46 Universiiy of (Jalifornia Publications. [botany 



prevailing on the higher mouutaius produee, to a large degree, 

 characters similar to those which are found on desert plants. 



On comparing the climatic conditions prevailing at high 

 altitudes with those of the desert region we at once recall the 

 vast difference in temperature; but it has been demonstrated by 

 a series of experiments carried on by Bonnier* and others that 

 the alpine vegetation is affected not so much by the extreme and 

 continued low temperature as by the the fluctuation between 

 warm days and cold nights. During the day the sun's rays give 

 to the mountain tops a reasonable amount of heat but, as evening 

 draws on, rapid radiation due to the rarified atmosphere sets in, 

 and the temperature is suddenly and very appreciably lowered. 

 Similar conditions prevail on the desert where excessively hot- 

 days are followed by cold nights, while at middle altitudes, 

 especially on the forested areas, radiation is less rapid and the 

 temperature more nearly uniform. 



It is also to be noticed that plants of the higher montane 

 regions adopt various devi(;es for the accumulation of moisture 

 and for protection against excessive transpiration. Sometimes 

 the xerophytic characters which they exhibit are quite similar to 

 those of desert plants: the root system is well developed, the 

 stems and leaves are often clothed with a dense pubescence, and the 

 leaf surface is reduced ; while a histological examination shows an 

 increase in schlerenchymatous tissue, a thickening of the outer 

 walls of the epidermal cells and comparatively few air spaces in 

 the leaves. It is difficult to say how much of this is due to the 

 lack of moisture and how much to other conditions, the most 

 important of which are undoubtedly the fluctuation in tempera- 

 ture and the strong insolation. 



As to the factors producing the arid conditions found on high 

 mountains, it need merely to be noted that, in addition to the 

 effect of a low temperature, which renders much of the moisture 

 unavailable to the plant, the water early escapes from the surface 

 soil, both as run-off and by percolation, and that as a result of 

 the rarified atmosphere evaporation from the soil and from 

 plants takes place very rapidly, while the constantly shifting- 

 air currents also accelerate evaporation to no small degree. 



*Comptes Rendus cxxvii. .'507 (1898). 



