62 PllESIDKXTIAL ADDRESS SECTION C. 



by J^lackman and Mattliaei,''' and by A. M. Smith in leaves 

 exposed to direct sunlight. Smith for example observed ia 

 Ceylon temperatures as much as lo^C. hiolier than the shade 

 temperature of the air. 



Analogous considerations are suggested by the disposition 

 and form of the leaves characteristic, of the maquis of the South 

 Western part of the Cape Province. The broader leaves are 

 usually inclined upwards or assume the profile position, as in 

 Piotca grandifiora. Their foliage, therefore, does not cast a dense 

 shade like the horizontally displayed leaves of most forest trees. 

 The leaves do not shade one another to the same extent but 

 allow more light to pass unfiltered, both between them and 

 reflected from their outer surfaces, to the leaves below. Like 

 the transparency of the palisade tissue, the transparency of the 

 foliage also means a diminution in the heating effect. 



Similarly, the foliage of plants with small or dissected leaves, 

 which are so characteristic a feature of this type of vegetation, is 

 relatively transparent. I have on a former occasion drawn 

 attention to the inadequacy of the traditional view that micro- 

 phyllj' is a xerophilous adaptation in that it reduces the 

 transpiring surface. Small leaved plants usually have very nu- 

 merous leaves and the total surface exposed may be very 

 considerable. Besides, the more slender the leaf the larger sur- 

 face it exposes per unit of volume. The same applies to such 

 dissected leaves as those of Afhanasia parviflora and of the 

 Australian Hakca spp. with cylindrical segments. These expose 

 as much surface relative to their volume as flat leaves of half 

 the thickness. 



The ecological intei-pretation of these leaf types is, however, 

 a complicated problem. I will merely refer again to the trans- 

 parency of foliage composed of such leaves and offer another 

 suggestion which appears to me to present the xerophilous aspect 

 of microphylly in a truer perspective. I base it upon some results 

 obtained by Yapp in his study of the vegetation of the Fens of 

 Eastern England, and on the recent work of FaiTiier on the 

 resistance of wood to the flow of water to the leaves. 



Yappf found that, when shoots of tlie Meadow Sweet 

 (Spir(ca ulmaria) were suffering from a deficiency of water, those 

 parts of the leaves which wei'e farthest from tlie main veias were 

 the first to dry. These parts are, therefore, less efficiently sup- 

 plied with water by the conducting channelis. The lower leaves 

 of this plant, produced in the shelter of surrounding vegetation, 

 are glabrous. The uppemiost leaves fully exposed to the sun and 

 wind are hairy. Intermediate leaves developing under less ex- 

 treme conditions bear hains on just those parts which ai'e the 

 first to dry up in case of drought. 



Proc. Eov. Soc. B. 76, 1905, p. 402. 

 t Annals of Botany. 26, 1912, p. 815. 



