CELTIS PALLIDA. 9 



With an increase in diameter of the stem characteristic changes take place, 

 more particularly in the cortex, which greatly affect the topography of the 

 chlorophyll apparatus. As the cortex becomes wider, rings of secondary 

 hard bast are formed within the primary ring; parenchyma, which for the 

 most part contains chlorophyll, extends between these rings. The groups 

 of bast are connected in part or always by medullary rays. As the stem 

 increases in diameter these groups are pushed farther and farther apart 

 and the intervening portion becomes filled with parenchyma which contains 

 chlorophyll. In this respect Cettis strikingly resembles Prosopis. The sec- 

 ondary hard bast of the former, however, is not placed as regularly as in 

 Prosopis, and the chlorophyll distribution, consequently, of Ce/tis is not so 

 symmetrical as in the other species. 



FIG. 3. Celtis pallida: A, section of branch 4.5 mm. in diameter; JS, detail of A, 

 showing presence of chlorophyll in secondary cortex; cam., cambium; cJi. b., cortical 

 band of chlorophyll; h. b., hard bast; med., medullary ray;///., phellogen;///. d, 

 phelloderm. 



But the greatest change in the chlorophyll apparatus occurs as a result 

 of the formation of phelloderm. The cork-cambium arises in the cells imme- 

 diately outside the chlorophyll band and by its activity gives rise to periderm 

 without and phelloderm within. The latter contains chlorophyll. When 

 the amount of phelloderm about equals the thickness of the chlorophyll 

 band no more appears to be formed. The result is that the chlorophyll 

 band of stems from 4 mm. to 1.6 cm. in diameter is about half phelloderm. 

 How this relation is in older stems was not learned. 



How long the chlorophyll remains active in the stem was not determined. 

 It is present in the outer portions of the pith in the stems 8.5 mm. diameter 

 after it has disappeared from the medullary rays, but whether this is a con- 



