. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. themum Forskalii of the Egyptiandesert approximately one-half of each succulent leaf is made upof water-storage tissue (Fig. 106); and in epiphytic species ofCodonanthe growing on a dry substratum nearly three-fourthsof the fleshy leaf is occupied by cells devoted to the storage ofwater (Fig. 81). The fleshy stems of cacti and some Euphor-biaceae are largely composed of the same kind of tissue. Inthe leaves of Ficus elastica the protoderm of the upper surface ig6 STORAGE OF FOOD
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. themum Forskalii of the Egyptiandesert approximately one-half of each succulent leaf is made upof water-storage tissue (Fig. 106); and in epiphytic species ofCodonanthe growing on a dry substratum nearly three-fourthsof the fleshy leaf is occupied by cells devoted to the storage ofwater (Fig. 81). The fleshy stems of cacti and some Euphor-biaceae are largely composed of the same kind of tissue. Inthe leaves of Ficus elastica the protoderm of the upper surface ig6 STORAGE OF FOOD AND WATER divides tangentially and gives rise to several cell-layers constitut-ing a typical water-storage tissue (Fig. 79). Water-storage tracheids sometimes occur as terminals of thefiner branches of leaf veins, as in Euphorbia splendens andTownsendia cespitosa (Eg. 107); and occasionally the meso-phyll cells have the characteristic wall thickenings of tracheidsand apparently serve in water storage. The tubers of thepotato and other fleshy underground parts serve for the stor-age of water as well as of FIG. 107.—Water-storage tracheids in the leaf of Euphorbia splendens. b, b, water-storage tracheids; d, mesophyll cells; c, branch from a milk tube. (After Haberlandt.) Frequently, and especially in xerophytes, cells or groups ofcells contain mucilage as a real cell content or as much thick-ened cell-walls, the inner layers of which have become transformedfrom cellulose to mucilage. Mucilage has a great affinity forwater, imbibing it with power and holding it with great the amount of mucilage is considerable, as is frequentlythe case in desert plants, such as the Aloes, cacti, certain speciesof Astragalus, and many others, it plays an important part inthe water-storage function. Characteristics of Water-storage Tissues.—The water-storage cells are characterized by having thin cellulose walls,or walls, if thickened, having many pits or thin places. These
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