34 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 
then from the medullary rays of the wood, beginning in the inmost portions; 
then from the wood parenchyma; after this, from the medullary rays of the 
cortex; and, lastly, from the chlorophyll band, or, more usually, the band 
itself is obliterated. In the exceptional event of chlorophyll in the epidermis 
of the younger stem, as in Parkinsonia, this may leave the stem before any 
other tissue is deprived of its chlorophyll. Generally speaking, however, 
with the exception of the wood parenchyma, the chlorophyll of the stem 
disappears in a centrifugal fashion. Possibly the exception is due to some 
favoring condition, as the proximity of better air-supply, or more water, 
or the light may be condensed by the water-content, so that a portion of the 
contiguous parenchyma may have better light relations than that in the 
medullary rays. 
The chlorophyll band remains active until its organic connection with 
the inner living portions of the cortex is severed by the formation of cork, 
_ or until destroyed by pressure occasioned by growth of the stem. In 
some forms, as Cereus and Parkinsonia, as arule, itis never destroyed during 
the life of the plant, but persists and gives the color characteristic of each. 
SPECIAL STRUCTURAL FEATURES. 
We may turn now from a review of what may be called the general con- 
dition of the stem as regards the distribution of chlorophyll and take note 
of phases of the distribution and other characters of the chlorophyll-bearing 
tissues not shared by all of the plants examined. 
Attention may first be called to a curious modification of the chlorophyll 
band which frequently accompanies increase in diameter of the stem. This 
is associated with the formation of cork. The chlorophyll band, properly 
so-called, is an integral portion of the primary cortex; in old stems, how- 
ever, the outer part of the band may be morphologically secondary cortex. 
This circumstance occurs in the following manner: When cork is formed, 
it is likely to take its origin in cells exterior to the chlorophyll band and 
very close to it. By the activity of the phellogen periderm is organized 
without and phelloderm within in the usvfal fashion and in the cells consti- 
tuting the phelloderm chlorophyll may be found. Consequently it happens 
that in older stems of certain plants, as in Celtis pallida, fig. 3, the outer 
portion of the chlorophyll band, in addition to being of different origin, may 
be more recently organized than the inner portion. There appears to be 
a limit to the thickness of the chlorophyll band brought about by this 
means, however, as the portion of the band which is of secondary origin 
has never been seen to be more extensive than the primitive part. 
Another point which has to do with the structure of the chlorophyll band 
relates to the similarity in some cases and the dissimilarity in others of 
the structure of the band in the stems and the structure of the chlorenchyma 
of the leaves of the same plant. In all plants studied palisade tissue of 
some sort was found in the leaves, but in part only of the plants was the 
