Delf . — Transpiration in Succulent Plants . 435 
moreover, in some species of Salicornia there appears to be every transition 
between lignified stereides and spirally thickened unlignified storage 
tracheides. 
II. Transpiration in Relation to Habitat. 
Probably the factors in the environment which most affect the tran- 
spiration of a plant are the water supply and the atmospheric conditions, 
although undoubtedly other climatic and edaphic influences are also con- 
cerned. We have now to consider the effect of the chief physical factors on 
the principal types of succulent plants. 
In the truly xerophilous succulents of desert regions we have a reaction 
to extreme scarcity of water, to strong illumination, and to the desiccating 
effects of warm, dry air ; adaptations are, therefore, found connected with 
both absorption and retention of water. The former process is facilitated 
by the development of deeply penetrating and often much branched root 
systems, and in many cases there is little doubt that the plants are for this 
reason almost independent of rainfall. Volkens, indeed, describes shrubs in 
the deserts of Northern Africa which seem to thrive where there is absolutely 
no rainfall for eight months of the year. In some cases the root absorption 
seems to be supplemented in an important way by absorption of dew by 
means of glands on the leaves, as in the Tamaricineae, also described 
by this author for the same region. 
It is, perhaps, on account of the efficiency of the extensive root systems, 
that in spite of the very limited water supply the desert plants examined by 
Cannon developed more conducting tissue when grown in a dry than in an 
irrigated place. The same explanation may apply to the statements of 
Prof. F. E. Lloyd that Fouquiera splendens and Verbena ciliata are charac- 
teristic of truly desert regions and yet form leaves which in structure 
resemble those of a slightly succulent mesophyte ; here, however, there 
is the additional explanation that the leaves are annual and are shed just 
before the hottest season. 
The retention of water within the plant is accomplished partly by the 
formation of protective structures in the epidermis, and partly by the 
accumulation of mucilaginous substances and salts of organic acids within 
the aqueous tissue. 
The most frequent protective adaptations are the development of 
a thick cuticle, which is often covered with a coating of wax, or with 
finely divided wax particles, the formation of relatively few stomata, 
which may also be sunken below the level of the neighbouring epidermal 
cells, protected with depositions of wax particles or surrounded by a raised 
wax collar as in Euphorbia Tirukalli l Less often the stomata are pro- 
tected by hairs. In all these cases there is a certain amount of reduction of 
1 Figured in Haberlandt’s Physiologische Pflanzenanatomie, 1896, p. 397. 
