168 MEMOIRS FROM THE DEPARTMENT OF BOTANY OF COLUMBIA COLLEGE. 
sented in the accompanying plate. The tracheides usually have oval border-pits, and 
vary in length from 250-350 u. Three kinds of vessels are here represented: first, those 
with annular markings, which either continue as such or merge into spirals; second, close 
spirals, and third, scalariform. These are sometimes accompanied by woody-fibres or by 
wood-parenchyma; the latter constitutes the great bulk of the inner sides of the bundles, 
and in cross section appears as large semicircular accumulations of round prismatic cells, 
which gradually or abruptly merge into the pith. The pith is composed of large pris- 
matic cells, 70-200 w long and with four to eight sides, having a maximum diameter of 
120 w; numerous resin canals occur in this tissue. 
SUBGENUS PERSICARIA. 
PotyGonuM PENNsyLVANIcUM. The species of this subgenus are usually much 
branched, thus bringing more or less strain on the main axes. To resist this strain and 
to maintain an erect position, we find a strong development of strengthening tissues. 
The epidermal and periblematic cylinders range from 120-150 u, while the pleromatic 
parts are especially stout, reaching a thickness of over 400 w. 
A simple epidermis occurs, although in some sections it appears to be double, but a 
long section of the same part shows the apparent second layer to be composed of cells of 
the primary cortex which have become somewhat larger than the rest. The epidermal 
cells are parallelopipeds, 5-15 uw thick, 30-50 w broad and 50-70 w high. 
The primary cortex is strongly developed, consisting in the specimen examined of 
from seven to ten layers of small oblong-lenticular cells, those near the epidermis being 
usually larger than the rest, as above stated; the cells vary in diameter from 5-20 « and 
in length from 20-40 uw. There is a great contrast between the hypoderma and the starch- 
ring; in this the cells are very large, measuring from 10-70 w in diameter and 70-120 4 
high; they are either square-angled parallelopipeds or irregular prisms; their normal 
distribution is in one layer, but between the bundles they break through the stereome 
cylinder and come into contact with the phloem, thus increasing to several layers. The 
stereome is strong here, but rather peculiarly arranged, and will be mentioned again in 
discussing the xylem. The stereids are in the form of long prisms, with transverse or 
oblique ends, ranging from 200-300 «. Their diameters vary from 15-20 u, and in most 
cases the walls are so thick that they almost obliterate the cavities. The amount of 
phloem just inside the stereome cylinder is remarkable; in the stem sectioned the zone 
was nearly continuous and measured from 40-100 u; the elements are variable in size, 
some being only 5 w in diameter, others reaching 15 wor more. In long section the sieve 
tubes appear to range from 150-250 uw in height, and the septa are either transverse or 
