THE FLORA 399 



whereby the leaflets may open out in moderate illumina- 

 tion and close together under conditions where transpira- 

 tion tends to be excessive, in strong wind or hot sun. 

 Another device consists in folding back the edges of the 

 leaves underneath (Cassiope tetragona, Ledum, Pinguicula) ; 

 and still another, in crowding them thickly together (Cas- 

 siope, Bryanthus) . All of these many modifications have 

 the one object of securing a reduced or reducible transpir- 

 ing surface, and almost all the plants of Labrador adopt 

 one or another of these methods of accomplishing it. The 

 examples given are only illustrative, and might be increased 

 many fold under almost every heading. 



4. Increase in thickness of the leaf and of its cuticle. 

 Many leaves are tough and leathery (Ericacece, Empetrum) ; 

 or have thick, strong cuticle (grasses and sedges) ; or develop 

 a waxy, resinous, or varnished coating on the under side 

 or on both (Andromeda, Vaccinium Vitis-Idcea, Pyrola, 

 some Salices, evergreens). 



5. Development of water-storing cells in stem or leaves, 

 the latter becoming thick and succulent. This is not of 

 very common occurrence. It is found, however, for ex- 

 ample, in saxifrages, Sedum, and Sphagnum. 



6. Protection of the stomata from the influences that 

 tend to cause evaporation through them. This may be 

 secured by (1) turning away the under side of the leaf from 

 sun and wind, as in the pinnately divided leaves men- 

 tioned already ; (2) sinking the stomata in the leaf-surface 

 (Andromeda, Empetrum} ; (3) covering the under side of 

 the leaf and sometimes also its upper side and the stem with 

 a protecting layer of hairs or tomentum, which may vary 

 greatly in length and thickness, from a mere silvery or 



