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 circ-umference 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 

 Ccltis and in Prosopis. The chlorenchyma is increased in amount by the 

 activity of the cork-cambium also. Peri derm 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. In a 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: 



