GENERAL INTRODUCTION 22J 



organs and put out her new shoots during the cloudy and 

 genial days which usher in the winter. 



A second cause are the violent winds that shatter any stalk 

 which ventures to lift itself even a little above the ground. 

 Lastly, radiation during the icy nights compels the plants 

 to seek shelter near the soil ; for the ground preserves 

 more warmth than the air, and also stores the moisture 

 which is needed against a time of drought. The mother 

 earth is her child's natural protector. 



The composition of mountain soil differs greatly from 

 that of the lowlands. In chinks of rocks, at the foot of 

 broken cliffs, on weatherbeaten ridges is found a black, 

 porous humus as rich in nutritive matter as it is poor in 

 organic constituents. Centuries may pass before the 

 minute accretions form a kindly bed where plants can 

 thrive. First come the Lichen-folk, those earliest colo- 

 nisers of the rocks, establishing themselves here and 

 there in barren places where permanent moisture is 

 secured. The Mosses follow, richer in tissue and more 

 exacting than the frugal Lichen. The Mosses decompose, 

 and on their ruins more developed types are sown, only 

 to pass away in their turn and to furnish soil for the 

 higher types of vegetable life. 



Thus vegetation advances and is installed over all the 

 hills, thanks to the Lichens, which, as the precursors 

 and promoters of vegetable life, play an important part 

 in the economy of nature. 



1 have mentioned that this rock-soil is dark, spongy, 

 always cool and humid. Mixed with it in varying 

 quantity are pebbles and sand. The physical composition, 

 then, is such as to promote growth. Considered chemi- 

 cally, soil is of different kinds ; it contains the detritus 

 either of calcareous or of granitic rocks, and in composi- 

 tion generally corresponds to the nature of the rock that 

 is dominant in the neighbourhood. In calcareous districts 



