166 MEMOIRS 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 uw, 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 uw. The tracheides and 
vessels are more or less prismatic, and range from 20-40 w 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 w 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 «), 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 win length. It may be well to note here that the pith of the see- 
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. 
PoLtyconum 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 u. The periblematic cylinder averages about 90 u. A stereome 
