DEPARTMENT OF BOTANICAL RESEARCH. 95 



extremely abundant representation of the annual species which appear 

 only during the periods of favorable moisture conditions. 



The Role of Climatic Conditions in determining the Distrihulion of Vegetation 

 in the United States, hy Burton E. Livingston and Forrest Shreve. 



The character of this investigation was described in the Year Book 

 for 1916 and the advanced state of its prosecution was mentioned. 

 The large body of data which has been brought together, showing the 

 extreme values for 38 climatic conditions in 115 distributional areas, 

 has been found to merit in itself a fuller examination and comparison 

 than was at first expected. A graphic method which has been devised 

 for the presentation of the climatic extremes has been found to facilitate 

 the study of these data and to furnish evidence as to which of the 

 climatic conditions are of greatest weight in deterinining particular 

 cases of distribution. The major conclusions of the work have not 

 been altered under further testing of their validity, and other conclu- 

 sions have been greatly amplified. The final stages of the investiga- 

 tion have shown still more conclusively that the principal types of 

 vegetation in the United States are controlled in then* distribution by 

 those moisture conditions which have most to do with the maintenance 

 of an equilibrium between absorption and transpiration in individual 

 plants. 



GENETICS AND TAXONOMY. 



Experimental Evolution in a Desert Habitat, by W. L. Tower. 



The analysis of the composition of the mutants arising from the 

 series C. H. 15.7 and C. H. 156.8 and the extensive and final testing of 

 the adaptation of introduced stocks to desert conditions have occupied 

 the major portion of my attention since the last report. 



ADAPTATION OF INTRODUCED SPECIES. 



Any desert environic complex represents probably the most variable 

 and diversified set of conditions that organisms have to meet. The 

 inhabitants, plant and animal, in any desert have come from progen- 

 itors that originated in more moist conditions. 



In the study of introductions of species from moist into desert envir- 

 onments, two problems are ever before one — the adjustments necessary 

 to survival and the adjustments necessary to reproduction. 



On introduction it is necessary to aid artificially the reproductive 

 process, as none of the introduced species could breed without some 

 degree of aid in meeting the conditions of the desert. This aid in 

 reproduction can be regarded as a step from the moist towards the full 

 natural rigors of the chosen desert environment. 



