1'ARKINSONIA. 



25 



be found in the wood. The wood parenchyma in the immediate vicinity 

 of the ducts may contain chlorophyll (fig-. 10). 



As to chlorophyll in the pith , it need only be said that it occurs sparingly 

 in stems 1 cm. in diameter and is not present in the older and larger branches. 



The chlorophyll disappears from the stem in a very regular sequence. 

 It leaves the epidermis first, then the pith, then the inner medullary rays; 

 after this the wood parenchyma, then the medullary rays of the cortex and 

 the median band, and finally, when cork is formed, the outer band. That 

 is, with two exceptions, the chlorophyll disappears from the stem in a cen- 

 trifugal direction. The exceptions were most marked in stem of P. torrcyana 

 8 mm. in diameter, in which practically all of the chlorophyll of the pith, 

 as well as of the medullary rays of the wood, had been removed, but the 

 deeply-placed wood parenchyma near the ducts still contained chlorophyll 

 in considerable quantity. 



The leading" departures from the chlorophyll conditions shared in common 

 by the three species of Parkinsonia are as follows: /'. aculcata: Branches 

 7 and 12 mm. in diameter had no chlorophyll in pith or in the inner part 

 of the wood. P. inicrop/iylla: A variation due possibly to differences in 

 water relations was observed in branches 1 cm. in diameter. One branch, 

 taken from a tree which was apparently poorly supplied with water, had 

 very little chlorophyll in the wood and none in the pith, while another 

 branch of the same diameter, taken from a tree that had been irrigated at 

 frequent intervals, had the maximum distribution of chlorophyll. P. tor- 

 irvana: As may be implied from a previous statement, this species appears 

 not usually to have so much chlorophyll in its branches as the other ones, 

 but an exception was noted in a stem 2.25 cm. in diameter, in which the 

 chlorophyll was in the epidermis and extended into the stem for a distance 

 of 6 mm. This tree was growing in a particularly favorable location in 

 the wash at the west base of Tumamoc Hill. 



The following measurements were made: 



