Reed and Smoot : Polygonum virginianum 379 



as in those species, like P. alpinum, which grow in dry situations, 

 but is better develeped than in species like P. scandens, which grow 

 in wet localities. In comparison with the other species, the 

 stereome cylinder of P. virginianum is compact and well developed. 

 The elements are small and very thick-walled, therefore they may 

 be regarded as contributing much of the rigidity necessary to sup- 

 port these tall, slender stems. 



The phloem bundles, although small, do not disintegrate as 

 they do in some other species. The xylem forms a comparatively 

 wide zone and contributes a great deal of mechanical strength to 

 the stem. It consists of five different elements, namely, woody 

 fibers, tracheids, spiral vessels, annular vessels, and w r ood paren- 

 chyma which gradually merges into the pith. 



The pith consists of large cells, hexagonal in longitudinal sec- 

 tion. Unlike the same kind of cells in other species, they are 

 usually broader than they are high. 



In a general way, the form and structure of the tissues in the 

 pedicel are similar to those in the stem. Tne most constant dif- 

 ference between them is that some of the individual elements of 

 the pedicels are more elongated than the corresponding elements 

 in the stems. About 0.75 mm. below the base of the achene, 

 the tissues of the mature pedicel are interrupted by a crescent- 

 shaped " separation-layer/' It is by virtue of this layer that the 

 fracture is readily effected when the achenes are thrown off. Its 

 characters will be more fully discussed later. The cells of the 

 epidermis, which at first are parallelopipeds, like those of the 

 cauline epidermis, become stretched from four to six times their 

 original length as the pedicel develops. 



The zones of stereome and woody tissue are reduced in width 

 and number of elements. The zone of stereome tissue is usually 

 thicker on the concave side of the pedicel than on the convex side 

 (fig- 3)- In all probability this may be regarded as a regulatory 

 thickening such as Newcombe ('95) described for mechanical tis- 

 sues in general. 



The fibro-vascular tissue never forms a zone of any considera- 

 ble thickness. There is no evidence that the phloem bundles ever 

 disintegrate, but are continuous throughout the pedicel, although 

 they decrease in size beyond the separation-layer. Opposite each 



