PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 



315 



The relations between the hairy covering on the upper side of the leaf and trans- 

 piration stand out, most strikingly, in those districts where plants during their 

 veo-etation period are, as a rule, exposed to dry air only for a few hours each day, 

 and where their activity is not interrupted by a long warm dry period, but by frost 

 and cold — as is the case, for example, in the Alpine region of mountain heights. On 

 the Alps, the drying up of flowering plants by the sun only occurs in a very few 



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ti^' 70 — l,deU\eiss {Gnaphalium Leontopodnim). 



cases, viz., where the scanty soil on the narrow ledges of steep projecting rocks, and 

 crags, and on rocky slopes, &c., is only watered by rain, mist, and dew. If no 

 showers fall for several successive days, and the south wind blows over the heights 

 with a clear sky day and night, these scanty layers of soil may dry up to such 

 an extent that they are unable to supply the necessary fluid food to the plants 

 rooted in them. Under these circumstances plants growing there have most 

 pressing need of means of lessening transpiration in the leaves. In places such as 

 these are to be found, almost without exception, plants whose leaves and stems are 

 thickly covered on all sides with hairs, together with succulent plants and saxi- 

 frages incrusted with lime. This is the habitat of the felted Whitlow-grass (Draha 



