FALL OF THE LEAF. 357 



be stopped by them, and which the plant carries out, or endeavours to carry out, 

 without immediate external stimulus. 



It is highly interesting, with respect to the acceleration or retardation of the leaf- 

 fall, to observe how the same species of plant will behave under various favourable 

 or retarding external influences; or how, in each region and locality, a selection has 

 been made to a certain extent of the plants best adapted to the given conditions. 

 First it is to be noticed that, under otherwise similar circumstances, the foliage 

 remains green for a longer time, and is retained longer on the branches in places 

 where the soil and air are more humid. In damp, shady, wooded glens, not only 

 ferns, but the leaves of birches, beeches, and aspens are still green while on the 

 sunny hillocks close at hand the brown leaves flutter down on to the withered 

 fronds of the Bracken Fern. 



The most remarkable fact, however, is that in elevated mountain regions a plant 

 loses its leaves much earlier than does the same species growing in the lowlands. 

 From the fact that in the Alps, the larches and whortleberry bushes, on the upper 

 limits of the woods, put forth their green needles and leaves about a month later 

 than in the valleys at a height of 600 metres above the sea, it would naturally be 

 expected that this considerable delay would be compensated for by a corresponding 

 postponement of the ending of the year's work, and that the fall of the foliage on 

 the upper limits of the wood would also be postponed for about a month. But this 

 is far from being the case. The same species of larch which becomes green a month 

 later, up on the mountain slopes, also turns yellow a month earlier in the autumn. 

 While the whortleberry bushes in the depths of the valley are still adorned with 

 dark-green leaves, the same species growing in the glades on the upper limit of the 

 wood, already, from the valley, appear to be shrouded in deep crimson. Their leaves 

 are becoming discoloured above, and are withering and dropping from the twigs. 

 The explanation of this phenomenon follows naturally from what has just been said. 

 In the high mountain regions where tall trees find their uppermost limit, the ground 

 is frequently covered with frost at the end of August; snow falls regularly in the 

 first half of September, and although this may be melted in snnny places, the soil 

 is nevertheless thoroughly cooled by the water so produced. The days rapidly 

 become shorter, and the sunbeams can no longer replace the heat lost by radiation 

 in the lengthened nights. The temperature of the soil in which the plants are 

 rooted consequently falls rapidly, and the immediate results are that the absorbent 

 roots stop working, the decolorization progresses, and the foliage-leaves, which are 

 no longer able to repair the loss caused by transpiration, wither and fall away. 

 Accordingly, on this upper tree limit, only those larches and whortleberry-bushes 

 can thrive which are organized to commence their year's work a month later, 

 and to finish it a month earlier, than those which have taken up their po* 



1400 metres below. 



This obviously applies not only to the larches and whortleberries, cited her 

 examples, but to all other plants whose range of distribution extends from the 

 lowlands up to the wood limit on the slopes of the mountains. It also applies 



