CHAP. XXX TRANSPIRATION IN LAND-PLANTS iii 



ronceals its stomata in hairy furrows. Movements associated with changes 

 in humidity are met with for instance, in Festuca ovina, Corynephorus 

 ranescens, many grasses growing in deserts, steppes, or on high mountains.^ 

 Dissected leaves, such as those of Artemisia campestris, often possess 

 very similar small, terete segments. 



5. The juncoid leaf is long, terete, devoid of furrows, and is seen in 

 species of J uncus, a number of Cyperaceae, and some alpine Umbelliferae 

 in South America. This form of leaf is mostly met with on wet, cold, 

 acid soil that is exposed to wind.- 



6. The succulent leaf may be mentioned here because, apart from 

 its thickness, it is often more or less terete, hnear, oblong, or spathulate, 

 devoid of teeth or other indentations ; examples are provided by Sedum 

 acre, Sempervivum tectorum and other Crassulaceae, species of Mesembry- 

 anthemum, Batis maritima, and other halophytes, and some Orchidaceae.^ 

 This form of leaf is characterized by the relative smallness of the surface 

 in comparison with the volume. Henslow's view that succulence is due 

 to the direct action of environment is probably correct.* 



7. The sclerophyllous or inyrtoid leaf. There are many other forms of 

 leaves not belonging to any of the preceding types, yet adapted to resist ex- 

 cessive transpiration ; among these may be specially noted the leaves of 

 plants that Schimper describes as being ' stiff-leaved ' or sclerophyllous.^ 

 The leaves may be small (as in Loiseleuria procumbens and Diapensia) ; 

 or narrow and stiff, more or less revolute (as in Lavandula, Hyssopus, 

 and other Mediterranean species) ; or broader (as in Myrtus communis 

 Nerium, Olea, Rhododendron), obovate, oblong, elliptical, lanceolate, or 

 of some other simple form, devoid of teeth or other indentations. They 

 are flat, coriaceous, and stiff, largely owing to the thick-walled epidermis, 

 and are evergreen. To protect themselves against excessive transpiration, 

 such leaves usually have additional contrivances which will be described 

 hereafter.^ Here we may include, too, the leaf-like cladodes of Ruscus 

 aculeatus and other species, also of Semele androgyna. 



The shoots possessing leaves of the forms enumerated, and especially 

 the pinoid, ericoid, and cupressoid, are usually extremely rich in leaves. 

 By increasing the number of leaves the plant strives to compensate 

 for the decreased assimilation that is caused by reduction in their size. 

 Furthermore, possibly the close aggregation of the leaves on shoots 

 with short segments may itself retard transpiration. 



(6) Forms of Shoots. 



Shoots with greatly reduced or caducous leaves occur in connexion 

 with many xerophytes. The foHage-leaf has vanished, and its functions 

 have been taken over by the stem, which produces palisade parenchyma. 

 The epidermis of the stem in this case functions for a number of years. 

 Such aphyllous shoots show several forms : 



1. The winged shoot is often aphyllous, and the light strikes its 

 assimilatory tissue at acute angles.' 



2. The switch, or Spartium form of shoot, is switch-shaped, erect, 



' See p. 109. - See Gobel, 1889-03, Bd. II. ' Sec Warming, 1897. 



* See p. 371. * Schimper, 1898, 



* For further information see papers by Vesque, Volkens, Gobel, Warming. Hen- 

 slow, and Schimper. ' Sec pp. 19, 113, 114. 



