74 CARNEGIE INSTITUTION OF WASHINGTON. 



48.1 hour-degree units of exposure, and the air-temperature by 103.9 

 hour-degree units. 



(d) From the data just given, it will be seen that from sunrise to 

 sunset the number of hour-degree units inclosed by the temperature 

 curve for a June day is very much greater than the number for a 

 March day, and that the increase is greater in the case of the meridional 

 joints than in that of the equatorial joints. The numbers of hour- 

 degree units inclosed by the curves of the meridional joints for March 

 9 and for June 2, 1916, differ by 146.1 hour-degree units; the numbers 

 inclosed by the curves of the equatorial joints differ by 117.3 hour- 

 degree units; and the numbers inclosed by the cuves of the air- tem- 

 peratures differ by 138.4 hour-degree units. 



(e) The loss of weight from February 28, 1916, to April 5, 1916, of 

 joints in a meridional position was 18.59 per cent, the loss of weight 

 of joints in an equatorial position was 16.30 per cent, and that of shaded 

 joints was 5.79 per cent, whereas the loss of weight from May 15 to 

 June 28, 1916, of joints in a meridional position was 24.70 per cent, 

 that of joints in an equatorial position was 26.23 per cent, and that of 

 shaded joints was 23.32 per cent. The dry weight of joints similar to 

 those used in these observations was 16.15 per cent on March 8, 1916, 

 and had increased to 17.70 per cent on April 5, an increase of 1.55 per 

 cent; whereas the dry weight of joints on May 17 was 29.37 per cent 

 and had increased to 36.38 per cent on July 10, 1916, an increase of 

 5.01 per cent. 



(/) The maximum temperatures reached by joints growing under 

 natural conditions were found to be 53.0° C. on July 24, and 55.0° C. on 

 July 25, 1916. These temperatures are higher by several degrees than 

 those reported by Askenasy or Ursprung for succulent plants such as 

 Opuntia, and it is interesting to note that Pfeffer states that ''Prolonged 

 exposure to a temperature of from 45° C. to 46° C. kills most Phanero- 

 gams" (Pfeffer' s Plant Physiology, vol. 11, p. 226). 



On the Relation between the Rate of Root-Growth and the Oxygen of the Soil, 



by W. A. Cannon. 



It is now known that the perennials of the Tucson region associated 

 in the same habitat, or growing under apparently similar conditions, 

 may be subject to an environment which in many particulars is unlike. 

 This follows from possible differences in root-habits. Thus, the cacti 

 have roots lying within a few centimeters of the surface of the soil, 

 while Prosopis, for example, forms roots which may reach to com- 

 paratively great depths. Among the environmental differences to 

 which such widely different root-types are exposed are those of soil- 

 moisture, soil-temperature, and aeration. 



A study of the response of the leading types of root-systems to the 

 environmental factors of the soil has been made with the object of 



