CHAP. XXX TRANSPIRATION IN LAND-PLANTS 115 



in South America. The woolly coat acts as a sunshade, and serves to 

 moderate changes in temperature, also to reduce transpiration. As 

 a particular form of hair may be mentioned the scaly hair, which when 

 abundant lends a metallic lustre to plants such as the Elaeagnaceae, 

 also some species of Croton and Styrax. The coating of hairs is almost 

 invariably densest on the lower face of the leaf where the stomata occur. 

 Young stems and leaves are frequently densely coated with hair, more 

 densely than when older, as at the former stage they have greater need 

 of protection against intense transpiration. Sometimes, in dry parts of 

 tropical countries, the leaves produced just after the dry season and 

 those developed later on differ widely in appearance, the former being 

 more hairy, and the latter larger and greener. ^ But one group of 

 xerophytes, namely, succulent plants such as Cacteae, species of Aloe 

 and Agave, are usually smooth and quite devoid of any coating of hairs ; 

 in this case other protective measures are adopted. 



The production of hairs, like all other self-regulatory devices of the 

 plant, is possibly a direct adaptation to external conditions. According 

 to Vesque, hairiness and dryness of the atmosphere increase side by 

 side. Following Mer, Henslow - attributes the production of hairs to 

 ocal supply of nutriment, which is correlated with suppression of paren- 

 chyma ; according to him, the more the parenchyma is checked the 

 greater is the compensatory production of hairs. But even if this 

 hypothesis be correct it does not carry us much nearer to the compre- 

 hension of the correlation between hairiness and dryness. 



B. Investing Leaves. 



The young parts of the shoot are usually protected against intense 

 transpiration and intense light by older leaves. It is a quite general 

 phenomenon for the youngest foliage-leaves to be protected by older 

 ones in so-called ' open ' buds ; on the other hand, there are numerous 

 buds that are provided with thick hud-scales, such as are met with in 

 deciduous woody plants, not only in temperate and frigid countries, but 

 also, though less frequently, in the tropics.^ By the production of cork, 

 hairs, resin, and the hke, they are adapted not only to protect the young 

 leaves resting within the bud from transpiration, but also during foliation 

 to guard the buds against change of temperature."* In certain climates bud- 

 scales are rare ; for instance, Coville ^ writes in reference to the Death 

 Valley, ' scaly buds are almost unknown in the desert shrubs '. The 

 same is true of Mediterranean countries, where the rainfall takes place 

 in winter, and of the tropical rain-forest.*^ The young bud-parts of 

 many xerophytic mosses are protected by white hairs that clothe 

 the tips of the old leaves.' 



Stipules and leaf-sheaths (the latter, for instance, sheltering the young 

 inflorescences of dune-grasses) may perform the same service, though tliey 

 are not strictly included in the category of bud-scales ; ^ in this way the 



' H. Schinz, 1893. ' Henslow, 1S94, 1895. 



' Illustrations by Warming, 1892. 

 Griiss, 1892 ; Feist; Cadura; Percy Groom, 1893. 



^ Coville, 1893, p. 53. " Schimper, 1898 (1903. pp. 329-5 0- 



' See pp. 109-10. See illu.strations in Warming, 1907-9- 



I 2 



