304 MAINTENANCE OF A FREE PASSAGE FOE AQUEOUS VAPOUR. 



view. It can hardly be doubted that the earlier mentioned plants of high moun- 

 tainous regions cease to transpire for weeks at a time in the wet seasons, when a 

 thick unbroken mist covers the slopes, and earth, stones, and vegetation are dripping 

 with moisture; and of course the conduction of food-salts to the green leaves is 

 interrupted to a corresponding extent. If one considers how short a period is 

 afforded to plants of high mountain districts in which to perform their year's work, 

 it will be understood how the most active means for promoting transpiration must 

 be brought into play, and how everything which might interrupt or limit this 

 process, so in portant to the welfare of the plant, must be avoided. A few months 

 after the last snow has melted on the heights, fresh snow again falls, and entirely 

 prevents growth and nourishment during the long winter. 



These climatic conditions account for the fact that so many Alpine plants, 

 almost all those having rolled leaves, are evergreen. It is necessary that every 

 sunbeam during the short vegetative period should be utilized, and that the leaves 

 retained from the previous year should be able to transpire and to form organic 

 materials on the first sunny day after the winter snow has melted, although the soil 

 may have become only slightly warmed. It may perhaps be urged against this 

 explanation that though, in the steppes the period of vegetation is restricted to the 

 brief space of three months, nevertheless evergreen plants with rolled leaves are 

 completely absent. But the conditions of moisture on the steppes during this three 

 months' vegetative period are essentially different from those of the high mountain 

 region. In the steppes, transpiration is never brought to a temporary standstill by 

 too much moisture; evaporation can take place uninterruptedly from the leaves, and 

 they have to be protected not from moisture, but from over-transpiration. With the 

 exception of the halophytes and a few other growths which are particularly well 

 protected, no plants, on account of the extreme dryness of the air, can retain their 

 green foliage in the height of summer on the steppes. 



Some of the plants which adorn the high mountains of southern regions make 

 their appearance in the lower plains of the extreme north. The same carpet of 

 Trailing Azalea, Dwarf Willows, and Dryas (Azalea procumbens, Salix reticulata, 

 Dryas octopetala) is found on the soil underfoot. In addition are other small plants 

 which remain green during the winter {e.g. Gassiojoe tetragona), which are similarly 

 provided with rolled leaves. Even if we were not informed by Arctic explorers 

 that the number of foggy days in the course of the short Arctic summer is much 

 greater than on the mountain heights of the south, and that therefore a help instead 

 of a hindrance to transpiration is required, the utmost use being made of the short 

 time in which it is possible to draw food-salts from the soil, we might infer this to 

 be the case from the frequent appearance of these small carpet-forming plants with 

 their evergreen rolled leaves. Apart from other considerations, and disregarding 

 the development of the various floral areas in point of time, the above signification 

 of the evergreen rolled leaves explains the similarity and partial identity of the 

 arctic flora with that of the heights mentioned. 



Let us turn now to the low-lying country along the North and Baltic Seas, and 



