STERLING: KRAMERIA CANESCENS GRAY. 369 
between the stem and root is found in the conspicuously large 
and numerous water tubes of the root xylem (fig. 30, plate VI). 
Excepting the tubes of the protoxylem, all of the water tubes 
in roots have bordered pores, which are oblong and lie horizon- 
tally in the thick, lignified walls (fig. 31, plate VI). The water 
tubes vary from 12 to 65 microns in diameter, and have walls 
from 1.5 to 3 microns in thickness. In a root 4.84mm. in diameter 
the area of water-tube cavity amounts to 31 per cent of the 
xylem or 6.8 per cent of the whole root. The total area of 
tracheid cavity amounts to 6.7 per cent of the xylem or 1.3 per 
cent of the whole root. Combining these amounts gives a 
total for both water tubes and tracheids of 37.7 per cent of the 
xylem, or 8.1 per cent of the whole root, devoted to the carry- 
ing and storage of water. Comparing this data with that given 
above for a stem 4.21 mm. in diameter, and having a total 
capacity for both water tubes and tracheids amounting to 
25.25 per cent of the xylem, or 8.75 per cent of the whole stem, 
shows that as a whole the stem has the greater capacity for 
holding water. 
THE LEAF. 
The small, narrow, simple leaves are developed in seasons 
favorable for growth, but in seasons of excessive drought they 
may be entirely wanting. They are bifacial and have the 
stomata standing longitudinally at right angles to their long 
axes (figs. 39 and 40, plate VIII). The stomata, averaging 95 
on a square millimeter, lie in the same plane with the cells of 
the epidermis (fig. 42, plate VIII). 
The epidermis, closely resembling that of the stems, bears 
numerous unicellular trichomes, which give to the leaves a fine 
silky appearance (fig. 47, plate VIII). The trichomes are 
usually bent near the base, so that they lie close to the surface 
and make a thick covering over the entire leaf surface. They 
vary in length from 150 to 600 microns, and, like the trichomes 
of the stem, are thick-walled and have the lower portions 
cutinized, while the upper portions are but weakly, or not at 
all, cutinized. Although the trichomes on the leaves are very 
numerous, averaging 450 to 500 on a square millimeter, the 
number is exceeded by the stems, which bear an average of 850 
on a square millimeter. ; 
Lying beneath the epidermis, and extending around the en- 
2—Univ. Sci. Bull., Vol. VI, No. 3. 
