112 AERATION AND AIR-CONTENT. 



to be regarded as adaptations to conditions without exception among 

 the endemic species of Carex do not always correspond to the present 

 habitats in the case of the derived ones. The steppe characteristics 

 shown by some grasses and grass-like plants of the flora have plainly 

 arisen in response to the great climatic extremes of their original 

 home, and not out of their present habitats in Germany. He re- 

 garded it as probable that this conclusion held also for the ericoid 

 leaf-forms, waxy coating, scale-hairs, and similar adaptations which 

 reduce transpiration and in particular protect the stomatal openings 

 and are apparently lacking among endemic plants. 



In contrast to the views of Warming and Schwendener, Kihlmann 

 (1890:80, 105) sought the explanation of the protective devices of bog- 

 plants in the factors of the habitat. He cited the opinion of Hartig 

 (1880), who emphasized the similarity between the effects of winter- 

 killing and those of drying-out arising from a lack of water, and stated 

 that very many cases of winter-killing are really due to the drying of 

 leaves and shoots at a time when the absorption of water from the 

 frozen soil is impossible. He also repeated and confirmed Sachs's 

 results with plants whose roots were surrounded with ice, finding that 

 they wilted quickly and completely in the sun, while control plants 

 suffered not at all. Consequently, transpiration was regarded as 

 the most important factor in retarding tree-growth in the north. It 

 is not the mechanical force of the wind itself, the cold, the salt-con- 

 tent, or the humidity that sets a limit to the forest, but chiefly the 

 uninterrupted drying-out of the young shoots at the time of the year 

 when replacement of the water transpired is impossible. 



While Kihlmann recognized the significance of the dry climate 

 of polar regions, and especially of the low humidity of the air, as 

 emphasized by Warming and others, he regarded the latter as not 

 sufficiently low in summer to alone explain the phenomena. In his 

 opinion, a more potent factor was the sudden and marked lowering 

 of the temperature of soil and air through the entire growing-period 

 by a sudden fall of snow or by an icy rain, while the strong winds 

 maintained transpiration at an active rate. The relatively small 

 snowfall in winter and its unequal distribution explain why the 

 drying-out of the plant-cover over wide stretches continues as in 

 summer and to an unusual degree. Consequently, the slightest dif- 

 ference in level can produce a sharp difference in vegetation. The 

 moisture of the underground ice is but slightly available, as a result 

 of the very slow melting in summer, and it can not protect the plants 

 from drying-out if they are not able to absorb and use the ice-cold 

 water. As a consequence, it is readily understood why so many 

 arctic plants, and among them the most universal and widely dis- 

 tributed, show a marked adaptation to drought, and especially to 

 dry air. 



