The Causes of Timber-line on Mountains. 181 



jr?* 



by their massing together in dense level topped societies, thus 

 affording each other mutual protection, and by the occurrence of 

 the upper outposts of the forest as clumps of dwarfs in local de- 

 pressions and sheltered spots. Wind cripples are characterized 

 by the fact that shoots which project beyond a critical line are 

 dead or dying, often by the entire loss of the conical form; by 

 their much branched and condensed habit, and frequently by- 

 one-sided growth. The primary condition which the tree must 

 fulfill in order to exist is to develop sufficient foliage within the 

 limitations set by the wind. 



2. Timber lines caused by snow. v Such are recognized 

 by the gradual resolution of the forest into clusters of relatively 

 tall trees, and by the occurrence of these groups at timber line 

 and above on local elevations. Snow cripples are distinguished 

 from wind cripples by the presence of healthy upper shoots, 

 by the possession, in their upper portions, of the normal conical 

 form, and by the fact that their lower branches are generally 

 dying or absent. Fungus mycelia abound enwrapping the 

 dead foliage. The primary condition which the tree must fulfill 

 in order to exist is to expand its foliage above the late melting 

 beds of wet snow. 



In the vSelkirks, wind cripples and a wind tree line also occur 

 some distance above the timber line. From the tree line upward, 

 between the snow fields, cold deserts prevail. The alpine grass- 

 land of the Selkirks is thus principally due to snow. 



In view of the strong general similarity between the vSel- 

 kirks and the Alps, it is suggested that the ideas here presented 

 in regard to the Selkirks are probably of application in the Alps 

 as well. 



NATURAL ALTERNATION OF VEGETATION IN ITS 

 RELATION TO PERMANENT PASTURAGE. 

 By L. X. Duncan. 

 On nearly every farm in the south we find more or less 

 land that is too rough to cultivate, or land that probably once 

 made profitable crops, but under ordinary farm practices it has 

 washed and the natural fertility has become exhausted until it 

 no longer makes profitable returns for the labor expended. 



