28 TOPOGRAPHY OF CHLOROPHYLL APPARATUS IN DESERT PLANTS. 



In the young- stem the medullarj' rays of the cortex are about one cell 

 wide, but as the hard-bast groups separate from each other with the gfrowth 

 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 modif}' 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 

 sejxiratcs 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 g-roups, 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 

 the entire chlorophyll apparatus 

 is welded into a sing-le tissue. 

 Later, however, the outer band 

 of chlorophyll becomes ag-ain 

 separated from the inner chlor- 

 enchyma by the further devel- 

 opment of the same stony tissue. 

 As in Parkinsonian chlorophj'll 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 ling-ers 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 chlorophjdl 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 chloro]:)hyll 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. 



i c> .Boo a^ 



cam . ^^ _ ^ 



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. 



