6o REPORTS OF INVESTIGATIONS AND PROJECTS. 



a year, while the floor of a hemlock forest near by received but 68,596 hour- 

 degree units, having a season practically 11 days shorter than the meadow 

 in the number of hours above the freezing-point. In the application of this 

 method to a plant v/ith a winter resting season, the exposure is calculated 

 from the solstice, or from the time of planting the seeds until maturity, 

 or until the attainment of some stage of development. Thus the red maple 

 (Acer rubrum) was found to need 3,209 hour-degree units to bring its flowers 

 to the stage of opening, while Draba verna, in the same locality, required 

 but 1,644 hour-degree units for the same process. 



Relative rapidity in changes of temperature, relative humidity, and great 

 difference in the insolation may be held to be accountable for the diversity 

 in plants at various elevations, such as those included by the series of planta- 

 tions described. 



CORRELATED PROBLEMS. 



Distribution and Movements of Desert Plants. — Dr. V. M. Spalding has 

 continued his investigations on this subject along the following lines: 



(i) An attempt has been made to determine more definitely the physical 

 factors upon which plant-formations are conditioned. The plants of oppo- 

 site slopes have been compared floristically and numerically, and records of 

 soil and air temperatures in both winter and summer have been made. The 

 data obtained in this way and by determinations of soil moisture may be 

 expected to furnish a realization of the essential features of aspect or ex- 

 posure as a factor in distribution. Differences in temperature and amount 

 of water present in the soil appear to be the controlling factors. 



(2) The study of limited areas in the domain of the Laboratory for ob- 

 servations upon competition and dissemination has been continued. Many 

 of the new areas have been demarked. Vanishing data in the movements of 

 species and all of the principal facts in these areas are preserved by serial 

 photographs and full records. The setting or general environment of the 

 observational areas has been recorded by a series of maps showing the present 

 actual distribution of characteristic species over the entire domain, which 

 covers 1.33 square miles. This mapping has been largely done by the effi- 

 cient assistance of Mr. J. C. Blumer, and a large number of dried specimens, 

 constituting a portion of the record, have been prepared by him and also 

 by Prof. F. E. Lloyd. 



(3) Data as to distributional movements of the widest amplitude have 

 been obtained by visits to various localities between the Desert Laboratory 

 and the Pacific Ocean, in the deserts of Arizona, the Salton Basin, in the 

 mountains of southern California, and in the coastal region. 



Attention is now being turned to a consideration of northward and south- 

 ward, or vertical, movements, barriers to distribution, and influence of under- 

 lying geological formations. 



