. A textbook of botany for colleges and universities ... Botany. 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 ( with the upper and under portions different in st
. A textbook of botany for colleges and universities ... Botany. 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 ( 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 Fig. 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 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- sidera
Size: 1169px × 2136px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
