DEPARTMENT OF BOTANICAL RESEARCH. 73 



edge of the grassland. All of these subdivisions of the desert are bio- 

 logically distinct and merit separation in vegetational work as much as 

 do the deciduous and the evergreen forests of the eastern States. 



The Effect of Position upon the Temperature and Dry Weight of Joints of 



Opuntia, by J. M. McGee. 



The flattened joints of the platyopuntias were held in a vertical 

 position, either edgewise or lengthwise. Terminal mature joints of 

 Opuntia hlakeana were chosen to test the relation of the position with 

 respect to sun, to the body-temperature, and dry weight. The 

 separated joints were held apex downward by impalement upon the 

 bulb of a thermometer, which in turn was fastened with wooden 

 clamps in such manner that the two surfaces of the joint faced north 

 and south and it was said to be in an "equatorial" position. The 

 second lot faced east and west, one such double set being exposed 

 during March, a second during June, and a third during the latter half 

 of July. The chief results may be summarized as follows: 



(1) Joints of Opuntia hlakeana in any position show temperatures 

 above the air-temperature while exposed to solar radiation. 



(a) The temperatures of joints in an equatorial position rise steadily 

 till 12 m., then more slowly till 2 p. m. when the maximum is reached. 

 After 2 p. m. the temperatures steadily decline, becoming the same as 

 that of the air soon after sunset and then falling slightly below the air- 

 temperature and remaining so during the night. 



(h) The temperatures of the joints in a meridional position rise 

 sharply after sunrise, reaching a maximum about 11a. m. They then 

 slowly drop until 12*" 30"° p. m., when they begin to rise again, reaching 

 the second and highest maximum point about 4 p. m., after which they 

 fall, at first slowly and then more abruptly, till sunset. After sunset 

 the temperatures slowly fall below the air- temperature, as in the case 

 of the other joints. 



(c) Computation of the area inclosed by each curve, using the 10- 

 degree hne as a base, shows that on March 9, 1916, the number of 

 hour-degree units inclosed by the air-temperature curve was 134.6; 

 by the curve of the joints in an equatorial position 211.5 hour-degree 

 units; and by the curve of the joints in a meridional position 230.8 

 hour-degree units. Hence it will be seen that the temperature of the 

 joints in a north-and-south position exceeds that in an east-and-west 

 position by 19.3 hour-degree imits and the air-temperature by 96.2 

 hour-degree units and that in these joints the temperature effects 

 would be accentuated. Similar computations show that on June 2, 

 1916, the number of hour-degree units inclosed by the air tempera- 

 ture curve was 273.0; by the curve of the equatorial joints 328.8 hour- 

 degree units; and by the curve of the meridional joints 376.9 hour- 

 degree units. The meridional joints exceed the equatorial joints by 



