=. SAY Eh Dakeeke, wae Al a atl ai Sees 
Histological Studies in the Artemisia Formation 9 
The leaves are narrowly cuneate, 1-4 cm. long, varying in size 
and thickness somewhat for the habitat, and are typically three- 
toothed at the truncate apex. Heads of 5-8 yellow flowers are 
very numerous on the shoots of the year. The wood is of a dark 
olive brown color, coarse grained, with conspicuous vessels in 
the spring portion of the narrow annual rings. The wood is not 
strong, is easily split or broken by twisting. The root system is 
usually composed of three to six or more main branches which 
subdivide soon, thus forming a rather narrow root system. 
The rate of growth is very slow. The largest plants at Fort 
Garland were fifty years old. The annual diameter increment 
was about 2.1 mm. (0.12 in.) for this locality. In the Estes Park 
-region where the plants were 6.3 cm. (2.5 in.) in diameter and 
twenty-five years old the annual diameter increment was 2.5 mm. 
(0.10 in.). The plants show a slightly better development near 
streams and irrigation ditches, though not to the degree that 
might be expected, especially when compared with Chrysothamnus 
graveolens, a common associate which makes a much more lux- 
uriant growth near the ditches. The size and the rapidity of 
growth of Artemisia tridentata is not much accelerated even when 
it grows near streams where the holard is considerably higher 
than in the normal habitat. This species is rarely found in the 
saline areas which are rather common in the formation in the 
South. It is replaced in such areas by the spiny chenopod, Sar- 
cobatus vermiculatus. 
The leaf structure coincides well with the common conceptions 
of such xerophytic organs. The leaf is covered both on the upper 
and the lower surfaces with a dense mat of interlocking T-shaped 
epidermal hairs, so protection is almost perfect in this regard. 
The structure of an individual epidermal hair is shown in figure 
9, plate I. These hairs are usually composed of three cells, two 
‘of which constitute the stalk, and the third, the terminal cell, is 
drawn out into the long cross-bar of the T. They rise from a 
single epidermal cell. The stalk cells regularly show the pres- 
ence of well-defined and apparently functional nuclei. The nu- 
cleus of the end cell was also found in a number of cases. The 
cells of the epidermis always contain prominent nuclei with a 
419 
