18 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 
Except in the very youngest branches, 7. ¢., those less than 1.13 mm. in 
diameter, no chlorophyll occurs either in pith or wood, but in a section of 
this diameter it was observed in both. It occurred in the outer cells of the 
pith and in the primary medullary rays of the wood, as well as in paren- 
chyma between the ducts. 
The formation of cork and the activity of the cambium make important 
modifications in the chlorophyll apparatus as above described. The cork 
cuts off all tissue exterior to the ring of hard bast. There does not appear 
to be a definite cork-cambium, but the cortical cells are directly converted 
into cork. About the time cork is formed the parenchymatous cells within 
the hard-bast ring become much enlarged, the chlorophyll content is 
greatly increased, and these cells replace in function the primary chloro- 
phyll band, which has become cork. Through the activity of the cambium 
more deeply placed chlorenchyma and hard-bast rings are formed, which 
eventually replace the secondary chlorenchyma much as the latter has 
replaced the primary chlorenchyma. The exfoliating process appears to 
be repeated several times, until in the oldest portions of the branch the 
portions cut off and those reformed no longer contain chlorophyll. In this 
repeated formation, destruction, and reformation of chlorenchyma /yranseria 
is peculiar among the plants observed. 
The following measurements were made: 
. epth of 
Diameter Jeane ee color: Width of 
of branch. | From tip. oo cortex. 
; mm cm. B& & 
f 1.13 1.5 Yh er cre 
1.23 5 26 294 
1.67 II 126 525 
2.46 15 231 588 
1 
K:BERLINIA SPINOSA. (Plate 3, A, and fig. 8.) 
Keberlinia, leafless except in seedling stage, occurs as isolated plants 
mainly in the bottom-lands of the river. It avoids for the most part the 
dry slopes of the mountains and the mesa. The plant studied is growing 
near the southeast corner of the cemetery at Tucson. It is about 1.5 m. 
high and extends horizontally, so that the diameter of the shrub may perhaps 
be 3m. The shrub has in consequence a squat appearance. 
In structure Keéerlinia shows several striking characters. A cross- 
section of a branch 3.5 mm. in diameter and 5 cm. from the tip has in the 
cortex four well-marked regions. It is bounded by an epidermis with a 
vety heavy cuticle, from 80 to 96 » thick, which is pierced by stomal canals. 
Immediately beneath the epidermis and reaching to it is a band of chlorophyll 
nearly 0.2 mm. in breadth. This band is bounded on its inner surface by 
