METEOROLOGICAL DISCUSSION 29 



native species the resulting dryness of the growing season would result 

 in desert conditions. Such effects are most marked in regions in which 

 the surface layers of the substratum consist of loose material not capa- 

 ble of retaining water in sufficient quantity for moisture loving spe- 

 cies. A striking example of this feature is offered by the area around 

 Crater lake, Oregon, as described by Mr. Coville. The surface 

 layers in this locality consist of powdered pumice apparently almost 

 devoid of humus, from which water drains with extreme rapidity. 

 Snowfall to a depth of about lo feet occurs in winter, but after this 

 melts the soil becomes extremely dry and the plants capable of endur- 

 ing the resulting drouth show marked protective adaptations, the vege- 

 tation consisting chiefly of such species as Phlox douglasii^ Spraguea 

 umbellata^ and Arenaria pumicola. ^ 



The above factors must be taken into account in the interpretation 

 of alpine districts in many parts of the world. The precipitation on 

 mountain summits is very gi^eat but in some instances it is in the form 

 of snow which melts and drains away very rapidly leaving the humus- 

 free soil extremely dry, while the air shows rapid alternations from 

 high to extremely low relative humidity. An example of this char- 

 acter is offered by the summit of Agassiz peak in the San Francisco 

 mountains of northern Arizona, an illustration of which is shown in 

 Plate XXIX, facing page 43. A further description of the meteorolog- 

 ical conditions on this mountain will be found on pages 38 to 45. 



The chief factor in the production of deserts is a lack of water as a 

 nutrient substance for vegetation. Deserts may be produced as a re- 

 sult of other defective nutritive and mechanical conditions as well. 

 Such conditions are to be found in areas in which the soil contains 

 harmful substances in injurious concentration in the soil, of which the 

 alkali lands are familiar examples. Sterile areas due to lack of nutri- 

 tive material and water, and to the unsuitable mechanical conditions of 

 the soil are offered by stretches of sand dunes and plains in many parts 

 of the world. In sand dunes the substratum is in constant motion of 

 greater or less rapidity, lacks a suitable water supply, and may be devoid 

 of other nutritive material. Even if the dune areas are supplied with 

 water in proper quantities, yet the peculiar character and niovemcnts 

 of the substratum result in some striking forms of vegetation. 



1 Coville, F. V. The home of Botrychium fumicola. Bull .Torr. Bot. Club, 

 28: 109. 1901. The August vegetation of Mount Mazama, Oregon. Mazaina, 

 I : 170. 1896. 



