104 ADAPTATIONS. OECOLOGICAL CLASSIFICATION SECT, in 



absorbing substance and thus promotes transpiration, but according to 

 others it does precisely the reverse. 1 



An epidermis with mucilaginous inner walls, which are gelatinous, 

 occurs in many land-plants, and specially in woody plants such as Empe- 

 trum, Arbutus Unedo, and other Ericaceae. The mucilage possibly 

 serves to depress transpiration ; 2 but it may perhaps function rather as 

 a water-reservoir. 3 



A fact of perhaps supreme importance, and one that is the cause of some 

 of the relations already mentioned in connexion with the epidermis, is 

 that wettable plant-parts wither much more rapidly than unwettable parts. 

 Wiesner regards the increase of transpiration in the former as due to a 

 peculiar swelling of the cell-walls, which consequently oppose less resistance 

 to evaporation. Many of the devices mentioned as decreasing transpira- 

 tion also serve to prevent the plant-parts from being wetted, and in this 

 way, too, prevent rapid transpiration. 



Cork, in virtue of its air-filled cavities and its other characters, depresses 

 transpiration, as has been proved by experiment. Its thickness is some- 

 times obviously and directly correlated with dryness of climate, as is 

 illustrated by the difference between the trees of the Brazilian campos 

 and of the adjoining forest. The desiccating action of fires occurring in 

 the campos appears to stimulate the development of cork, and thus to 

 provide an example of self -regulation. 4 Very thick investments of cork 

 occur in a number of desert-plants, for instance, Dioscorea (Testudinaria) 

 Elephantipes in South Africa, and Cocculus Leaeba in Egypt. 



In the aerial roots of some Orchidaceae and Araceae a mechanism 

 designed for absorbing water assumes the form of a velamen, which clothes 

 the root with an envelope of cells, usually of several layers in thickness : 

 the cells resemble the water-absorbing cells of Sphagnum ; they are thin- 

 walled, with annular, spiral, or reticulate thickenings. When these cells 

 are filled with air the velamen is white ; but when they are occupied by 

 water the chlorophyll-containing tissue of the root becomes more or less 

 visible. Liquid water is rapidly sucked up by the velamen, and can be 

 transported to the conducting tissue. It is possible that water in the 

 form of vapour may also be absorbed by the velamen. Wehmer, how- 

 ever, expresses another view, namely, that the velamen acts as a protection 

 against transpiration. It is possible that both views are correct. 5 



Here it may be mentioned that the roots of many xerophytic land- 

 plants produce a very strong endodermis, which probably protects against 

 desiccation. 



B. Stomatal Transpiration and the Aerating System. 



Intercellular spaces are also seats of transpiration ; the transpiring 

 surface of a plant is constituted not only by the surface exposed to the 

 external atmosphere, but also by the cell-walls bounding all intercellular 

 spaces; it may therefore be anticipated that the air-containing inter- 

 cellular spaces of land-plants, and especially of xerophytes, will sharply 



1 See Chapter V, pp. 20-1. Volkens, 1890. 



Pfitzer, 1870-2; Radlkofer, 1875, p. 100 ; Vesque, 1884: Walliczeck, 1803: 

 Westermaier, 1880; H. E. Petersen, 1908. 



4 Warming, 1892. Burgerstein, 1904, p. 69. 



