We MEMOIRS FROM THE DEPARTMENT OF BOTANY OF COLUMBIA COLLEGE. 
parenchyma, open spirals appear. There may be either double or single spirals, and as 
was the case in other subgenera, the single spirals are always the heavier. The wood- 
parenchyma grades into the pith; it is plentiful about the inner sides of the bundles, and 
its cells have a length of 200-300 «. 
The pith differs somewhat in character from any described under the foregoing sub- 
genera; the cells are much more elongated, and their form is either cylindric or pris- 
matic, averaging about 200 « in length, or three to five times as long as broad. 
SUBGENUS DURAVIA. 
Potyconum Cairornicum. ‘This section, containing the smaller plants of the 
genus, exhibits a very peculiar structure, differing in some respects from all the others. 
The plants are low, slender and wiry, and often seem to strive to spread and grow erect 
at the same time; they have, however, a strong anatomical structure, but this strength 
seems unnecessary. 
The epidermis averages about 15 uw. ‘The primary cortex ranges from 10-90 uw, this 
great variation being caused by the existence of wings and ridges which will he discussed 
later. The xylem varies from 40-100 «. . 
Here, as in the case of P. alpinum, we find strikingly prismatic epidermal cells, and 
here, too, their long axes are usually radial in cross section. The minute size of the stem 
will be understood when the accompanying plate is compared with the others, for it is 
drawn to the same scale. The cells of the epidermis in P. Californicum exceed some and 
rival others of those of much larger plants; their length is from 40-60 u. Depressed 
conical trichomes are borne on some of the cells, and appear almost like papillae. 
As a second peculiarity we meet a curious form of primary cortex. The cells out- 
side of the starch-ring are flat-lenticular and placed with their long axes radial. Their 
tangential diameter averages about 5 u, while the radial varies from 10-30 4. In long 
section they appear to be arranged more or less obliquely; this seems to depend, however, 
on the rate of growth of the surrounding parts. We observe a modification in this species 
seen only in the one preceding: at four or more points (usually four) a bundle of scleren- 
chymatous cells exists just outside of the starch-ring. Opposite to these bundles the 
primary cortex is thickest and forms ridges or wings. The elements of these bundles in 
long section appear to be the same as those composing the stereome in other subgenera ; 
their diameter is a little less than 10 w, and the length varies from 80-120 w; the septa are 
either transverse or oblique. One layer of oblong-prismatic cells makes up the starch-ring ; 
their length ranges from 20-30 u, and their breadth is about one-half of their length. 
Small and indistinct fascicles of phloem exist in their normal places inside of the 
