1917.] 



THE FLORA OF THE SWISS ALPS 



43 



cies of its genus ; a classical example of a primeval but nevertheless 

 still vigorous type with great power of expansion. 



All these alpine shrubs show a very slow growth of the stem ; the 

 annual layers are often very narrow; 0.07 mm. in the case of the 

 Azalea, so that a stem 55 years old has a diameter of only 7 to 8 

 mm. The comparison of the alpine rate of growth with the tropical 

 growth is striking: in comparing a cross-section of a tropical Acacia 

 6 years old with the stem of our Azalea 55 years old, the growth in 

 the Alps is seen to be 615 times slower than in the tropics. 



The last link in the chain of more and more reduced dwarf shrubs 

 is the dwarf Willow (Salix herbacea L.), called by Linnaeus "the 

 smallest of all trees. ' ' It thrives in our Alps in a belt from 6,000 up 

 to 11,000 feet above sea-level; it can reach the age of 40 years, but 

 the whole stem and the branches are completely hidden in the 

 ground (Fig. 18) ; only the tops of the twigs come above the soil, 

 bearing two little leaves and a delicate catkin, male or female. This 

 mat-forming tree is the strongest expression of the adaptation of a 

 tree to high alpine conditions. 





Fig. 19. "The Smallest Tree of the World"— Dwarf Willow (Salix herbacea 

 L.). A pure association seen from above. The ground is completely covered 

 with the short twigs of the plant, bearing two rounded leaves and a little cat- 

 kin. The rest of the tree is hidden in the ground. ' ' Snow Valley ' ' at Pasture 

 Di Lagalb, near Bernina hospice, at 2,400 meters above the sea. 



Photo von Ostrom. 



