24 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 



distribution, but in older stems, owing to changes in structure incident to 

 growth and development by which the various chlorophyll-bearing tissues 

 are eliminated or lose their chlorophyll contents, this distribution is greatly 

 modified. 



The epidermis is usually or at least frequently well supplied with chloro- 

 phyll. This applies to stems 1 cm. or less in diameter, although a branch 

 of P. tor r cyan a was examined which was 2.25 cm. in diameter and which, 

 nevertheless, still had chlorophyll in the epidermis. It may be remarked in 

 passing that this branch showed another characteristic which is unusual in 

 Parkinsonia the woody cylinder did not contain chlorophyll. As will 

 appear later, in the ordinary se- 

 quence of the disappearance of 

 chlorophyll from the stem, the epi- 

 dermis leads, followed by the pith 

 and the wood. 



The most prominent mass of 

 chlorophyll-bearing tissue in the 

 stem, and the one that gives the 

 color characteristic of the tree, is 

 the outer cortical chlorophyll band. 

 Also, this chlorophyll tissue is the 

 most enduring. It has been iden- 

 tified in stems 8 cm. in diameter, 

 and is present in the oldest parts, 

 even in some or perhaps most instances within a few centimeters of 

 the very base of the tree. It varies in width from 83 /" to 246 p- and its outer 

 surface lies from 83 /* to 500 /* beneath the surface of the stem. In structure 

 the chlorophyll band is wholly of spongy tissue. The cells are cuboid and 

 thin- walled. 



Within the outermost band of chlorenchyma is a ring of mechanical tissue 

 composed of alternating groups of hard bast and of heavy-walled paren- 

 chyma (which later become grit-cells?). A second band of chlorenchyma 

 lies immediately within this mechanical stratum, which for convenience 

 will be termed the median band of chlorenchyma. In the younger stems the 

 median band is practically continuous, but in the older ones it becomes 

 broken up into distinct masses. From the median band there passes inward, 

 like the spokes of a wheel, the medullary rays of the inner part of the cortex. 

 These rays in the younger branches are well supplied with chlorophyll. 



Turning now to the woody cylinder, we find that the medullary rays, a 

 portion of the wood parenchyma, and the pith are chlorophyll-bearing. In 

 branches 1 cm. in diameter the entire medullary ray from cortex to the pith 

 is so well provided with chlorophyll that the cut end of the branch under 

 a hand-lens appears grass green. In much larger stems, however, and in 

 smaller ones from a less healthy plant no chlorophyll, or scarcely any, is to 



FIG. ii. Parkinsonia torreyana: Segment 

 of stem 7 mm. in diameter, to show the 

 distribution of chlorophyll. Lettered as in 

 preceding figures. 



