22 



TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 



Olneya TESOTA. 



OIncya is a small tree of whieh but a sin.i;-le specimen is gTowins" near 

 the Laboratory domain. There is a grove of this speeies 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 

 collench^-ma, and by a relatively narrow inner cortical portion. The wood 

 has a largfe 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 

 4 mm. 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 parench\'ma, 

 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. 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: 



Width of 

 cortex. 



Width of Depth of 



chlorophyll i chlorophyll 



band. band. 



415 

 498 

 780 

 1079 

 1 162 

 1660 



48 



So 



54-4 

 118. 4 

 160 

 160 

 160 



