20 Raymond John Pool 
edge of a stream where the holard was 27.5 per cent. The plants 
in the moister situation made a much more luxuriant growth, the 
leaves were broader and the stems thicker than those from the 
dry soil. ‘ 
Figure 33 represents a whole transverse section of the leaf of 
the dry form. The leaf is free from epidermal hairs, but is cov- 
ered by a fine corrugated cuticle. The chlorenchyma is entirely a 
palisade, the cells being very small and the intercellular air spaces 
being reduced to mere lines. The median storage portion is made _ 
up of globose cells in several layers completely surrounding the 
bundles, and in the region of the midrib they extend laterally to 
each epidermis. Opposite each bundle on the lower side and 
within the storage region is a large resin canal. The development 
of these canals will be described presently. 
The leaf of the moist form (fig. 34) shows some variation 
from the above structure. In the first place it is much broader, 
but not any thinner. The chlorenchyma is composed of few, more 
loosely aggregated palisade cells, and the storage region is con- 
siderably reduced. The storage cells here are oblate and they 
resemble sponge cells rather closely. In the young leaf they 
actually contain chloroplasts. The resin canals are equally typ- 
ical for this form and they occupy the same position. 
4 
THE INTERCELLULAR RESIN CANALS OF CHRYSOTHAMNUS 
GRAVEOLENS 
As stated above, these canals are found singly opposite the 
vascular bundles in the lower portion of the leaf. They traverse 
the leaf and petiole longitudinally as tiny tubes when they are 
mature. They usually acquire a circular transverse section. The 
canals were not traced farther than the petiole, but they probably 
continue into the cortex of the twigs and stems. 
The development was followed from the first appearance to 
the mature canal. The position in the leaf tissue is seen from 
either figure 33 or 34, plate V. There are about four great stages © 
in the development of the canals. Figure 38 shows the structure 
in detail through a vascular bundle and one of the canals. 
These structures are lysigenic in origin, i. e. they arise by the 
430 
