22 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 
OLNEYA TESOTA. 
Olneya is a small tree of which but a single specimen is growing near 
the Laboratory domain. There is a grove of this species at Robles Pass, 
Tucson Mountains, and another east of Pima Canyon, Santa Catalina Moun- 
tains. These habitats are rocky lower mountain slopes; it does not occur 
in this vicinity on the bottom-lands of the river or on the mesa. 
Branches 1.5 mm., 4 mm., 5.5 mm., 9 mm., 1.15 cm., 1.35 cm., and 
2 cm. in diameter were studied; the sections were cut the following dis- 
tances from the tip: 1, 20, 35, 50, 65, 90, and 120 cm. 
A stem 1.5 mm. in diameter is characterized by an epidermis not well 
defined, by a chlorophyll band that is frequently interrupted by masses of 
collenchyma, and by a relatively narrow inner cortical portion. The wood 
has a large proportion of wood parenchyma. The medullary rays extend to 
the chlorophyll band of the cortex through the gaps in the hard-bast ring. 
In addition to there being chlorophyll in the so-called chlorophyll band 
of the cortex, it is to be found in branches 1.5 mm. in diameter in the 
medullary rays of the cortex, but not in the wood or the pith. In branches 
4mm. in diameter and 20 cm. from the tip, however, chlorophyll was seen 
in the medullary rays of the wood and in the wood parenchyma. 
A characteristic change in the distribution of the chlorophyll in the stem 
and in its relations to various tissues takes place with increase in diameter. 
As the circumference becomes greater the groups of hard bast are pulled 
farther and farther apart, the spaces between are filled with parenchyma, 
and as this tissue is really the distal ends of the medullary rays, the latter 
in older stems become fan-shaped. This condition recalls that observed in 
Celtis and in Prosopis. The chlorenchyma is increased in amount by the 
activity of the cork-cambium also. Periderm is to be seen in stems 9 mm. 
in diameter. It is formed by the subepidermal phellogen, which also gives 
rise to phelloderm that contains chlorophyll. In the older stems,the chloro- 
phyll band is about one-half periderm and one-half primary cortex. 
The chlorophyll early disappears from» the woody cylinder.. Ina branch 
9 mm. in diameter it could be found in neither pith nor wood, and in 
branches 2 cm. in diameter it was confined to the outer portion of the cortex 
and did not appear to be functional. 
The following measurements were taken: 
, Width of | Depth of 
Distance | Diameter | Width of 
from tip. | of branch. | cortex. ie so aa say a 
cm. mm. be & B 
I Ls 160 48 38.4 
20 4 415 80 32 
35 5-5 498 54-4 48 
50 9 780 118.4 70.4 
65 1.15 1079 160 64 
go 1.35 1162 160 64 
120 20 1660 160 96 
