io6 ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect, iii 



Ephedra, Acanthosicyos horrida, and species of Genista. The furrows 

 occur on the upper face of the leaf in many steppe-grasses, and others, 

 such as Weingaertneria canescens, Festuca ovina, Psamma (Ammophila) 

 arenaria, Aristida, Stipa, Sporobolus spicatus, Cynodon Dactylon ; in 

 such cases the furrows may be narrowed above, and the stomata more 

 completely enclosed by the rolling up of the leaf.^ Furrows or broader 

 channels clothed with hairs occur on the lower face of the leaf in many 

 plants, such as Empetrum, Phyllodoce caerulea, Calluna, species of Erica, 

 Loiseleuria procumbens. Ledum palustre, Cassiope tetragona,^ and 

 Dilleniaceae.^ To this category may be added leaves, such as those of 

 Dryas octopetala, with margins revolute to a smaller extent and with 

 stomata on their hairy under-surfaces. 



If leaves are permanently and steeply directed upwards as well as 

 appressed, so that the lower face is the more strongly illuminated, then 

 the lower side may be differentiated like the normal upper side and possess 

 palisade tissue ; in such cases the stomatiferous furrow occurs on the 

 upper face, as in Passerina filiformis, Ozothamnus, and Lepidophyllum.* 

 In these plants, therefore, the easy egress of aqueous vapour is checked in 

 more than one way. 



That these features stand in direct relation to dryness of climate 

 is shown by species such as Ledum palustre and Andromeda pohfolia, 

 whose leaves are smaller and more revolute the more they are exposed 

 to wind and drought.^ 



The cases last mentioned form transitions to flat, broad leaves in 

 which the sole screen over the stomata is formed by a dense investment 

 of felted or peltate hairs, as in Olea, Rhododendron, and Elaeagnaceae, 

 or some other kind of tomentum on the lower face of the leaf. Sometimes 

 leaves of this kind have veins strongly projecting on the lower face, and 

 as the stomata lie in the meshes of the network of veins, they are to a 

 certain extent sunk below the general surface, as for example in the West 

 Indian Lantana involucrata. 



When stomata are in secluded cavities containing much aqueous 

 vapour, or lie under a dense tomentum, they are usually raised above 

 the adjoining surface, just as in the leaves of plants that live as a whole 

 in contact with moist air. 



It may also be noted that stomata are enclosed in cavities, or sheltered 

 under tomenta, in order that they may be protected from occlusion by 

 water.' 



(h) Intercellular spaces. 



Respiratory cavities may be structurally fitted to regulate transpira- 

 tion ; they may have cuticularized walls, or be surrounded by special 

 cells as in the Restiaceae,^ or may be very small. In many instances 

 cuticle extends from the outer face of the epidermis, through the stoma, 

 and down over the walls of the respiratory cavity.^ 



The width of intercellular spaces varies with the external conditions ; 



' See p. 267. 



^ Warming, 1889; Gruber, 1882; Ljungstrom, 1883; H. E. Petersen, 1908. 



^ Steppuhn, 1895. * Lazniewski, 1896; Gobel, 1891. 



Warming, 1887, p. no. " See p. 254. 



' Kerner, 1887. * Pfitzer, 1870-2. ' A. de Bary, 1877, p. 79. 



