18 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS.. 



Except in the very youngest branches, /. e., 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 chlorenchyirui and hard-bast ring's are formed, which 

 eventually replace the secondary ehlorenehyma much as the latter has 

 replaced the primary ehlorenehyma. 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 ehlorenehyma I'ranseria 

 is peculiar among the plants observed. 



The following measurements were made: 



SPINOSA. (Plate 3, A, and fig. 8.) 



l\irbcrlinia, 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 3 m. The shrub has in consequence a squat appearance. 



In structure K<cberlinia 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 

 very 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 



