MEMOIRS FROM THE DEPARTMENT OF BOTANY OF COLUMBIA COLLEGE. 175 



80-100 fi in length ; their shape is a little more regular than that of the elements of the 

 preceding tissue and often approaches a cylindric form. 



The stereome cylinder is strong and continuous, varying from three to five cells deep 

 and from 80-150 ^ in thickness. The stereids are prismatic, sometimes tapering, many- 

 sided and, as a rule, large, some reaching 50 ^ in diameter; 250-350 ^ covers their 

 length, and the septa are oblique. The large ones between the bundles are often incon- 

 spicuous both on account of the width of the cells and the extent of their cavities, while 

 in others the cavity is almost obliterated. The stereome is mostly followed by the xylem, 

 but small fascicles of phloem do occur in their normal positions. These fascicles measure 

 from 40-100 ft in diameter and often contain only a dozen elements. In favorable long 

 sections the sieve-tubes are distinct, varying from 150-250 fj. in length. As the xylem 

 appears in cross section, one would expect to find the tracheae strongly marked ; the 

 woody fibers are remarkably large, thick-walled and short ; in cross section they appear 

 as square or oblong cells (long axes tangential) ranging from 10-40 ^ in diameter; the 

 walls and cavities are usually of about the same thickness ; in long section some of the 

 tracheids appear as vermiform fibres ranging from 150-250 fi in length but strongly 

 marked with border-pits. No large tracheides were observed in the stem sectioned, but 

 vessels were plentiful ; they are of irregular prismatic shapes and placed in rather an irre- 

 gular manner ; their diameter ranges from 20-60 fi, and we find large and small spiral, 

 large annular and scalariform vessels. Wood-parenchyma is almost abundant towards 

 the inner sides of the bundles, and is usually quite distinctly differentiated from the 

 pith. A row of cells similar to those in subgenera Perbicaria and To vara separate 

 these two tissues. 



A regular and large-celled pith occurs. The cells are oblong prisms with a diameter 

 of from 40-120 [i and a height varying from 150-200 [i. 



Grouping the subgenera, according to their anatomical structure, we can bring the 

 eight under five divisions. First : We see that F. bistortoides and P. alpinum, represent- 

 ing subgenera Bistorta and Aconogonon, agree in these respects : the primary cortex is 

 much the same, although that of P. alpimom is somewhat more, angular ; the stereome 

 cylinder is continuous in both, and its elements in cross section have a more rounded 

 and less angular appearance than in any other subgenus ; phloem appears in the same 

 kind of fascicles ; the xylem is practically alike in both, its tracheides and vessels exist in 

 about the same position, and the inner sides of the bundles have the same semicircular 

 shape and pass into the pith through a row of similar parenchymatous cells. 



Second : The next natural grouping brings in P. Pennsylvankum and P. Virgini- 



