328 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 



of Africa and the East Indies. These plants are exposed, far more than the 

 thick-leaved plants, for the greater portion of the year to extraordinary dryness. 

 Their usual habitats are dry sandy and stony plains, waste rocky plateaus, and 

 crevices of rocks which are almost completely wanting in soil. They always inhabit 

 regions where no rain falls for about three-fourths of the year, and which usually 

 belong to the driest parts of the earth. The whole organization of these plants 

 corresponds to these conditions of their habitat. Dry scales and hairs are produced 

 instead of foliage-leaves, or these are often metamorphosed into thorns which 

 project in great numbers from the thick stem -structures, and efficiently protect 

 them from the attacks of thirsty animals. The epidermis of the pillar-like, disc- 

 shaped, or spherical stem-portions is thickened on the outer wall, so as to almost 

 resemble cartilage, and frequently it forms a coat of mail round the deeper-lying 

 green tissue by the abundant deposition of oxalate of lime (as much as 85 per cent). 

 Most of the succulent plants, whose cell- walls, which are in contact with the air, are 

 fortified by oxalate of lime, silicic acid, or suberin, have in their tissue peculiar 

 aggregates of cells which apparently serve for the storing-up of water for the dry 

 season, and which have been termed " aqueous tissue " . The water in these reser- 

 voirs is always so apportioned that it lasts from one rainy season to another; that 

 is to say, the adjoining green tissue which exhales the stored-up water does not 

 suffer from drought during the dry season. Also, it is contrived in these plants 

 that, immediately after the fall of the first rain, the reservoirs are again filled with 

 water, and that the emptying and filling of the cells and the decrease and increase 

 of their volume exercise no harmful influence on the adjoining tissue. Succulent 

 plants have been not inaptly compared to camels, the " ships of the desert " , which 

 provide themselves with a large quantity of water, and are then able to dispense 

 with further supplies for a long time without injury. The cells of the aqueous 

 tissue are comparatively large and their walls thin; the active protoplasm within 

 forms a delicate layer round the walls that is to say, a sac whose cavity is filled 

 with watery, often somewhat mucilaginous, fluid. In the cactuses the aqueous 

 tissue is hidden as much as possible in the interior of the thick rod-shaped or 

 spherical stem; also in many thick-leaved plants, such as some of the European 

 species of the genus Sedum (e.g. Sedum album, dasyphyllum, glaucum); in South 

 African species of the genera Aloe and Mesembryanthemum (e.g. Mesembry- 

 anthemum blandum, foliosum, sublacerum), the aqueous tissue is concealed in the 

 interior of the leaf, and is usually composed of cells surrounding vascular bundles 

 there situated. In Sedum Telephium, known by the name of Orpine, as well as in 

 species of House-leek (Sempervivum), and many salsolas growing on steppes, the 

 ramifications of the vascular bundles are enveloped in a mantle of green tissue, 

 and the bundles, which are, as it were, overlaid with green cells, are so arranged 

 with regard to the colourless aqueous tissue, that to the naked eye they look 

 like green strands in a transparent matrix which is. as clear as water. In the 

 Mexican Echeverias the aqueous tissue is inserted as broad stripes in the green 

 tissue, and in the thick-leaved orchids it appears as if sprinkled between the green 



