1 70 MEMOIRS FROM THE DEPARTMENT OF BOTANY OF COLUMBIA COLLEGE. 



850 n in length. The starch-ring is made up of elements much larger than those; of the 

 two preceding zones. The cells are prismatic, often quite irregular, varying from 20-50 ,« 

 in diameter, while the average length is about 50 fi ; they are disposed in one row except 

 between the bundles, where they become several rows deep, and here and there at Inter- 

 mediate points we find them either two deep or wanting, in which case the stereome and 

 hypoderma meet. Where the starch-ring interrupts the stereome cylinder it comes into 

 contact with the xylem, and as in the case of P. Pennsylvanicum the starch-ring some- 

 times meets the phloem; in this case, however, the latter is the projecting part. 



A stout sheath of stereids makes up a stereome cylinder and begins the plerome 

 derivatives; the elements are very similar to those of the preceding subgenus in size, 

 shape, and thickness of the wall; the cavities are usually almost obliterated, and the 

 diameter of the cells varies from 10-30 (i, while their length ranges from 200-300 fi ; the 

 septa are either square or oblique. The phloem occurs as small fascicles, oblong in cross 

 section. The amount of this tissue is very small, compared with subgenus Persicaria, 

 and the elements are quite angular; the sieve-tubes measure from 140-210 fi in length. 

 The xylem is strongly developed and similar to that of the preceding, except that the 

 woody fibres and wood-parenchyma are slightly larger. The woody fibres are typical 

 and have, with the exception of some near the stereome cylinder and about the larger 

 elements, their long diameters radial in cross section. Here the tracheides are somewhat 

 shorter than in P. Pennsylvanicum, but the markings are about the same ; Ave find both 

 annular and spiral markings, the former further out in the bundles ; all the spirals are 

 close, and no open nor double ones such as exist in Persicaria were found. 



It is significant that the pith merges into the surrounding tissue through the same 

 kind of cells as it does in P. Pennsylvanicum, and more than this, in most of the sections 

 the parenchymatous cells of the pith are more or less oblong, but here they are almost 

 invariably broader than high and somewhat hexagonal. 



SUBGENUS AVICULARIA. 



Polygonum aviculare. Many species of both prostrate and erect habit are in- 

 cluded in this subgenus. A prostrate plant has been selected for a representative here 

 because most of our other species are of erect habit. The stems are more or less wiry 

 and tough, but are not called on to withstand much strain except at the ends, where 

 they are somewhat assurgent ; however, as growth continues they become prostrate. We 

 find a strong development of tissues here, with the exception of the stereome parts. The 

 epidermis averages about 15 ^u, the derivatives of the periblem 50-80 n, and those of the 

 plerome vary from 200-250 fx. 



