380 



Reed and Smoot : Polygonum virginianum 



phloem bundle there is a group of from five to seven annular and 

 spiral vessels. Between these groups of vessels the space is filled 

 up with wood parenchyma in such a way as to form a continuous 

 cylinder. The woody tissue never, reaches any considerable de- 

 velopment, hence this cylinder is thin-walled even at maturity. 



Just above the separation layer the fibro-vascular bundles branch 

 dichotomously. The outer branches continue their course into the 



persistent calyx, the inner 

 branches meet and fuse 

 directly below the achene. 



(Figs. 2 and 3.) 



A longitudinal section of 

 the pedicel shows that the 

 cells of the pith differ in size 

 and shape from the corre- 

 sponding cells in the stem. 

 The latter were described as 

 being hexagonal in longitu- 

 dinal section and distinctly 

 broader than they were high. 

 The pith cells in the pedicels, 

 on the other hand, are rec- 

 tangular in longitudinal sec- 

 tion and have a length from 



Flr , rv r " ^-i . *. 1 two to six times as & reat as 



t ig. 2. Diagram of a median longitudinal . . \ TVi 



section of a young pedicel shortly after the differ- tlleir width (fig. 5). 1 ne 

 entiation of the separation- layer. A, epidermis; longest Cells are formed near 

 B, cortex; C, sclerenchyma ; D y fibro-vascular the base Q f the pe dicel ; the 



bundles; E, pith; /; separation -layer ; 

 cushion of thin-walled pith-cells. 



/, 



shortest ones near the sepa- 

 ration - layer. Subsequent 



changes in the form and structure of the pith cells will be described 



later. 



The achene is attached to the distal end of the pedicel by a 

 broad base, which does not become weakened as the fruit matures. 

 This accounts for the fact that when the achene is cast off the frac- 

 ture occurs, not at its base, but at the specially developed separa- 

 tion-layer mentioned above. 



The wall of the achene consists of two layers. The outer and 



