1917.] THE FLORA OF THE SWISS ALPS 39 



Mill. var. prostrata Tubeuf ) are the dominant elements. We force 

 through these thickets, and now we stand on the free alpine pasture 

 covered with thousands and thousands of bright-colored flowers. 

 "Nothing- in the world equals this splendid spectacle," says Dr. 

 Christ; "we hesitate to advance fearing to crush under our feet 

 these delicate beings." In ascending we see by degrees the continu- 

 ous sward dissolving into isolated green patches. They begin to min- 

 gle with white patches of perennial snow, the outposts of the snow- 

 belt, and finally we enter the dominion of eternal snow, the recent 

 glacial period. 5 



II. 



Previous to the study of the typical plants of the alpine belt, we 

 will take a brief view of its climate. Its principal features are the 

 following ones : 



The shade temperature falls lower and lower as we ascend, but 

 this loss of warmth is more than compensated by the enormous in- 

 crease of the effects of sunshine. We see this difference in the clear- 

 est manner in comparing a thermometer in the shade with a ther- 

 mometer in the sun. Frankland has made such experiments with the 

 following results : He found at Witby in England (66 feet above 

 the sea), 91° Fahrenheit in the shade, 100° Fahrenheit in the sun, 

 a difference of 9°. At Pontresina, 6,000 feet above sea-level, the sun- 

 thermometer showed already 31.5° Fahrenheit more than the shade- 

 thermometer, and finally at the Diavolezu, at 10,000 feet above the 

 sea, the thermometer showed 43° in the shade, 139.1° in the sun, 

 thus a difference of 96° Fahrenheit. Dr. Riibel found at the Bernina 

 hospice, 8,000 feet above the sea, a still greater difference of 111.6° 

 Fahrenheit, 12.2° in the shade, 122.9° in the sun. Saussure found on 

 Mont Blanc even a difference of 162° Fahrenheit ! 



This plenty of light and warmth that the alpine sun spreads to 

 the alpine plants is the key to understand their flourishing growth. 

 But there is one great drawback, the shortness of the period of 

 vegetation. The period shortens nine days for every 333 feet of as- 

 cending. In the alpine belt it has a decreasing duration of from five 

 months to only three weeks ; in this short lapse of time high alpine 

 plants must perform all their biological duties. 



On the other hand, this shortness of the vegetation time is a little 

 compensated by another important difference between lowland cli- 



5 Lately, I. Braun, Die Vegetation der Schneestufe in den Bhatische-lepon- 

 tische Alpen, Ein Bild des Pflanzenlebens an seinen ausseresen Grenzen ("Neue 

 Denkschriften der Schweiz. Naturf. Ges. Band XLVIII. Basel, 1913' '), has in 

 an excellent paper proposed the following division of the snow-belt of the Swiss 

 Alps: 



Pionirrascu (isolated patches of mats), — up to 150 m. above the snow-line. 

 Area of Dicotyledons (mostly cushion plants), — up to 550 m. above the 



snow-line. 

 Belt of Thallophytes, — from the last Phanerogams to the highest peaks. 



