MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 293 



It should be particularly noticed here that plants with two-coloured leaves, 



x such as those whose upper surfaces are green, smooth, free from stomata and easily 



wetted, while their under surfaces, covered with gray or white hairs, and rich in 



stomata, which cannot be wetted, are generally to be found on the banks of rivers 



and streams. 



In the open woods which skirt the banks of rivers in the valleys of moun- 

 tainous districts, i.e. in places where mist rises on summer evenings, and all the 

 twigs, leaves, and stalks are covered with drops of water, the most characteristic 

 plants are the Gray Alder (Alnus incana) and the Gray Willow (Salix incana), 

 and as undergrowth everywhere the Raspberry— all plants adorned with the two- 

 coloured leaves just described. Leaving the region of woods growing on river 

 banks for the neighbouring meadows, through which ripples fresh water from a 

 spring, and where everything drips with dew from evening until the middle of 

 the following day, we come to the natural home of herbs and shrubs with leaves 

 green on the upper and white on the under sides. There Fuller's Thistles (Cirsium 

 heterophyllum and canum) grow luxuriantly, and the Meadow-sweet, with its 

 large two-coloured leaves; whilst the whole course of the brook is bordered by the 

 Colt's-foot (Tussilago Farfara) with leaves which may be considered typical of this 

 group. 



What a contrast does this present to the lofty vaults of a dense forest, perhaps 

 only a thousand paces away, where on the shady ground little or no dew is 

 formed, and where the leaves which canopy the brown soil are never exposed to a 

 thorough wetting! No parti-coloured leaf is to be found there, no leaves whose 

 upper surface is green and smooth, while the under side is covered with white 

 hairs; and plants which exhibit a thick coating of wax on their under surface, like 

 the Primula farinosa of the moors, are also absent. On the other hand ferns are 

 here, as for example the Hard Fern (Blecknwm Spicant), whose leaves are furnished 

 with stomata which open quite without protection on the tops of projecting undula- 

 tions. This contrast between the leaves of plants in the open marshy country and 

 in the interior of forests is found, not only in the colder territories of the north, 

 but also in tropical districts. Moreover, plants whose leaves are covered with white 

 hairs on the under surface are never to be found under the close leafy roof of huge 

 trees which prevent nocturnal radiation and the formation of dew. Here occur, 

 rather, plants having totally unprotected stomata opening on slightly raised areas 

 of the surface, as for example in Pomaderis phylicifolia, and on the leaves of the 

 Pepper family, e.g. Peperomia arifolia (see fig. 64 3 and 64 4 ). 



A very remarkable contrivance by which stomata are protected from moisture 

 consists in providing the stomata of the upper surface with countless papillae and 

 cone-shaped projections ; between them, of course, being innumerable hollows and 

 depressions. Falling water-drops roll off such surfaces; the water cannot displace 

 the atmospheric air in the depressions, and therefore the leaves and stems, in so far 

 as their epidermis presents the aforesaid irregularities, appear covered with a thin 

 layer of air. As the stomata are situated in the small hollows, they always remain 



