302 MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 



of the leaf. Oa the right and left of the middle ridge lie two deep grooves, wlneh 

 are apparent to the naked eye as light stripes between the dark gi-een projecting 

 portions. This is the case, for example, in the leaves of the Azalea 'procumhens, 

 also in one of the Ericaceae known by the name of Loiseleurea, which covere the 

 soil with a close-matted carpet wherever it makes its appearance, and is widely 

 distributed through Labrador, Greenland, Iceland, Lappland, and generally through 

 the whole Arctic region, as well as over the high mountain.s of Scandinavia, the 

 Pyrenees, Alps, and Carpathians. The annexed figure 72 represents a transveree 

 section through a single rolled leaf of Azalea, a hundred and forty times its natural 

 size. 



Occasionally several strong anastomosing ribs project from the under side of 

 the rolled leaf, inclosing small pits and depressions in whose depth stomata are 

 situated, as may be seen in the leaves of the widely distributed Willow, Salix 

 •reticulata (see fig. 71 *). 



Although all these rolled leaves have an appearance of firmness and solidity, 

 and fre(juently remind one of the needle-like leaves of the conifers, they are, unlike 

 these, filled up with a very loose spongy parenchyma, which takes up far more 

 room than the palisade tissue lying beneath the epidermis of the upper side. The 

 upper epidermis of all rolled leaves is easily wetted, fre(juently uneven and finely 

 wrinkled, destitute of any waxy covering; the cells strongly thickened on their 

 outer walls, and pressed closely together, so as to leave no spaces between them. On 

 the under side it is very different. Here stomata ai-e present in great number, and 

 the epidermis is either covered with wax, as in the Marsh Andromeda, the Whortle- 

 berry, and the Reticulate Willow {Andromeda, iiolifolia, Oxijcoccos palustris, and 

 Salix reticulata), or it is clothed with a fine felt- work, as, for example, in Ledum 

 palustre. Very often peculiar rod-shaped or filamentous projections of the cuticle 

 are pi-esent, which at first sight might be taken for hairs, but which differ from 

 hairs in being solid, not hollow. Figs. 72 and 71 '■ -• ^ show these structures (which 

 may be considered as counterparts of the cuticular pegs on the bamboo leaf) on 

 the under side of Azalea p)''''Ocumhens, Erica caffra, and Andromeda tetra(]ona, 

 as well as on the edges of the fissure which leads into the hollow leaf of the Crow- 

 berry {Empetrum nigimm). These structures are to be found almost without 

 exception in the heathers of the northern moors as well as in the Mediterranean and 

 Cape flora. The importance of this continuous delicate coat lies chieflj^ in the fact 

 that air adheres to it as to the cuticular l^egs of the bamboo leaf, and indeed so 

 firmly that even water, under considerable pressure, is not able to displace it. On 

 placing a leaf of Azcdea procumhens under water, two elongated air-bubbles are 

 seen along the two longitudinal furrows, wliich glisten like two strips of silver. 

 Even shaking the leaf to and fro will not dislodge these air-vesicles, and even if the 

 branch has been left submerged for a week, this air will still cling to the depressions 

 in whose depths the stomata occur. If the branch be removed from the water it 

 will be seen that the upper side of the leaves is wet, while water has been kept 

 away from the stomata of the under side. And as with Azalea procumhens, so is 



