8 



THE FACTORS 



[Part I 



as well as by excessive transpiration. But even this assumption in no way explains 

 the suitability of the above-mentioned structures. This depends on an adaptability 

 gained in the struggle for existence, and, like all true vital phenomena, it does not 

 yet admit of any physical explanation. 



The most important natural regions and habitats where physiological 

 dryness prevails and only xerophytes therefore thrive, are grouped as 

 follows, according to their physical characters : — 



i. Deserts, Steppes, and other districts with a dry substratum and dry air, 

 occasional or persistent great heat, and intense illumination. 



2. The Bark of Trees, Rocks where there is rapid drying up of the 

 substratum, owing to deficient depth. 



3. Sandy soil, Gravel, and the like, on account of the rapid drying up of 

 the substratum owing to its great permeability. 



Fig. 9. Xerophilous structure. Dry hot climate of Algerian ana Moorish deserts. Zygophyllum 

 comutum. Plant with succulent leaves. Natural size. After Engler. 



4. Sea-shores, Solfataras, which have abundance of soluble salts in 

 the soil. 



5. Peat-bogs, because of the humous acids in the soil. 



6. Polar zones, Vicinity of Glaciers in high mountains, where the 

 temperature of the soil is low. 



7. Alpine Highlands, which are under rarefied air and strong insolation 

 characteristic of the alpine climate. 



The plants of all these stations are provided with devices for the safe- 

 guarding of their transpiration ; they are xerophytes. Reduced surface is 

 very general in their case. With increasing physiological dryness, the 

 leaves become smaller in surface but proportionally thicker, more leathery 



