12 Raymond John Pool 
source of the odor so characteristic of the species. No micro- 
chemical tests of the contents were made, so that the chemical 
nature of the secretion is not known. 
The greatest development of glands in number and size takes 
place under the most arid conditions and in the highest light in- 
 tensities. This might indicate that there is some correlation be- 
tween the development of epidermal glands and the factors of the 
habitat, although I am not yet ready to indicate the exact forma- 
tive causes. They may be in some measure protective. 
Artemisia frigida. Plate I, fig. 5. 
This plant shows about the same variation in leaf structure as 
that found for its larger sister species. The species belongs to 
the dissected leaf division of the genus. Figure 5, plate 1, shows 
the structure of the leaf of A. frigida from the same situation as 
A. tridentata, the leaf structure of which is shown in figure I. 
The epidermis is covered with a fine canescent coat of single- 
celled, needle-shaped hairs. In addition to this protection a thin 
cuticle is present. There are no epidermal glands. The chloren- 
chyma corresponds closely to that of A. tridentata, being of two 
rows of close prolate palisade cells on each side of a narrow me- 
dian layer of storage cells. The vascular bundles are encased 
in a sheath of storage cells. Plate I, figure 6, shows the structure 
of the same species from a shady brook bank where the holard 
was 19 per cent and the light intensity was considerably lower 
than in the open formation. The plants were taller and the leaves 
broader than those in stronger light. The cuticle is still present, 
but the epidermal hairs are much fewer in number. The palisade 
is not quite so closely aggregated as in figure 5. There is no true 
water storage tissue except as a very narrow sheath about the 
vascular bundles. The median zone of cells here contains chloro- 
plasts, the individual cells being more or less oblate, and they 
apparently perform the functions of ordinary sponge cells. 
Artemisia canadensis. Plate I, fig. 7. 
This material was taken from a rocky hillside where the light 
was unity and the holard was Io per cent. The epidermis is com- 
paratively free of hairs, but the species has compensated for loss 
422 
