makes the existence of the plant less dependent on external 

 conditions. 



Another point which has already been referred to is the 

 low stature and stunted character of so many Alpine plants. 

 Undoubtedly there are marked exceptions, such as the tall 

 Gentians ; but, generally speaking, it must be admitted that 

 mountain plants are short stemmed and dwarfed, the vegetative 

 part of the plant being often merely represented by a wreath 

 of leaves. This is also very largely the result of the peculiar 

 conditions of plant-life at high altitudes. The bright sunlight 

 by day and intense cold at night have both a retarding effect 

 on the growth of the stem. As regards light, the converse 

 result is well seen in the long, lanky shoots of a potato grow- 

 ing in a dark cellar. But there is also another factor of some 

 importance. The green leaves of plants are constantly giv- 

 ing out as watery vapour the fluid which has been absorbed 

 by the roots. The decreased atmospheric pressure at high 

 altitudes, the general dryness of the air, and the bright 

 sunlight by day all unite to accelerate this process. During 

 the day the ground is warmed even to a larger extent than 

 the air, and thus the absorbtion of water by the roots is 

 correspondingly hastened. During the night the intense 

 radiation rapidly cools down the soil on which the plant 

 grows, but the surrounding air is not cooled to anything 

 like the same extent. The result is that the absorption of 

 fluid by the roots is diminished in a much larger degree than 

 the effusion of watery vapour from the leaves, and the plant 



xvii 



