INFLUENCE OF HEAT ON CONFIGURATION AND DISTRIBUTION OF PLANTS. 523 



summer, as, for example, Lamium album, produce late in the autumn, under a 

 very low temperature (if they bloom a second time), corollas whose upper side is 

 tinged with red; and that in the winter, and in frosty habitats, the ray -florets 

 also of many Composite, as, for example, of the well-known Daisy (BelUs perennis), 

 are coloured red on that side which is turned towards the sky when the capitulum 

 is closed, and towards the ground when the capitulum is open. 



INFLUENCE OF HEAT ON THE CONFIGURATION AND DISTRIBUTION 



OF PLANTS. 



On high mountains near the snow-line, and generally in all those districts 

 where the heat supplied to the plants is extremely scant, there occurs, together 

 with a production of anthocyanin, a dwarf and tufted habit. Usually this 

 phenomenon is explained by the large amount of snow, which must have a great 

 effect in these frosty heights during the long winter, and it is believed that high 

 Alpine plants are protected by this form and position of their stems and leaves from 

 injury by the pressure of snow. It cannot indeed be denied that the pressure of 

 the snow has some influence on the form and direction of the stem-structures, and 

 this influence will be explained fully in the following pages in a particularly 

 instructive example, viz. in the mountain pines. But this nestling on the ground 

 of plants growing in the high Alps can only be partially referred to this cause. 



It is a mistake to suppose that the annual snow-fall increases with the height. 

 The amount of snow which falls attains a maximum at about 2500 metres above 

 the sea-level. This height marks only the upper limit of mountain pines, dwarf 

 junipers, alders, and rhododendrons. Above this the fall diminishes, and at a height 

 of 3000 metres the snow is no deeper than far down in the valleys. Even where 

 the maximum fall occurs trees are still met with; there are yet larches and AroUa 

 pines, which, on account of the great elasticity of their branches and the downward 

 direction of their older boughs, can bear very heavy weights of snow without 

 becoming broken or crushed. The willows of mountain regions, characterized by 

 the way in which their elongated stems and branches are pressed to the earth 

 (Salix serpyllifolia, S. retusa, Jacquiniana, reticulata), and which are represented 

 in fig. 131, grow, however, far above the tree limit, at a height above the sea 

 where the depth of snow, already beginning to diminish, is in no case greater than 

 in the valleys, where Purple and Sweet Willows, and other species of large-leaved 

 wiUows raise their straight stems several metres high above the ground on the 

 banks of streams. It must also be remembered that the woody growths close to 

 the ground in high Alpine regions are very often established on steep places, where 

 the snow could not easily lie, could in no instance be deeply piled up, and could not 

 exert a pressure on the stems and branches. The delicate Thyme-leaved Willow 

 (Salix serpyllifolia) nestles with an especial predilection to the surfaces of rocks, 

 and covers them with an actual carpet, and the Buckthorn (Rhamnus pumila) is 

 found exclusively on steep declivities, where it roots in the crevices of the narrow 



