CHAP. XXX TRANSPIRATION IN LAND-PLANTS 109 



or during the dry or cold season, and remain leafless for a long time ; 

 such species are described as deciduous, in opposition to evergreen. In 

 these plants during the unfavourable season all parts above ground 

 are usually protected from transpiration by means of cork or bud-scales, 

 the latter being covered with cork or other bodies that check evaporation. 



In all these plants the structure of the foliage-leaf is usually not at all 

 or only slightly xerophytic, but is mesophytic, if the vegetative season 

 be sufficiently moist. In Egypt ^ and in the lowland of Madeira,^ where 

 the atmospheric humidity is small even in winter, annual herbs growing 

 on uncultivated land adopt protective measures against drought quite 

 different from those employed by weeds in the irrigated fields. Protection 

 against drought is more pronounced the more a species prolongs its 

 vegetative season beyond the commencement of the dry season. Accord- 

 ing to Kerner^ the fohage of deciduous trees on the Austrian coast is 

 very hairy on the under-side, because the summer is exceedingly dry. 



The transpiring surface is reduced in quite a different manner in other 

 plants, for example, in grasses whose leaves in dry weather become 

 rolled up, so that they form tubes, and thus appear filiform or bristle- 

 like. Such is the case with Psamma (Ammophila) arenaria, Weingaert- 

 neria (Corynephorus) canescens, species of Festuca, and many other 

 grasses inhabiting dune or heath ; and, in Mediterranean countries, 

 Avith species of Stipa, Lygeum, Aristida* ; rolled-up leaves are particularly 

 characteristic of steppe-grasses, and are also met with on sahne soil 

 in Triticum junceum and other grasses. As the air becomes drier the 

 leaf rolls up so that the transpiring upper surface, where the stomata 

 mainly or solely occur, is less exposed to transpiration ; the stomata 

 thus become enclosed in a space in which the air is more or less motion- 

 less. In moist weather the leaf unfurls. Among Cyperaceae similar 

 though less considerable movements are exhibited. In these movements 

 a part is played by the hinge-cells lying in fun^ows on the upper face 

 of the leaves of grasses ; these cells are deeper than the other epidermal 

 cells, and their cellulose walls are easily folded as the leaf curls. The 

 motive force would appear to reside in the bast-tissue, which is usually 

 near the under-face of the leaf, and either absorbs or gives out water, 

 thus swelling or contracting. But the turgor of the mesophyll seems to 

 play an important part, at least in some cases.^ 



Similar movements are exhibited by a number of Dicotyledones, 

 including Hieracium Pilosella, Antennaria dioica, Crepis tectorum.'' 

 West-Indian species of Croton,'^ and Euphorbia Paralias,^ which grows 

 on the dunes of western and southern Europe. The leaves of Erica 

 Tetrahx,^ and Ledum palustre are less rolled on moist than on dry 

 soil. Among Cryptogamia may be mentioned some ferns ^^ and mosses, 

 including species of Rhacomitrium, Tortula, and Polytrichum. The 

 leaves of Rhacomitrium canescens and Tortula ruralis in dry weather 

 are folded together, and the shoots quite grey with densely set, long 

 hairs ; but when the weather or soil is humid they are extended in a 



' Volkens, 1887. ' Vahl, 1907 b. * Kcrncr, 1886. 



* See Duval-Jouve, 1875; Tschirch, 1882; Warming, 1891. 

 ' Duval-Jouve, 1875; Tschirch, 1882. ' Willc. 1887. 



' Warming, 18996. ' Giltay, 1886. ' Grabncr, 1895. 



' See Wittrock, 1891. 



