i6 



THE FACTORS 



[Part I 



of sea-shores, epiphytes) ; in districts with cold winters, they fall off both 

 in number and dimensions, and only those species that shrink up consider- 

 ably in winter appear to withstand low temperature for any length of time. 



A considerable reduction in 

 the transpiring surface, a 

 formation of thorns, a dense 

 coating with hairs (Fig. i8) 

 are rather characteristic of 

 dry air, whilst in moist air 

 the leaves are more fre- 

 quently well developed and 

 glabrous. Warm districts with 

 prolonged periods of drought, 

 such as tropical and sub- 

 tropical deserts, are char- 

 acterized by the presence of 

 endochylous succulent plants, 

 whereas a perichylous con- 

 struction indicates a plant 

 accustomed to a more fre- 

 quent though a scanty water-supply, such as characterizes the sea-shore 

 and the epiphytic conditions within humid forests. Mobile pinnate leaves 



Fig. 1 8. Xerophilous structure. Dry climate of the 

 Egyptian desert. Heli.-inthemum kahiricum. Transverse 

 section of a very h.iiry leaf. Magnified 40. After 

 Volkens. 



Figs. 19 and 20. Xerophilous structure. Cushion-like growth. Left hand : Raoulia mani- 

 millaris from New Zealand. One-sixth natural size. Right hand : Dionj'sia sp. from the mountains 

 of Persia. Part of a cushion. Natural size. 



point to rapid changes in the environment; vertically arranged leaf-surfaces 

 to strong insolation. The leathery foliage of sclerophyllous plants is found 

 indeed in all formations of xerophytes, but is commonest in mild temperate 



