THE FACTORS OF DISTRIBUTION 87 



5. The air becomes more rarefied, and there is con- 

 sequently greater radiation. Above the cold height 

 previously alluded to (2) this results in a drying of the 

 air, an absence of cloud, and a great reduction in 

 precipitation. 



6. Light is intensified. 



7. The soil becomes progressively colder. 



The responses of plant-form to these conditions are: 

 i. The general development of strong and deep root- 

 systems and of the perennial habit. 



FIG. 6. Merope, a Composite of the Higher Andes. 

 (From Graebner's " Lehrbuch der Pflanzengeographie.") 



2. The prevalence of woody plants, whether trees or 

 shrubs, with a shortening of the internodes, more copious 

 branching, and a stunted, more horizontal, or prostrate 

 direction of growth (Fig. 6). Such wiry -stemmed 

 " cushion -plants " as the Vegetable Sheep of New 

 Zealand (Rauolia) and Azorella and many others in 

 South America are very characteristic of wind-swept, 

 sub-alpine plateaux. 



3. Leaves are commonly smaller, evergreen, thick, and 

 often inrolled at the edges, with few sunken stomata, 

 and large air-spaces. These sclerophyllous characters 

 may serve as protection against cold, and they certainly 

 serve to minimise transpiration when, as a rule, the moist 

 air and cold will lessen both it and food-absorption. At. 



