CLIMATIC FACTORS 



117 



a heavy dew and by evening of the same day fell to 87 per cent. These 

 figures show the value of a ground layer of mosses for reducing evapora- 

 tion from the soil. 



Volkens (1887) believed that dew made possible the short-lived 

 vernal vegetation of the Lybian desert. One advantage of dew over 

 rain is its regularity (rf. Hiltner, 1930). 



Fig. G4. — Washing awa\- ot the vegetation by i.iiu i \ptien-marl of the Oued 

 Cheiuour, Province Coustautinc). A few cleander bushes remain as relicts. {Photo 

 hy Daguin.) 



Snow. — Snow is injurious in warm temperate climates because it 

 beats down plants, especially the broad-leaved evergreens, but it is 

 valued for its water-storing capacity. The significance of snow 

 increases toward the poles and toward the summits of mountains. In 

 the arctic and arctic-alpine zones, snow becomes the leading factor in 

 halting the march of vegetation. 



