MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 305 



to the lowlands, which extend as far as the northern slopes of the Alps. Where man 

 has not transformed the ground into arable soil, only moor and heath, heath and 

 moor, are seen in wearisome monotony. On the moors especially are always the 

 same plants — various Heaths (Calluna vulgaris, Erica Tetralix, Erica cinerea), 

 Black Crowberry (Empetrum nigrum), Whortleberry (Oxycoccos palustris), Marsh 

 Andromeda (Andromeda polifolia), Wild Rosemary (Ledum palustre) — all plants 

 with evergreen rolled leaves, just as on the mountain heights. Some of these small 

 evergreen bushes, viz. the Crowberry and the common Ling (Calluna vulgaris), may 

 be traced in an unbroken range from the plains up to a height of 2450 metres on 

 the slopes of the Alps. Strange to say, these plants do not blossom much earlier on 

 the lowlands than on the high Alpine regions, and it has actually been shown that 

 Calluna blooms rather sooner at a height of 2000 metres than in the northern portion 

 of the Baltic lowlands. How is this ? The winter snow has long disappeared from 

 the lowlands, while the hill-sides above are still concealed under their cold white 

 covering. The winter snow has gone, to be sure, but not the winter! While every- 

 thing around is already in blossom, while the ear is already visible on the stalks of 

 rye, the neighbouring moor is still dismal, waste, and lifeless. A month or so later 

 there is a stir on the dry soil of the cold moor, and the absorbent roots of the plants 

 which have evergreen rolled leaves commence their activity. When the warm days 

 of midsummer arrive and the sun sends down its powerful rays, the temperature of 

 the soil quickly increases, and indeed rises far more than would be thought possible. 

 The clamp cushion of bog-moss at mid-day feels quite warm; and a thermometer 

 placed 3 cms. below the surface in the uppermost mossy layer of a moor on a 

 cloudless summer day (22nd June) showed a temperature of 31° C. while the tem- 

 perature of the air in the shade was 13°! An unpleasant vapour rises from the damp 

 earth, which settles on the surface, and makes a walk over the moor particularly 

 disagreeable. Scarcely has the sun set in glowing red on the horizon when this 

 vapour condenses into patches of mist which settle over the dark expanse; stems, 

 branches, and leaves are covered with drops of water, and next morning everything 

 is as thoroughly soaked as if it had rained throughout the night. This process, 

 which is regularly repeated during the fine weather, is only interrupted when a 

 damp wind from the sea blows, driving masses of cloud over the heath, or when 

 copious rain saturates the soil. It needs no further showing that under such condi- 

 tions an abundant and continuous transpiration from plants is impossible, and that 

 in the short intervals which are allowed to the leaves for transpiration, the outlets 

 from the wide-meshed spongy parenchyma must not be obstructed; and it does 

 not need further proof that the evergreen rolled leaf is the form most suited and 

 adapted to these conditions. 



The flora of the Cape of Good Hope may not unjustly be compared with that of 

 the Baltic lowlands — countless low bushes which are very like Heaths, Wild Rose- 

 mary, and Crowberry in appearance — all with small rigid evergreen leaves, and entire 

 rolled-back margins; the upper side of the leaf usually dark green, the under side 

 having the same arrangements as shown in the rolled leaves of plants growing on 



