MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 



303 



it with all other rolled leaves, whether they belong to Cape plants or to heath plants 

 of the Baltic lowlands. 



It cannot be doubted that the mechanism of rolled leaves, as just described, 

 furnishes a protection for the storaata against moisture, and keeps open a passage 

 for aqueous vapour and excreted gases. The question is now only how it comes 

 about that this arrangement is to be met with in plants of such widely distant 

 countries and under such differences of climate ? 



In order to understand this clearly, let us imagine ourselves in some of the 

 regions which are specially characterized by the abundance of plants with rolled 

 leaves. First, on one of the high ridges of the Central Alps, where the low-lying 

 Azalea spreads a thick covering over the soil, where Erica carnea in great quantity 



Fig. 72. —Vertical Section through a Rolled Leaf of the Trailing Azalea {Azalea procumbens) ; x 140. 



covers broad slopes, where Di^yas octopetala, Salix reticulata, Homogyne discolor, 

 Saxifraga ccesia, and many other plants which possess evergreen rolled leaves 

 weave their carpet over the stony earth. The ground in which all these plants are 

 rooted, and from which they draw their fluid nourishment, has many natural dikes 

 and retains a large quantity of water, not only from the melting of the heavy 

 winter mantle of snow, but also from the abundant rain of summer. For weeks 

 together the heights are wrapped in a cold mist which saturates everything with 

 moisture, and drops of water hang from the stems and leaves, unable to 

 evaporate as long as the air remains so supercharged with vapour. At length the 

 sky clears again, and the water on the plants begins to disappear. But even during 

 the fine night following, all the plants become covered with a very heavy dew in 

 consequence of their rapid cooling and radiation, and this not unfrequently remains 

 until the middle of the next day. Transpiration at last occurs when the sun shines, 

 and particularly if dry winds sweep over the heights. But who knows how long 

 this state of things will continue ? Each moment is precious, and every hindrance 

 to the evaporation, so important for the plants, would be a distinct disadvantage. 

 The outlets for aqueous vapour on the under side of the leaves especially should not 

 be obstructed, and the above described contrivance is arranged with this end in 



