. Carnegie Institution of Washington publication. 24 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT 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 rcyan a was examined which was cm. in diameter and which, nevertheless, still had chloroph
. Carnegie Institution of Washington publication. 24 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT 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 rcyan a was examined which was 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
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