1G8 MEMOIRS FROM THE DEPARTMENT OF BOTANY OF COLUMBIA COLLEGE. 



seiited in the accompanying plate. Tiie tracheides usually have oval border-pits, and 

 A-ar}' in length from 250-350 (.i. Three kinds of vessels are here represented : first, those 

 Avith 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 

 A\-ood-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 (/ long and with four to eight sides, having a maximum diameter of 

 120 u ; numerous resin canals occur in this tissue. 



SUBGENUS PERSICARIA. 



PoLY(;oNiTj[ Pennsylvank'um. 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 fi, while the pleromatic 

 parts are especially stout, reaching a thickness of over 400 fj.. 



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 parallelepipeds, 5-15 ft thick, 30-50 n broad and 50-70 fi 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 [i and 

 in length from 20-40 fi. There is a great contrast between the hypoderma and the starch- 

 ring; in this the cells are very large, measuring from 10-70 fi in diameter and 70-120 fi 

 high; they are either square-angled parallelepipeds 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 fi. Their diameters vary from 15-20 (j., 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 tiie stem sectioned the zone 

 was nearly continuous and measured from 40-100 [i; the elements are variable in size, 

 some being only 5 fi in diameter, others reaching 15 f^ or more. In long section the sieve 

 tubes appear to range from 150-250 fi in height, and the septa are either transverse or 



