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. In a 

 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. As a result 

 the outer band is joined to the 

 medullary rays and practically 



555 



h. b. 



y. 



^yfcJcx^f^: 



tetdtf^' 



C33 



FIG. 14. Prosopis velutina: Segment of stem 

 1.5 cm. in diameter, in which is shown ar- 

 rangement of rings of hard bast and their 

 relation in the chlorophyll apparatus. 



the entire chlorophyll apparatus 

 is welded into a single tissue. 

 Later, however, the outer band 

 of chlorophyll becomes again 

 separated from the inner chlor- 

 enchyma by the further devel- 

 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 in the 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. In a 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. 



