194 BOTANY 



animal sweat-glands, actively press out the water ; or, on the other 

 hand, they may simply allow it to filter through them when the 

 internal pressure has attained a certain strength. 



It would almost seem that, in case of inactive transpiration, such 

 exudations of liquid water supplied the place of evaporation, were it 

 not that the out-pressed liquid is not pure water, as in transpiration, 

 but always contains salts and, sometimes, also organic substances in 

 solution. In fact, the quantity of salts in water thus exuded is often 

 so abundant that after evaporation a slight incrustation is formed on 

 the leaves (the lime-scales on the leaves of the Saxifrages). In 

 some instances, also, the substances in solution in the water seem 

 to be exuded with a purpose, as in the case of the SECRETIONS OF the 

 nectaries and of the DIGESTIVE glands of insectivorous plants 

 (p. 215), and of the discharges of the viscid stigmatic FLUID. The 

 superfluous water is discharged by a few plants, the Pumpkin, for 

 example, into the cavities of their stems and leaf-stalks, and is again 

 absorbed from these reservoirs when needed. 



Special Contrivances for regulating the Water-supply. — Almost all the higher 

 plants possess in the power to close their stomata a special means of checking 

 transpiration during a temporary insufficiency of the water-supply. In districts 

 subject to droughts of weeks or months' duration, only such plants can flourish as 

 are able either to withstand a complete drying up without injury (p. 179), or to 

 exist for a long time on a scanty supply of water. This last case is only rendered 

 possible by the extreme reduction of transpiration, or by the formation of organs 

 in which, in times of a superfluity of water, it may be retained for later use. 



Such protection against excessive transpiration is afforded by the formation of 

 cork or cuticular coverings, by the reduction in the number and size of the stomata, 

 and also by their occurrence in cavities or depressions. The rolling up of the 

 leaves, as well as the development of thick growths of hair and the assumption of 

 a vertical position to avoid the full rays of the sun, are also measures frequently 

 adopted to lessen transpiration. The most efficient protection, however, from too 

 great a loss of water by transpiration is undoubtedly obtained by the reduction of 

 the transpiring surfaces, either through a diminution in the size of the leaves or 

 through their complete disappearance. 



Theupright position of the leaves, or the substitution of expanded, perpendicularly 

 directed leaf-stalks for the leaves (Phyllodia), particularly characterises the flora of 

 Australia. A clothing of hair, on the other hand, protects the leaves of many South 

 African Proteaceae {e.g. Zeucadendron argenteum). Some of the Gramineae (Stipa 

 capillata, Festuca alpcslris, Sesleria tenuifolia, S. punctoria, etc. ) roll or fold their 

 leaf-blades, in times of drought, by means of special hinge-like devices, into narrow 

 tubes, and so maintain a sufficient supply of water by diminishing the transpiration 

 from their stomata. Reduction of the leaves is illustrated by the desert forms of 

 Genista and Sarothamnus and by the Cypress-like Conifers. A complete disappear- 

 ance of the whole leaf surface takes place in most Cacti, in which also the stems 

 become swollen and converted into water-reservoirs (Fig. 25). A similar develop- 

 ment of succulent swollen stems frequently occurs in the Euphorliaceae (Fig. 181), 

 in the Compositae (Kleinia articulata), Stapeleae, and many other plant families 

 found in arid regions. It has been estimated that the amount of water evaporated 

 by a Melon-Cactus is reduced by its succulent development to msta of that given 



