338 CARNEGIE INSTITUTION OF WASHINGTON. 



plains grassland of the Santa Rita Range Reserve south of Tucson to determine 

 the effective factors in the growth of the communities of winter annuals, and in 

 the differentiation of the structure of the climax itself. 



The Phytometer Method, by F. E. Clements, G. W. Goldsmith, and J. E. Weaver. 



Additional tests of the phytometer method have been made in order to 

 perfect sealing methods and to determine the limits of individual variability 

 in transpiration. Although paraffin-cloth has furnished a satisfactory sealing 

 method, ceresin applied in the same manner has proved to be somewhat better 

 adapted to the high surface temperatures encountered in the region. Plants 

 and blanks sealed with ceresin on cloth gave very satisfactory results when 

 protected against the midday heat by a cloth insulator, the latter preventing 

 the cracking of the seal with the contraction and expansion incident to tem- 

 perature changes. Poured wax-seals were unsatisfactory when used for 

 containers of more than a decimeter in diameter, owing to the effect of tem- 

 perature changes. Plastocene modeling clay covered with tin-foil gave results 

 as satisfactory as those obtained with ceresin, but was much harder to apply 

 to the phytometers and was more expensive. 



Individual sunflowers were found to vary as much as 20 per cent in the 

 transpiration per square decimeter under identical conditions of soil and expos- 

 ure. However, the effect of this variation was eliminated by selecting the 

 plants for each battery in such a way as to give the three batteries the same 

 average transpiration. The individual variation in transpiration is being 

 studied in pedigreed wheat and oats with the object of selecting a strain 

 showing the minimum of variability, and it is hoped to find a strain of native 

 sunflowers that will yield the same result. The application of the phytometer 

 method during the year has been made chiefly in the slope-exposure studies 

 described below. 



Slope-Exposure Studies, by F. E. Clements and Dolly Lutjeharms. 



In continuing the investigation of the efficient factors on north and south 

 slopes, the three original stations have been maintained and three new ones 

 established. The latter are located on the same slopes, but in such fashion 

 as to avoid the wind-drift over the summit, with the possibility of changing 

 the normal conditions. Air and soil temperatures, water-content, humidity, 

 evaporation, wind, and rainfall have again been measured throughout the 

 season for the six stations. The soil temperatures have been determined in 

 the new series at depths of 4 and 12 inches, and the light intensities measured 

 by means of chemical photometers, as well as the selagraph. Each station 

 was also provided with a battery of phytometers installed and handled as 

 during the preceding year, but compensated for individual variability in 

 transpiration. 



The instrumental results show that the air temperature and humidity of 

 the two slopes were similar during the first half of the summer, but later the 

 south slope exhibited a wider fluctuation than the north, as well as an average 

 higher temperature and lower humidity. The station at Ruxton Brook is 

 much more equable, with the air temperature regularly a few degrees lower 

 and the humidity higher than on the north slope. The soil temperature at 

 both 4 and 12 inches on the south exposure averages from 12 to 20 degrees 

 higher than on the north, with the brook station again a few degrees lower than 

 the latter. 



