166 MEMOIES FROM THE DEPARTMENT OF BOTANY OF COLUMBIA COLLEGE. 



many-sided and closely packed, and the angles are either obtuse or somewhat rounded. 

 The smallest and strongest stereids exist just at the outside of the bundles. Here the 

 thickness of the wall or that of the two contiguous walls exceeds the diameter of the 

 cavity ; indeed many of the cavities are almost obliterated and only between the bundles 

 do the cells become larger and thinner-walled. At these places, the stereome often inter- 

 rupts the xylem and meets the pith. The septa are almost invariably transverse. The 

 phloem occurs as fascicles between the stereome and xylem ; in cross section it appears 

 as either rounded or linear areas, and in the specimens studied it either remained as very 

 angular cells (in cross section) or it had all disintegrated leaving only large cavities. 

 There is a variable development of xylem. The woody fibres are typical, varying in 

 length from 150-400 ^u, and the ends taper in the great majority of cases. In cross 

 section these elements appear as square or oblong, in which case their longer axes are 

 radial and the diameters usually keep within the bounds of 5-10 fj.. The tracheides and 

 vessels are more or less prismatic, and range from 20-40 [i in diameter. They especially 

 characterize the region just external to the protoxylem. These elements are surrounded 

 by either wood-parenchyma or woody fibres. The tracheides average about 400 l^ in 

 length, and have transverse or oblique septa. They are not plentiful and usually have 

 small diamond-shaped border-pits. The arrangement of the tracheides and vessels in 

 this subgenus generally appears as represented in the accompanying plate. Toward the 

 outside of the bundles we find spiral vessels, the spiral markings being either close or 

 open. Further in are small vessels with heavy annular markings, while still deeper we 

 find vessels which begin with annular markings which grade into spirals. 



The wood-parenchyma, except near the pith, consists of long cells (200-350 fi), which 

 sometimes almost rival the woody fibres in length, but they usually have square ends 

 and thin walls. At the depressed semi-circular inner sides of the collateral bundles the 

 cells of this tissue are larger, but shorter, and gradually or abruptly pass into the pith. 

 The parenchymatous cells of the pith are either parallelopipeds or rarely cubes, and 

 measure from 70-150 ^ in length. It may be well to note here that the pith of the sec- 

 tion illustrated is from the vicinity of a node and therefore more angular than it would 

 appear if taken from the middle of an internode. 



SUBGENUS ACONOGONON. 



Polygonum alpinum. In this case we have a plant that agrees with the preceding 

 species in habitat, but instead of being simple the stem is more or less branched, as well 

 as the inflorescence, and therefore the axes are subjected to greater strain. The epidermis 

 varies from 20-30 ^. The periblematic cylinder averages about 90 fi. A stereome 



