ADAPTATIONS FOR REDUCING TRANSPIRATION. 389 



(a) Thickening of the cuticle of the epidermis cells, which is par- 

 ticularly noticeable in the large grasses and some of the woody plants 

 with mostly evergreen leaves, as Smilax spp., Quercus virgi?iia?ia, 

 Gelsemium sempervirens, etc., but is common to nearly all the strand 

 species. A surface which exhibits great thickening of the cuticle is 

 usually not otherwise protected. Where a dense covering of hairs or 

 other means of protection exists the cuticle is apt to be comparatively 

 thin, e. g., in Oenothera humifusa. The polished surface which usu- 

 ally accompanies such thickening may possibly be useful to the plant 

 by reflecting some of the light rays which fall upon the leaves. A 

 rough surface of the cuticle, due either to wrinkles or to warts, char- 

 acterizes some species of this formation, e. g., Lonicera sempervirens 

 and Galium hispidulum. In the latter plant the thick cuticle is both 

 strongly wrinkled and papillose. It has been suggested that such 

 roughening is useful by diverting some of the incident light rays. 



(b) A thin coating of wax, giving the surface a glaucous appear- 

 ance. Panicum amarum is glaucous all over, while Smilax glauca 

 and Lonicera sempervirens have the lower leaf surface conspicuously 

 wax- coated. 



(c) A dense covering of hairs. These may be simple, in which case 

 they are often elongated, and form a villous or tomentous covering: 

 on both surfaces of the leaf in Oenothera humifusa (stomata about 

 equally numerous on both surfaces), Hudsonia tomentosa, and Lechea 

 maritima; only on the under leaf surface in Rubus cuneif alius. Other 

 species have hairs that are irregularly branched, as Physalis viscosa, 

 or stellate and scale-like, as on the leaves of Helianthemum canadense 

 and the lower leaf surface of the live oak ( Quercus virginiana). Short, 

 stout, thick- walled, unicellular hairs also line the walls of the furrows 

 on the ventral (upper) leaf surface (where most of the stomata are 

 placed) in such grasses as Uniola paniculata, Ammophila arenaria, 

 and Spartina patens, thus preventing a rapid movement of the air 

 about the stomata. Similar hairs also occur on the dorsal leaf sur- 

 face of Lonicera sempervirens and on both surfaces in Galium hispi- 

 dulum, but probably not in sufficient numbers to be of service in 

 reducing transpiration. As noted above, the cuticle is usually thin 

 where the surface is protected by a dense coat of hairs. 1 



(4) Modifications of the internal structure of the leaves. In the 

 few species of the Sand Strand formation whose leaves are orthotropic 

 (vertical) or approximately so, there is a tendency to isolateral struc- 

 ture of those organs. In other words, the two faces of the leaf, ven- 

 tral and dorsal, are alike or nearly alike. Thus the leaves of Smilax 



1 The bicellular hairs of Lechea and Hudsonia, of a type apparently peculiar to 

 the family Cistaceae, are described and figured (for Lechea maritima) in the 

 chapter on Anatomy. Such hairs, together with glandular and stellate ones, also 

 occur in Helianthemum canadense, the third representative of this family in the 

 sand strand of the Dismal Swamp region. 



