Delf. — Transpiration in Succulent Plants. 415 
A large number of succulent plants, therefore, show but little develop- 
ment of cuticle, and can endure a rapid rate of transpiration per unit area 
of surface without harm. 
Many succulent plants, however, in addition to the possession of a more 
or less developed cuticle, have a coating of wax which acts as a further 
protection against water loss. Garreau 1 offered experimental evidence as 
to the protecting power of wax coverings in the case of Centranthus ruber , 
Syringa vulgaris , Seduni verticillatum > and some others, where the rate of 
transpiration was increased from 1*5 to 3 times by removing the wax. 
Frequently the wax not only coats the free surface of the epidermal cells, 
but also blocks the apertures of the stomata more or less completely. 
Dr. F. Darwin 2 has shown that, generally speaking, the stomata of a plant 
tend to become aggregated in the areas covered by the wax or bloom. 
Thus, if bloom be only found on one side of a leaf, the stomata are often 
either less numerous or altogether lacking on the side which is not thus 
protected. The association of stomata with a wax covering may, however, 
also be connected with the prevention of the blocking of the stomata by 
rain or heavy dew, since the maintenance of gaseous exchange is of great 
importance in the economy of the plant. 
A wax covering is chiefly characteristic of plants inhabiting dry regions, 
as the observations of Schimper, Volkens, and others show. Holtermann 3 
found that in Ceylon those epiphytes which lived in the warm damp air of 
the tropical forests were glabrous, whereas those which were found in ex- 
posed, sunny places were frequently coated with wax. Wiegand, 4 from his 
experiments with artificial surfaces, concluded that a wax covering would 
form an equally effective protection whether in still air or in wind. 
Most halophytes which live in damp or marshy situations are glabrous, 
as, for example, Rhizophora , Salicornia , and Aster Tripolium. Suaeda mari - 
tima is an exception, for the small leaves are covered with a fine bloom, and 
it is often found in marshy places. P sammophilous halophytes are frequently 
covered with wax if sclerophyllous, or very slightly succulent, as in Elymus 
and Eryngium maritimum ; but if succulent the leaves are commonly 
glabrous, as in Cakile maritima , which also has only a very slight develop- 
ment of cuticle. 
Hairs, whether glandular or protective, are not common in succulent 
plants ; they are, indeed, much more characteristic of sclerophyllous vegeta- 
tion. We may, however, instance Sempervivum arachnoideum , a succulent 
alpine plant, the rosettes of which are clothed, and perhaps also protected 
by long loose hairs. Some species of Avicennia are hairy, and Evolvulus 
1 Quoted by Dr. A. Burgerstein in Die Transpiration der Pflanzen, 1904, p. 39. 
2 Darwin, Dr. F. : Stomata and Bloom. Journ. Linn. Soc., 1887. 
3 Holtermann : Der Einfluss des Klimas auf den Bau der Pflanzengewebe, 1907. 
4 Wiegand : Relations of Hairy and Cutinised Coverings to Transpiration. Bot. Gaz., 1910, 
