176 THE BIOLOGY OF FLOWERING PLANTS 



on the stems of switch plants, in which the leaves are much 

 reduced, as in Ephedra, or fused with the stem, as in 

 Casuarina, or small and short-lived, as in Spartium junceum. 

 A similar feature is found in the leaves of Empetrum and of 

 many Ericaceae. The stomata are confined to the lower 

 surface, and the inrolling of the leaf margins places them in 

 a partially enclosed space. 



The formation of pits is arrived at in a variety of ways. 

 In Nerium Oleander the pits are depressions in the leaf 

 surface in which groups of stomata occur. In Pinus and 

 Juniperus, single stomata are sunk by the overarching of 

 the neighbouring cells of the epiderm and hypoderm. 

 This is the case, too, in the Australian Hakea suaveolens. 

 The guard cells are, as a rule, smaller than the epiderm cells, 

 and, if the disproportion is great and they are close to the 

 inner edge of the latter, a pit is formed as in Spartium 

 jtinceutn. Perhaps the most frequent case is that in which 

 the pit is formed by the presence of an abnormally thick 

 cuticle. A thick cuticle is the commonest feature of 

 xerophytic foliage, and is responsible for the glistening 

 appearance of leaves like those of the holly or cherry laurel. 

 The thick cuticle depresses cuticular transpiration. Renner 

 reckons that in leaves such as those of the rhododendron 

 cuticular transpiration is practically nil. It is commonly 

 associated with a hard leathery leaf, with much sclerenchyma- 

 tous mechanical tissue, which prevents flagging, and a close 

 structure with reduced intercellular spaces ; this may 

 reduce evaporation from the internal surfaces. Combined 

 with a distribution of stomata on the under surfaces of the 

 leaves only, these features give a type of leaf highly resistant 

 to drought. It is the characteristic foliage of evergreen 

 trees, especially of those which retain their leaves through a 

 hot dry summer, as, for example, the evergreen oaks of the 

 Mediterranean and California, and many trees and shrubs 

 of the tropical thorn forests and scrub. This vegetation 

 is described by its leaf type as sclerophyllous. In such 

 shrubs as the holly the cuticle forms a layer not quite so 

 thick as the epidermal wall, but frequently it attains a much 



