CHAP. XXX TRANSPIRATION IN LAND-PLANTS 105 



(^ontrast with, and be narrower than, those of water-plants in which they 

 are usually very large.^ At the same time there is an extreme difference 

 in regard to the stomata present. 



(a) Stomata. 



Stomata, as Leitgeb and Schwendener have proved, are adapted by 

 their mobihty and structure to regulate transpiration. They close when 

 excessive transpiration is threatening, when leaves are withering because 

 of lack of moisture in the soil, also while the leaves of many plants are 

 resting during winter ; and they re-open when there is no further danger. 

 The guard-cells of certain desert-plants are mobile only in young leaves ; 

 but in the old leaves they become immobile owing to strong thickening 

 of their walls, and the stomata may become blocked with wax or resin.''' 

 Floating-leaves of water-plants have stomata on the surface exposed to 

 the air, but these assume a pecuHar form and soon lose their power of 

 movement.'^ 



The number of stomata depends upon the nature of the environment. 

 As a general rule, the drier a habitat is the fewer are the stomata, as may 

 be seen best when comparison is made between closely alhed species.^ 



The distribution of stomata is most intimately connected with transpira- 

 tion and with the conditions of moisture. Meadow-grasses and other 

 mesophilous land-plants, as a rule, have stomata on both faces of the 

 leaf ; steppe-grasses only on the furrowed upper face ^ ; other xerophytes 

 usually only on the lower face, where the stomata are often concealed 

 in such a way as to render transpiration more difficult. 



Stomata of land-plants are frequently sunk beneath the general 

 surface in pits, furrows, and the hke, which are often hned with hairs. 

 There is thus formed a space containing air, which escapes only with 

 difficulty, is screened from the wind, and becomes charged with aqueous 

 vapour ; the result is that transpiration is retarded. Various arrange- 

 ments of this kind are described in the succeeding paragraphs. 



The simplest device is the production outside a single stoma of a 

 saucer-, urn-, or funnel-shaped cavity, which is formed either by cuticular 

 processes giving rise to the outer stomatal cavity, or by the adjoining 

 epidermal cells projecting above the stoma, which is sunk within an 

 external respiratory chamber,^ as in Pinus sylvestris and some Proteaceae. 

 In Euphorbia Paralias,'' also in various grasses and sedges,*^ the stoma is 

 surrounded by low papillae. 



Groups of stomata in pits, whose narrow apertures are almost occluded 

 by hairs, occur on the lower face of the leaves of Nerium, Banksia, and 

 other xerophytes. 



In numerous plants stomata are lodged in longitudinal furrows, and 

 then are usually confined to the furrows, whose margins are often more 

 or less beset with hairs. Many stems, especially switch-shaped ones, have 

 deep furrows to which the stomata are limited, as is the case in Casuarina, 



' See p. 98. ' Wilhelm. 1S83; Volkcns, 1890: Gilg, 1891. 



' Habcrlandt, 1904, p. 412. 



* Pfitzer, 1870-2; Zingeler, 1873; Czech, 1869; Tschirch, 1881; Volkcns. 

 1 881; Altenkirch, 1894. 



' Pfitzer, 1870-2. Tschirch, 1881, 1882. ' Giltay, 1886. 



* Volkens, 1890; Kihlman, 1890; Kaunkiiir, 1895-9. 



