LEAVES 



629 



(i) salt marsh and other succulents (which include the most representa- 

 tive forms) having sap of high osmotic pressure, and (2) succulents 

 whose water retentiveness is due to structural or other characters, 

 notably cutinization, as in the cacti. It is probable, however, that no 

 explanation thus far given accounts for all cases 

 of extreme water retention in the presence of 

 conditions favoring transpiration. 



Structural features. 

 While most leaves are 

 dorsiventral (i.e. with 

 the upper and under 

 portions different in 

 structure), many succu- 

 lent leaves are cylindrical 

 and almost radially sym- 

 metrical, like stems and 

 roots, instead of having 

 only one plane of sym- 

 metry, as do most leaves 

 (fig. 923). * However, 

 while the epidermis, 

 chlorenchyma, and 

 colorless parenchyma, as seen in cross sec- 

 tion, are essentially uniform in aspect in 

 the entire leaf cylinder, the conductive tract 

 is dorsiventral, as in ordinary leaves, the 

 xylem being above and the phloem beneath 

 (fig. 926). In succulent leaves the veins 

 commonly are buried so deeply as to be in- 

 conspicuous from without. 



Water tissue. All the living plant tissues 

 are composed chiefly of water, but the term 

 water tissue is employed especially in succulent 



plants to designate regions of turgescent parenchyma cells with delicate 

 cellulose walls, thin peripheral layers of cytoplasm, and few or no chloro- 

 plasts. In many succulents the water tissue is not sharply delimited 

 from ordinary chlorenchyma, and may be made up entirely of turgescent 



Fro. 923. A 



branch of Senecio sp., 

 a desert xerophyte, il- 

 lustrating extreme leaf 

 succulence, the very 

 fleshy leaves present- 

 ing a small surface in 

 proportion to their 

 volume. 



I 



FIGS. 924, 925. Cross sec- 

 tions through the leaf of Coty- 

 ledon, a succulent xerophyte: 

 924, a diagrammatic section, 

 showing that the leaf is rela- 

 tively thick in proportion to its 

 width ; 92 5, a cross section, con- 

 siderably magnified, showing 

 relatively uniform chloren- 

 chyma cells, except that the 

 outermost cells (0) are rounded 

 and the innermost cells (i) an- 

 gular; note that stomata (s) 

 occur on both surfaces and that 

 chlorophyll and air spaces are 

 more abundant in the outer- 

 most than in the innermost 

 cells. 



1 Most dorsiventral leaves present two distinct surfaces and hence may be called bi- 

 facial, contrasting with cylindrical equilateral leaves. 



