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. torreyana was examined which was 2.25 cm. in diameter and which, 
nevertheless, still had chlorophyll in the epidermis. It may be remarkedin 
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 eee eae 
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 i fe 
the outer cortical chlorophyll band. i i i i 
Also, this chlorophyll tissue is the Scio : i? ee 
most enduring. It has been iden- 37 Jtcim'y mm. in diameter, to show the 
tified -in stems 8 cm. in diameter, distribution of chlorophyll. Lettered as in 
and is present in the oldest parts, preceding figures. 
even in-some or perhaps most instances within a few centimeters of 
the very base ofthe tree. It varies in width from 83 # to 246 » 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 
