara separate 
these two tissues. 
A regular and large-celled pith occurs. The cells are oblong prisms with a diameter 
of from 40-120 « and a height varying from 150-200 w. 
Grouping the subgenera, according to their anatomical structure, we can bring the 
eight under five divisions. First: We see that P. bistortoides and P. alpinum, represent- 
ing subgenera Brsrorra and AconoGonon, agree in these respects: the primary cortex is 
much the same, although that of P. a/pimum is somewhat more angular; the stereome 
cylinder is continuous in both, and its elements in cross section have a more rounded 
and less angular appearance than in any other subgenus; phloem appears in the same 
kind of fascicles; the xylem is practically alike in both, its tracheides and vessels exist in 
about the same position, and the inner sides of the bundles have the same semicircular 
shape and pass into the pith through a row of similar parenchymatous cells. 
Second: The next natural grouping brings in P. Pennsylvanicum and P. Virgini- 
