630 



ECOLOGY 



green cells (as in various thin-leaved succulents), quite as in most leaves 

 except for the evident fleshiness of the organ; or the leaf may be thick 

 with the chlorophyll gradually decreasing toward the center, the cells 

 otherwise being essentially similar in aspect, as in most Crassulaceae 

 (figs. 924, 925); or the leaf may be thick with chlorophyll decreasing 

 toward the center, but with the outermost chlorenchyma cells elongated, 



representing true 

 palisade cells, while 

 the cells toward the 

 center become more 

 and more isodia- 

 metric and also 

 poorer in chloro- 

 phyll (as in the 

 century plant and 

 in various cacti). 

 Another kind of 

 water tissue char- 

 acterizes more ex- 

 treme succulents (as 

 Salsola and other 

 forms with cylin- 

 drical leaves) and 

 may be regarded as 

 more representa- 

 tive ; in these plants 

 the water tissue, 

 which is composed 

 of large isodiamet- 

 ric cells, is centrally 

 placed and is more 



Fig. 926. — A cross section through a succulent xerophytic 

 leaf, that of the Russian thistle {Salsola Kali tenuifolia), illus- 

 trating peripheral palisade chlorenchyma (p) and central 

 water tissue (w); note the relatively thin cuticle (c), and 

 the sharp delimitation between the chlorenchyma and the 

 water tissue, the latter being characterized by the large size 

 of the cells and by the absence of conspicuous air spaces; 

 such a leaf is equilateral and approximately radially sym- 

 metrical, thus having a small surface exposure in proportion 

 to the leaf volume; dorsiventrality is exhibited alone by the 

 vascular bundle, the hadrome or xylem (h) lying above the 

 leptome or phloem (/) ; highly magnified. 



or less sharply delimited from the peripheral chlorenchyma cylinder 

 whose cells usually are relatively small and of palisade shape (figs. 

 926, 927). 



A third kind of water tissue differs from all the rest in its peripheral 

 position, the cells belonging to the epidermis rather than to the meso- 

 phyll. All gradations occur between an ordinary epidermis with a single 

 layer of colorless water-containing cells and a many-layered epidermis 

 of similar but more turgescent cells, as in Begonia and Peperomia, where 



