DEPARTMENT OF BOTANICAL RESEARCH. 



81 



coefficient of transpiring power to precede the highest evaporation 

 at the former elevation and showed the two to coincide at 6,000 feet. 

 These data confirm our knowledge of a check to the rate of transpira- 

 tion which is applied before the daily maximum of the evaporative 

 power of the air. This check appears to be applied later and later in 

 the day with increasing altitude, and to be eliminated at 5,000 feet for 

 some species and at 6,000 for others. The actual rates of water-loss, 

 and sometimes even the coefficients of transpiring power, are higher 

 at the lower altitudes. The ''reduction of transpiration," of which so 

 much has been said regarding desert plants, is not to be discovered in 

 the maximum absolute water-losses, which are greatest in the desert 

 plants, but is to be detected in the time of the daily check in rate. 



ENVIRONIC RELATIONS: PHYTOGEOGRAPHY. 



The Osmotic Pressure of Vegetable Saps in, relation to Local Environmental 

 Conditions in the Arizona Deserts, by J. A. Harris. 



An extensive series of cryoscopic determinations of the osmotic 

 pressure of saps of four principal growth-forms, trees and shrubs, 

 dwarf shrubs and woody twiners, perennial herbs, and winter annuals, 

 characteristic of the foot-hills, caiions, cliffs or rocky slopes, bajadas, 

 arroyos, and saline areas, was made for the purpose of ascertaining 

 whether or not any definite relation between sap-density and environ- 

 ment prevails. The data secured indicate that the lowest osmotic 

 pressure is to be found in the sap of plants from the arroyos. As might 

 be expected, the highest concentrations were found in the plants of 

 the salt spots. Eight determinations of plants from this habitat gave 

 an average of 37.1 atmospheres, but a very wide range of variation 

 is exhibited among the species as well as within any single species. 



The species of the bajadas without exception stand next to those of 

 the saline areas in concentration of sap, and the lowest concentrations 

 are to be found in the plants taken from the beds of the arroyos or 

 washes. The principal results are summarized in the following table: 



Osmotic pressure in atmospheres of growth forms in five habitats of the Tucson region. 



Climatic Cycles and Succession, by F. E. Clements. 



The further analysis of the fundamental processes and principles 

 of succession has led to a clear recognition of the distinction between 

 the ontogeny and phylogeny of plant formations. The ontogenetic 

 process is represented by the unit succession or sere, in which develop- 



