CHAP, xxxii STORAGE OF WATER BY LAND-PLANTS 121 



play some part in the rolling and unrolling of the leaf, and probably may 

 function as water-reservoirs. 1 



Mucilage is present in the epidermis of not a few desert-plants, includ- 

 ing Cassia obovata, Malva parviflora, Peganum Harmala, Zizyphus Spina- 

 Christi, and other plants in the Egyptian desert 2 ; in many plants mucilage 

 arises in all the epidermal cells, but in others only in some of these. The 

 mode of origin of the mucilage is not known in all cases ; often it arises 

 from the inner epidermal walls. These swell to such an extent in some 

 xerophytes that the lumen of the cell seems not to be more than about 

 half the volume of the wall or, at least, not so large as the latter ; this 

 is the case in Empetrum, a number of Ericaceae, Loiseleuria procumbens, 3 

 Egyptian species of Acacia and Heseda, and species of Rosa. 4 



Hairs functioning as water-reservoirs form water-bladders. They 

 occur in a number of African desert-plants, including Mesembryanthemum 

 crystallinum, Malcolmia aegyptiaca, Heliotropium arbainense, Hyo- 

 scyamus muticus, Aizoon, some Resedaceae, 5 in many Chenopodiaceae, 

 including Atriplex coriacea, A. Halimus, 6 A. (Halimus) pedunculata 

 and A. portulacoides, 7 also as mealy hairs in other Chenopodiaceae, 

 possibly also in Tetragonia expansa, 8 and Rochea falcata, 9 and 

 others. In their typical form they are large, clear, watery vesicles, 

 which project above the epidermis and glisten in the sunlight ; as their 

 contents are gradually consumed they dry up ; in Atriplex Halimus and 

 some other Chenopodiaceae, as well as in Oxalis carnosa, 10 the shrivelled 

 hairs form an air-containing covering over the lamina. Whether or no 

 all the hairs mentioned function to the same extent as water-reservoirs 

 requires investigation. 



Hairs of a most remarkable form occur, according to Haberlandt, u 

 on the roots of an epiphytic fern, Drymoglossum nummulariaefolium. 

 These hairs shrivel during the dry season ; the protoplasm withdraws to 

 the base of the hair and shuts itself off from the dry part by a cell-wall ; 

 when rain falls, the hairs grow out in a few hours and once more become 

 filled with water. 



Voluminous peripheral water-tissue may arise either by tangential 

 division of the epidermis or by the formation of a hypoderma. It is 

 situated mainly on the upper face of the leaf, and if present on the lower 

 face it is less developed. It checks the penetration of heat-rays rather 

 than of luminous rays, and thereby retards transpiration in addition to 

 acting as a water-reservoir. 



A multilamellar epidermis occurs frequently among xerophytes, but 

 particularly among lithophytes and epiphytes ; there may thus arise a 

 voluminous tissue, the thickness of which far exceeds that of the chloren- 

 chyma, as in species of Peperomia, Begonia, Ficus, and Gesneraceae. 12 



Hypodermal water-tissue occurs in other xerophytes. It is composed of 

 one layer of cells in certain Genisteae, 13 Velloziaceae, 14 and Orchidaceae 16 ; 



Duval-Jouve, 1875; Tschirch, 18826; Volkens, 1887. 



Pfitzer, 1870, 1872; Volkens, 1887. * Gruber, 1882; E. Peterson, 1908. 



Vesque, 18820, b, 1889-92. ' Volkens, 1887 ; Henslow, 1894; Schinz, 1893. 

 Volkens, 1887. 7 Warming, 1891, see illustrations, 1906. 



W. Benecke, 1901. * F. Areschoug, 1878. ' Meigen, 1894. 



Haberlandt, 1893. " Vesque, loc. cit. ; Pfitzer, 1870, 1872. 



Schube, 1885. " Warming, 1893. IS Kruger, 1883. 



