Char III] MOUNTAIN REGIONS IN THE TROPICS 



745 





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grey, or blackish as if charred, they cannot add any material variety to the 

 monotonous sandy desert V 



The alpine desert of the punas, however, supports, especially on its stony 

 tracts, numerous dwarf-shrubs, which largely belong to the Compositae and 

 naturally are of marked xerophilous structure (Figs. 409, 440). Rosette- 

 plants of typical alpine habit (Figs. 409, 410, 413, 441), but especially 

 cushion-plants, are present in great variety. These cushion-plants are often 

 more than 50 centimeters high, as for instance species of the umbelliferous 

 genus Azorella (Fig- 44-)- Verbenae, and even Cacti, form large cushions; 

 whereas in the region of perpetual snow small cushions and other dwarfed 

 forms of pronounced alpine 

 habit alone prevail (Fig. 409). 



Tschudi 2 gives the follow- 

 ing description of the climate 

 of the puna in Peru : — 



' Almost throughout the year 

 cold west and south-west winds 

 blow from the icy heights of the 

 Cordilleras over the surface, and, 

 for four months, bring with them, 

 as regularly as they do in the 

 Cordilleras,violent daily thunder- 

 storms, accompanied by heavy 

 falls of snow. The mean tem- 

 perature during the cold season, 

 the so-called summer (because it 

 seldom snows), is, approximately, 

 at night - 5 R., in the afternoon 

 9-7° R. ; duringwinterthemercury 

 rarely sinks below freezing-point 

 and stands between i° and 0° R., 

 but at noon goes up only to 7 R. 

 It is however almost impossible 



to give the mean temperature of these districts, as there is, often within a few 

 hours, a difference in temperature of 18 to 20 degrees Reaumur, which is so much the 

 more trying to a traveller on these mountains, as the fall of temperature is usually 

 accompanied by sharp cutting winds.' 



ii. MEXICO. 



The eastern and western slopes of the enormous highland of Mexico 



belong to the basal and montane regions. The alpine region appears only 



on the volcanos, for instance Popocatepetl (Fig. -147), 5420 meters high, 



and Orizaba, 5,384 meters high, both of which have perpetual snow on their 



summits. 



1 Tschudi, op. cit. 2 Id. 



Fig. 442. Andine alpine flora of the puna. Azorella 

 diapensioides, very common in Peru and Bolivia. 1. Habit, 

 very much reduced. 2. A flowering branch. Natural size. 

 After Weddell. 



