76 



crous at an elevation of 2300 m. as on the plains. On this account labiate 

 plants play a considerable role on high mountains. Also the increased diffi- 

 culty of insect fertilization is partly equalized by the fact that an asexual 

 reproduction also takes place (Polygonum v'wipanim, Foa alpina, Saxifraga 

 ccrnua) ; further, ten-elevenths of all kinds of small bushes and even Viola 

 iricolor, an annual with us, become perennial in the Alps. 



Besides this, reference should be made to the fact that, with unlimited 

 cultural experiments at high elevations, short-lived mountain varieties are 

 formed which, to be sure, furnish seed in smaller amounts but more satis- 

 factory in quality. This ofifers greater possibilities of yielding a good har- 

 Aest in the mountains and (according to Schiebler)^ has the advantage 

 of retaining at lower levels its shortened period of growth and there- 

 fore can be used advantageously in Northern climates. 



Development of the Aerial Axis of Woody Plants. 



In contradiction to a widespread opinion, it should be mentioned, that 

 dzvarf growth in high mountains is not to be ascribed to the pressure of the 

 snow since we have tree-like forms in those regions where the most snow 

 falls. It is known that the snow cohering does not become thicker, the great- 

 er the elevation of the mountain, but with us increases up to perhaps an 

 elevation of 2500 m., that is, only to the upper boundary of the dwarf coni- 

 fers, dwarf alders and the Alpine rose. Higher up the amount of precipi- 

 tation decreases. Spruces, larches and the cembra-pine suffer less from 

 snow pressure when they stand alone or scattered because their elastic, slop- 

 ing older branches let the accumulated snow slip off more easily when the 

 wind blows. Other trees, like SalLv serpyllifoUa and Rhamnus pumila, fre- 

 quently escape excessive snow pressure by their growth on steep rocky cliffs 

 from which the snow slides rapidly. However, trees exposed to the full 

 pressure of the snow can scarcely be made to grow closer to the earth be- 

 cause of the burden of the snow or of windy weiather. Rather, we may as- 

 sume with Kerner that it is the soil warmth which, in the immediate prox- 

 imity of the earth, affords them the best conditions for existence. In the 

 higher Alpine regions the soil is much warmer than the air which absorbs 

 less sunlight on account of its increasing thinness and its rapidly decreasing 

 water content. The above quoted author cites that, for example, on the top 

 of Mt. Blanc (4810 m.) the intensity of the sunlight is 26 per cent, greater 

 than at the level of Paris. On the Pic du Midi (2877 m.) a temperature of 

 33.8°C. was observed in the soil on which the sun shone while the air showed 

 a temperature of only io.i°C. This warmth of the soil together with the 

 intensity of the light explains the speedier development and blooming of 

 Alpine plants. 



Vochting-, in opposition to Kerner, thinks, on the ground of his 

 observations with Mimulus Tilingn, the young branches of which at a defi- 



1 Schiebler, Die Pflanzenwelt Norwegens. AUg. Teil. Christiania 1873. 

 - Vochting-, H., Ueber den Einflufs niedriger Temperatur auf die Sprofsrichtung. 

 Ber. Deutsch. Bot. Ges. XVI. ]8?8, p. 37. 



