DEPARTMENT OF BOTANICAL RESEARCH. 59 



rainy season has alone made possible the recording of all new individuals 

 and has betrayed large differences in the number of germinations from year 

 to year. These differences are not in accord with the seed-crops of the years 

 in question and will require further investigation. The record of the soil- 

 moisture conditions has been used in this work, so that it is now possible 

 to correlate the rates of seedling mortality in different seasons with the 

 changing soil-moisture content. 



Influence of Cold-air Drainage on Distribution, by Forrest Shreve. 



A comparison of the minimum-temperature records for the Desert Labo- 

 ratory and the Breeding Laboratory, which are respectively situated on 

 Tumamoc Hill and in the Santa Cruz Valley at its base, together with the 

 comparison of data from several stations in the Santa Catalina Mountains 

 differing in elevation and in topographical situation, has given fresh data as 

 to the importance of cold-air drainage as a factor determining plant occur- 

 rence. Gradients of temperature change with altitude have been worked out 

 separately for ridges and valley bottoms, showing that these topographic 

 features, when situated at the same altitude, have minimum-temperature 

 conditions which are as different as those of two ridges or of two valleys 

 which are 2,000 vertical feet apart. 



The Soil-moisture Evaporation Index and Its Relation to Vegetation, by 



Forrest Shreve. 



The observations and instrumentation carried on in the Santa Catalina 

 Mountains, mainly in the summers of 1910 and 191 1, have been collated and 

 are in preparation for publication. The leading features of this work have 

 been the determination of the general facts of plant distribution in relation 

 to altitude and topography, the securing of instrumental measurements of 

 the physical factors which seemed to be of chief weight in relation to the 

 vegetation, and the subsequent correlation of these two bodies of data. 



The vegetation of the Santa Catalina Mountains presents three well- 

 marked belts or altitudinal regions — desert, chaparral, and forest. The ver- 

 tical limits of these belts, like those of the component species, are about 

 1,000 feet higher on south-facing than on north-facing slopes. The six sta- 

 tions for instrumentation were situated at vertical intervals of 1,000 feet, 

 two being located in each of the vegetational belts. The data from the six 

 stations have been expressed in the form of gradients, which show the rela- 

 tion of altitude to the changes of the several factors. Interest has centered 

 largely in this work in the gradients of rainfall, soil-moisture, humidity, and 

 evaporation. The rainfall at the forested altitudes is from three to four 

 times as great as that of the desert, yet the protracted drought periods of 

 the desert are of equal length in the two regions. In the forested region a 

 large portion of the principal annual drought period precedes the opening of 

 the growing season, and its force is lessened by this as well as by other fac- 



