28 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 
In the young stem the medullary rays of the cortex are about one cell 
wide, but as the hard-bast groups separate from each other with the growth 
of the stem the rays broaden to fill out the resulting gaps until the ends 
are many cells wide. The most striking effect is associated with the pri- 
mary rays. They feel the effects of the growth sooner than the other rays 
and of a consequence the ends of the primary medullary rays are fan- 
shaped and present in cross-section a very striking appearance. From this 
manner of differentiation and development of tissues the amount of chloren- 
chyma in the young cortex is much increased. 
Growth of the stem works also 
to modify the relations of the 
outer chlorophyll band in a 
way that may be noted. Ina 
branch 1.3 mm. in diameter an 
unbroken ring of hard bast 
separates the chlorophyll band 
from the inner cortical tissues. 
In a branch 4.5 mm. in diameter 
the hard-bast ring becomes bro- 
ken up into groups, as was de- 
scribed above. The connecting 
cells at first with thin walls be- 
come, finally, stony tissue and 
contain chlorophyll. Asa result 
the outer band is joined to the 
medullary rays and practically 
the entire chlorophyll apparatus 
is welded into a single tissue. 
Later, however, the outer band 
; ; of chlorophyll becomes again 
Fie, 14-—Prasopit weluing: Segment of ste separated from the inner chlor 
rangement of rings of hard bast and their enchyma by the further devel- 
relation in the chlorophyll apparatus. opment of the same stony tissue. 
As in Parkinsonia, chlorophyll occurs in the medullary rays of the wood, 
in the parenchyma of the wood, and inthe pith. In order of disappearance 
it leaves the pith and the inner medullary rays first; it lingers behind in 
the parenchyma surrounding the ducts. The exact time, however, that 
the chlorophyll leaves the woody cylinder was not learned. Ina branch 
4 cm. in diameter no chlorophyll was to be seen in the wood, and it did not 
extend deeper than 0.5 mm. beneath the surface of the stem. 
The later history of the chlorophyll apparatus is connected with the for- 
mation of bark. This is one of the factors which brings about the changes 
in appearance of the stem which are characteristic of it at different times 
during development. 
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