64 REPORTS ON INVESTIGATIONS AND PROJECTS. 



havior in pedigreed strains, in hybridizations, and under various environic 

 conditions. The other includes a consideration of the mechanism by which 

 an environic agency affects the physical bearers of heredity. Both com- 

 prise important and interesting possibilities in evolutionary science. 



Evaporation and Soil Moisture, by Dr. B. B. Livingston. 



The study of the evaporating capacity of the air as a controlling factor in 

 determining plant transpiration has been continued ; the preliminary survey 

 of this subject by quantitative methods, as originally planned, is now near- 

 ing completion, so that the various phases of the problem may soon be 

 brought together. A newly investigated factor in the control of transpira- 

 tion is the diurnal variation of the moisture content of leaves when subjected 

 to normal sunlight. It has been found that as the day advances the water- 

 loss from such leaves is relatively decreased by the drying of the tissues, a 

 process which would lead to wilting if continued. This phenomenon has 

 been termed incipient drying, and it promises to become an important topic 

 in plant physiology. The influence of sunshine upon water-loss has been 

 quantitatively studied for a number of plants. 



A study of the relations between the absorption of water and the general 

 activities of the plant has been begun. It has been experimentally shown 

 that the lower limit of available soil-moisture (commonly termed the wilting- 

 point) is not the constant for a given soil plant that it has often been assumed 

 to be, but that it depends primarily upon the evaporating power of the air. 



Another factor upon which this supposed critical point depends has been 

 experimentally shown to be the water retentivity of the soil. Dr. W. H. 

 Brown, of the Michigan Agricultural College, very efficiently assisted in the 

 experimental work along these lines during the summer of 1910. Prof. J. S. 

 Caldwell, of the University of Nashville, has been employed as assistant in 

 this work for the summer of 1911. 



Studies on the relation of climatological conditions to the distribution of 

 vegetation in the United States, a collaboration between Dr. Livingston and 

 Dr. Shreve, have been in progress throughout the year. The discovery of 

 several new methods for handling the existing climatological data, in order 

 to arrive at approximate climatological descriptions of the various vegeta- 

 tional areas, has brought about delay in the preparation of the manuscripts 

 and charts, but the work should be ready for publication in a short time. 



The Influence of Atmospheric and Soil Conditions on the Transpiration, 

 Water-content, and Anatomical Structure of the Palo Verde {Parkinsonia 

 microphylla), by Mrs. Bdith B. Shreve. 



The daily curve of transpiration of young and full-grown trees at differ- 

 ent seasons of the year has been found and compared with that of hothouse- 

 grown young plants, record being kept of soil-moisture, humidity, air- 

 temperature, evaporating power of the air (as measured by the porous-cup 



