FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 329 



cells. The epidermis in numerous other thick -leaved plants serves as a store-house 

 for water in a marvellous way. Individual epidermal cells are then greatly 

 enlarged and project beyond the others in the form of sacs, clubs, or bladders, as 

 shown in the picture of Rochea (fig. 79). These bladders either fit together into a 

 one-layered extended coat of armour, or they are frequently placed irregularly side 

 by side or above one another. In some instances they form isolated groups or occur 

 singly, and appear then to the naked eye like protuberances on the green stems and 

 leaves, where they glitter and sparkle in the sunshine like an embroidery of dew- 

 drops. Many leaves and branches — as, for example, those of the widely-distributed 

 Ice-plant (Mesembryanthemu/m cristallinv/m) — have the greatest resemblance to 

 candied fruit covered with clear, colourless, sparkling sugar crystals. 



When the walls of the enormously-distended vesicular or bladder-shaped cells of 

 the epidermis are silicified, as are those of the repeatedly-mentioned Rochea, it is 

 easily understood that the watery cell-sap which they contain is not exhaled into 

 the air; the fluid is, so to speak, inclosed in a glass bottle and can only be given off 

 in the direction of the green tissue. But when the walls of the bladder-like giant 

 cells are not silicified, and not even particularly thickened, what is the result? 

 From the aspect of the Ice-plant one would think that a single warm dry day would 

 suffice to shrivel and dry up the watery vesicles. But this is certainly not the case. 

 Leafy twigs cut from the Ice-plant may be left all day on the dry ground in dry air 

 and sunshine, and the large bladder-like cells on the surface will not lose their 

 aqueous contents. After a week they become collapsed, having given up their 

 water, not to the atmosphere, but to the green tissue covered by this swollen coat. 

 Without doubt this phenomenon is to be associated with a peculiar formation of the 

 cell- wall; but it is as certain that the constituents of the cell-sap, which fills the 

 vesicles are also important, and it must be assumed that substances are dissolved in 

 this aqueous fluid which restrict the evaporation of the water. 



These substances, which hold water with great energy, and thereby enable the 

 plants in question to survive through periods of the greatest dryness, are partly 

 viscous, gummy, and resinous fluids, partly salts. It is well known that the sticky, 

 watery pulp of crushed mistletoe berries, used in the manufacture of " bird-lime ", 

 may be exposed to the air for months without quite drying up, and the mucilaginous 

 juices of many cactuses and thick-leaved plants behave in a similar manner, espe- 

 cially those of the Cape aloes, which exhale no water, and enable the plants 

 possessing them to withstand the drought for months. In the thick-leaved plants 

 of the salt steppes and deserts, the fluids are rarely resinous or gummy, but they 

 frequently contain a surprising quantity of salts dissolved in water, such as common 

 salt, chloride of magnesium, and the like; and these also obstinately retain water in 

 proportionately large quantities. It is one of the most surprising of phenomena to 

 see the thick-leaved salsolas rising above the soil of salt steppes, green and succu- 

 lent, when the ground is at its driest in the height of summer, when for months no 

 clouds have tempered the sun's rays and not a drop of rain has fallen, and when 

 almost all other plants have long ago turned yellow and faded. The large amount 



