20 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 
and more of the surface of the stem becomes covered with cork the breadth 
of the chlorophyll band decreases until in the older portions it likely quite 
disappears. During these processes very little phelloderm is organized, 
and exactly how the formation of cork cuts off the chlorophyll band was 
not determined. It is probable, however, that the secondary phellogen 
strikes deeply into the cortex and cuts out segments of the band. Asa 
result of this kind of cork formation the bark has a characteristic scaly or 
shaggy appearance. 
Chlorophyll disappears from the stem rather early. Ina branch 1.5cm. 
in diameter it was barely demonstrable in the inner cortex, although the 
cortex was only 0.75 mm. in thickness. 
The following measurements were made: 
‘ - . Width of | Depth of 
Diameter | Distance Width of = Thickness 
of branch. | from tip. | cortex. cpt ovens: io eae ha N) of cuticle. 
mm cm. Leg & BK be 
3.5 5 381.8 144 80 80 
4-5 20 415 112 88.4 88.4 
4-5 35 464.8 192 96 96 
8 45 747 112 64 64 
15 60 747 192 80 80 
KRAMERIA CANESCENS. (Plate 1,4, and fig. 9.) 
Krameria occurs on the mesa and low hills as scattered individuals; rarely 
are there several of them growing together in groups. The plant selected 
for study is growing on the aerial mountain delta and below to the west of 
the Laboratory. The shrub attains a height of about 50 cm.; the branches 
spring from the base, which may be either the primitive stem or the bases 
of older branches which have died. The newer growth is covered with hairs 
and beneath the hairy covering by its green color the stem shows that it con- 
tains chlorophyll. During dry seasons the branches are bare of foliage, but 
in favoring times they are fairly well provided with small, narrow leaves. 
Branches of a plant about 50 cm. high were studied, and as usual sections 
were taken from several at measured intervals from the tip. In cross- 
section a branch 1.5 mm. in diameter and 5 cm. from the tip presents five 
well-defined regions, namely, the epidermis with its rather thin cuticle, a 
broad subepidermal chlorophyll band, the inner portion of the cortex with 
its groups of hard bast and occasional crystal-bearing cells, and the wood 
and the pith. Regarding the wood and the pith there is nothing note- 
worthy from the present point of view. 
Chlorophyll is to be found from the epidermis to the pith in practically 
all of the parenchyma. It occurs sparingly in the epidermis in stems 
2.5 mm. in diameter, although it was not seen in one 1.5 mm. in diameter. 
Parkinsonia also has chlorophyll in the epidermis of the young branch; this 
is an unusual condition in the desert perennials. The most important 
