66 



Journal of the Mitchell Society 



[August 



The close correlation between altitude and sap density within the 

 different associations is very apparent. Similarly, the concentration 

 of the sap for individual species decreases with an increase in alti- 

 tude. This may be readily illustrated by the common sagebrush 

 {Artemisia tridentata) which was found to cover an extreme alti- 

 tudinal range of over 5,000 feet. In the sagebrush association at an 

 elevation of 5,800 feet the sap of this species had an osmotic pressure 

 of 35.2 atmospheres which decreased with an increase in altitude until 

 in the spruce-fir association at an elevation of 10,700 feet the osmotic 

 pressure was only 14.5 atmospheres. Other species exhibited similar 

 tendencies. The sap densities of the different species showed varia- 

 tions corresponding to the xerophytism of the associations in which 

 the species were growing, especially when the determinations were 

 made at or near the critically dry part of the growing season. It is 

 evident, therefore, that (1) the water relations of the plant have a 

 vital bearing on the succession of plant associations and (2) the 

 causes of these phenomena are associated with the osmotic properties 

 of the plant juices and the soil solution. The common progressive 

 succession is from the high osmotic pressures in drought-resistant 

 species of the xerophytic' habitats of the lower mountains, through 

 the mesophytic associations composed of species having successively 

 lower sap concentrations, to the climax spruce-fir association of uni- 

 formly low osmotic pressures. 



