ZIZYPHUS PARRYI. 



31 



ZizYPHUS PARRYI. (Plate 5 and fig. 15.) 



Zizyphus occurs sparingly on the river-bottoms or elsewhere where the 

 water conditions are fairly good. It is a grayish-colored (due to hairy 

 covering) spiny shrub from 1.5 to 3 m. high and bears during favorable 

 seasons a considerable leaf-surface. As is the case with the most of the 

 desert perennials, the leaves fall away with the advent of dry conditions and 

 leave the shrub bristling with spines in a condition very much like the 

 ordinary condition of Koeberlinia emoryi. 



Fig. 15. — Zizyphus parryi: Section of stem 3 mm. in diameter, 

 to show distribution of chlorophyll, ch. (J., cortical chlorophyll 

 band; h. b., hard bast; ca/n., cambium. 



The young- stem of Zizyphus shows the customary divisions into cortex, 

 wood, and pith, of which the latter is especially abundant. The main 

 divisions of the cortex are the epidermis with its rather heavy cuticle, a 

 chlorophyll band beneath the epidermis, a hard-bast ring- which is discon- 

 tinuous, and the g-roimd tissue between it and the cambium. 



The distribution of the chlorophyll is very much as in other plants. The 

 outer cortical parenchyma, most of the inner cortical parenchyma, the 

 medullary rays, and the outer pith-cells contain chlorophyll. The outer 

 chlorophyll band, which is interrupted at frequent and fairly reg-ular inter- 

 vals by masses of collenchyma, has a palisade structure, at least in the 

 outer part, of short cells very like the cells in the leaf. The inner chloren- 

 chyma cells are cuboid. The parenchyma and the collenchyma between 

 the chlorophyll band and the groups of hard bast deeper in the cortex con- 

 tain chlorophyll. The medullary rays of the cortex are also chlorophyll- 

 bearing-. The rays either end in the groups of hard bast or pass outward 



