1917.] THE FLORA OP THE SWISS ALPS 53 



A very typical group of rock-plants are the cushion-plants, 

 forming thick hemispherical cushions, covered with short-stalked 

 and closely adhering flowers. Their twigs radiate from a center; 

 they are thickly covered with little closely-set leaves, which remain 

 withering on the stem, filling the whole interior of the cushion with 

 decaying material, forming a sort of spongy mine of humus. The 

 living leaves form a continuous covering over the compact interior, 

 and are hairy or leathery. The whole structure of these cushions is 

 to be understood as a manifold protection against drought and in- 

 tense wind : transpiration is checked by the hairy or leathery struc- 

 ture of the leaf, by the low growth near the soil, where the wind is 

 less intense, and by the compact structure of the interior. The 

 spongy mass of humus forming the interior with its thousand and 

 thousand capillary cavities works as a sponge retaining the water ; 

 it holds the soil underneath in a damp state and prevents high tem- 

 perature. 



We find the cushion form in many plants of seemingly very dif- 

 ferent stations. On the stormy treeless shores of the sub-antarctic 

 islands, especially Kerguelen, grow cushions of some meters in di- 

 ameter (Bolax gumifera (Lam.) Spreng.) ; on the wet but cold 

 peat-moors of the Andes, as well as in the dry hot sands of the Sa- 

 hara, we find cushions; on crests and summits; so especially the 

 Swiss Androsace (Androsace helvetica L.) exclusively found in the 

 calcareous Alps, an autochthon product of them and much more 

 characteristic of our Alps than the Edelweiss. 



On the whole earth we find 338 species of cushion-plants in 34 

 different families. That is a classical example of convergence of the 

 fact that plants from the most different families adopt very similar 

 habits through the influence of similar conditions. Indeed, if we 

 compare the different stations cited above, they are all, for plant 

 life, to be characterized as dry, as menacing the plant with drought. 

 Their soil can well contain much water, be physically wet ; but this 

 water is only with difficulty available for the plant (retained by hu- 

 mus or because the soil is cold). The soils are physiologically dry 

 (peat moors, cold alpine soils), or the stations are exposed to con- 

 stant loss of water by intense wind; so on the wind deserts of the 

 sub-antarctic islands and on the exposed summits of the Alps. Here 

 in the Alps especially the winter with its dry air and frozen soil be- 

 comes a danger so much the more as many of our alpine cushions, 

 especially the Swiss Androsace, prefer the most exposed positions 

 where the snow is constantly blown away. 



I come to the end ; I wish finally to recall to you that we have 

 convinced ourselves that the total features and entire household of 

 alpine plants is a most faithful expression of alpine conditions of 

 life. The principal characteristics of the alpine flora, the dwarfy 

 growth, are of a double nature ; on one hand they are direct effects 

 of climate by means of plentiful light and the cold nights, on the 



