170 BOTANY 



PART I 



plants of the Mediterranean region, such as the Laurel and Myrtle). 

 The small size of the intercellular spaces in the mesophyll is 

 characteristic of the leaves of well-marked sclerophylls (Fig. 193); 

 there is often no spongy tissue, but frequently several layers of 

 palisade cells beneath both upper and lower epidermis so that the 

 structure of the leaf becomes bilaterally symmetrical. Some xero- 

 phytes are independent of such protections against transpiration, since 

 their highly concentrated cell sap enables them to absorb water from 

 very dry soil (p. 228). 



These anatomical arrangements are usually associated with 

 morphological peculiarities of the external form. 



Many xerophytes with small leaves have the branches crowded 

 together to form a dense cushion (e.g. many Alpine plants, Fig. 191) : 

 not only is transpira- 

 tion checked by this, 

 but a protection 

 against too strong in- 

 solation is obtained. 



A very effective 

 protection against 

 transpiration and 

 light is obtained by 

 the leaf surface being 

 placed vertically 



FIG. IQl.Baoulia mammillaria from New Zealand, showing the (Australian Acacias 

 cushion-like shape of the individual plant. (From SCHIMPER'S and MyrtaCCae) ; this 

 Plant-Geography.) ig of { en agsociated 



with a reduction of 



the lamina and a flattening of the petiole (PHYLLODES, Figs. 136, 192). 

 A similar position of the leaves is met with in some of our native 

 plants such as Lactuca scariola, the Compass Plant in which all the 

 leaves stand vertically and in the direction of north and south. Such 

 leaves avoid more or less completely the rays of the sun when this is 

 at its highest, and excessive heating and transpiration are thus 

 prevented. 



Very commonly the leaf surface is reduced. This takes place in 

 the grasses of exposed situations by the mi-oiling of the upper surface 

 (Fig. 194). In the Ericaceae, Genisteae, Cupressaceae, and some 

 New Zealand species of Veronica (cf. also Fig. 195), it is effected by 

 reduction of the lamina, which is completely lost in Cactaceae, in tree- 

 like species of Euphorbia, and in some Asclepiadaceae. With the 

 reduction in the leaf-surface the assimilation of carbon is also 

 diminished, and a compensatory development of chlorophyll-containing 

 parenchyma takes place in the stems of such plants. The twigs of 

 the Broom (Sarothamnus scoparius), which bear only occasional leaves 

 that are soon shed, are elongated and green (sclerocaulous plants). 



