Chap. II] REGIONS OF VEGETATION 717 



latitudes. Thus, Mediterranean and South African species occur on the 

 alpine highlands of tropical Africa, and polar species on those of the north 

 temperate zone. 



The occurrence of many polar species of plants on temperate high 

 mountains has led to the assumption of a complete analogy between the 

 alpine and arctic floras, and between the effects on plant-life of the alpine 

 and arctic climates. Yet H. Christ, the greatest authority on the European 

 alpine flora, has already protested against the existence of such a far-reaching 

 agreement, and Gaston Bonnier, on the evidence of careful observations, 

 has recently proved that alpine and arctic individuals of the same species 

 differ essentially in structure from one another, so that only very plastic 

 species can exist in both climates. The differences between tJie arctic and 

 alpine floras of the north temperate zones arc therefore to be attributed, 

 not Die rely to historical causes, but, in the first place, to physiological causes 

 that arc still in operation. 



The alpine climate in all latitudes shares with the polar climate abun- 

 dance of light and the low temperature of the air. The conditions of illu- 

 mination, however, are not identical, as in the polar climate it is a matter 

 of continuous weak light, in the alpine climate of discontinuous intense 

 illumination. The action of the low temperature of the air on plant-life 

 in alpine climates is arrested during sunshine by the intense insolation, but 

 is reinforced in the shade and at night ; polar plants are not exposed to 

 such fluctuations of temperature. Finally, the danger of scarcity of water 

 in alpine plants is chiefly, even if indirectly, due to the rarefaction of the 

 air ; in polar plants, to the coldness of the soil. 



Corresponding with the dissimilar physiological action of the .'.lpine and 

 polar climates, there are differences in the external and internal structure 

 of the plants. Thus, according to Bonnier, the epigeous parts of Salix 

 polaris and Saxifraga oppositifolia are more weakly developed in polar 

 than in alpine specimens. But specially characteristic of polar plants, in 

 contrast with alpine plants of the same species, is the possession of thicker 

 leaves with less differentiated histological structure and larger intercellular 

 spaces. This feature of polar plants, however, as has been already shown, 

 is to be ascribed to the action of continuous weak illumination, whereas 

 the intense but discontinuous illumination of the alpine climate of lower 

 latitudes on the contrary favours the histological differentiation of the 

 leaves. 



iv. THE UPPER LIMIT OF PLANT-LIFE IN HIGH MOUNTAINS. 



The upper limit of vegetation in high mountains naturally varies with 

 the climate, and, in the tropics, is in general higher than in temperate zones 

 where the temperature is lower for the same altitude. It appears not to 

 be impossible that lower cryptogams inhabit even the very highest summits. 



