Histological Studies in the Artemisia Formation II 
rows of palisade show a more or less sponge-like nature, stomata 
are fewer in number per unit area, and the epidermal glands are 
almost lacking. 
Figure 4, plate I, shows the structure of the leaf from the same 
species growing in the shade of a cluster of spruce trees in Estes 
Park. The chlorenchyma is even looser here, and the water stor- 
age function has about reached the minimum for the species. 
Soil structure has not left any histological impress on the spe- 
cies. Those individuals growing in the sandy or sedimentary 
soils and those in the granitic soils under the same conditions of 
light and moisture exhibit the same leaf anatomy. The structure 
shown in plate I, figure 1, probably is to be taken as that of the 
-normal leaf of Artemisia tridentata since this is the structure 
found in the more typical situations. 
The epidermal glands are always found in the pits in the epi- 
dermis, and in the natural state they are surrounded and over- 
topped by the T-shaped epidermal hairs. Plate I, figure 8, a, b, 
c, d, shows the gland in position in four stages of development. 
The sections are transverse sections of the leaf; hence the glands 
which are soon cut off by a transverse septum, figure 8, a, are cut 
longitudinally. The gland first appears as an upward protrusion 
of a single epidermal cell. This protrusion extends farther into 
a regular globose or dome-shaped structure, which remains uni- 
cellular until it is about three times the thickness of an epidermal 
cell. About this time a second cell is formed at the tip by a pe- 
culiar bud-like division, figure 8, b. The end cell then continues 
to enlarge at the point of contact with the lower cell, which may 
be termed the stalk cell, until it forms a cap-like covering over 
the distal end of the stalk cell, figure 8, c. By this time the stalk 
cell has elongated considerably and has divided again, so that we 
now have a two-celled stalk covered by a calyptra-like distal cell, 
figure 8,c. Soon the distal cell is divided by radial partitions into 
three or four cells as shown in figure 8, d, and the outermost cells 
have grown down, enclosing the whole stalk except the lowermost 
cell, which always remains free. This is the most common form 
of the gland. The cells of the glands are filled with a dense gran- 
ular, mucilaginous, volatile substance. This doubtless is the 
421 
