18 



TOPOGRAPHY OF CHLOROPHYLL APPARATU.S IN DESERT PLANTS. 



Except in the very yoiing-est branches, /. i\, those less than 1.13 mm. in 

 diameter, no chlorophyll occurs either in pith or wood, but in a section of 

 this diameter it Avas 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 activit\- of the cambium make imi^ortant 

 modifications in the chlorophyll apparatus as above described. The cork 

 cuts off all tissue exterior to the ring- of hard bast. There does not api)ear 

 to be a definite cork-cambium, but the cortical cells are directly converted 

 into cork. About the time cork is formed the parench>-matous cells within 

 the hard-bast ring- become much enlarged, the chlorophyll content is 

 g-reatly 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 ring-s 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 chlorench\-ma Franscria 

 is peculiar among the plants observed. 



The following- measurements were made: 



KcEHERLiNiA SPiNOSA. (Plate 3, A, and fig. 8.) 



Kirlnr/liiia, leafless excei)t in seedling stage, occurs as isolated plants 

 mainly in the bottom-lands of the river. It avoids for the most jxirt the 

 dry slopes of the moimtains and the mesa. The plant studied is growing- 

 near the southeast comer of the cemetery at Tucson. It is about 1.5 m. 

 high and extends horizontally, so that the diameter of the shrub may perha]xs 

 be 3 m. The shrub has in consequence a squat appearance. 



In structure Kccberlinia 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 m 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 btninded on its inner surface by 



