594 Thoday . — Behaviour during Drought of the Leaves of two 
Plant. 
Epidermis. 
Outer Wall. 
Passerina Jilifortnis ) 
„ cf . falcifolia ) 
Stilbe vestita 
A*. 
60-95 
Ii. 
Up to 23 
60-100 

Protea lepidocarpodendron 
80 
30 
Lobostemon sp. 
30 
— 
Ivy 
20 
9 
Helianthus {? annuus ) 
10-20 
Stilbe vestita , like Passerina , has ericoid leaves. Protea lepidocarpo- 
dendron has large, thick and tough, lanceolate leaves. Lohostemon 
(Boraginaceae) has thick and rather fleshy, coarsely hairy leaves. All 
these are evergreen plants of the Cape Peninsula maquis. In the first 
three the thickness of the epidermis is of the same order. They may all 
be described as sclerophyllous, the leaves being tough. The toughness is 
largely due to the thick outer wall and cuticle, so that thick outer wall and 
thick epidermis are correlated in these instances. The data are too few, 
however, for generalization, and show the need for extensive comparative 
observations. 
Marloth, from long and extensive acquaintance with the Cape flora, has 
emphasized the view that protection from intense insolation is of great 
importance in this climate . 1 He points to the large number of leaves 
which assume the vertical position, and suggests that the screening effect 
of white hairy coverings in other leaves may be as important as the protection 
they afford against drying winds. 
A large-celled epidermis °will act as a screen to an extent which must 
vary as the height of the cells and perhaps also depend upon the nature of 
the sap. On the latter point research is required. It is of interest, in this 
connexion, that solutions of tyrosine have been shown to absorb ultra-violet 
radiation and so reduce the toxicity of light to bacteria and Paramoecium , 2 
and it is not improbable that other substances commonly occurring in plants 
may be found to have a similar action. 
In Passerina , the height of the epidermal cells may be important 
in connexion with the closing of the groove, for the greater the 
distance between the outer and inner walls the greater is the difference of 
curvature and the greater therefore will be the effect of the approximation 
of them. Moreover, the greater the height of the cells, the greater the 
leverage at the disposal of the cohesion tension drawing the lateral walls 
together. 
Other Anatomical Features. 
Raised Stomata . The stomata, as already described, are raised into 
1 R. Marloth : Die Schutzmittel der Pflanzen gegen iibermassige Insolation. Ber. d. D. Bot. 
Ges., xxvii, 1909, p. 362. 
2 F. J. Harris and H. S. Hoyt in Univ. of California Pnbl. in Path., ii, 1919, pp. 245-50 ; see 
Physiol. Abstr., iv, 1919, p. 203. Entry 1822 also i. p 686. Entry 2751. 
