SEASONAL VARIATIONS IN THE CARBOHYDRATE-CONTENT. 49 



duced by changes in the external conditions. The driest portion of the 

 year is the arid fore-summer (April to July) with no significant rainfall, 

 high evaporation, and increasing temperature. The arid autumn (October 

 and November) is as dry as the fore-summer, but, as will be seen, the 

 effects of this season on the carbohydrate economy are somewhat moderated 

 by the lower temperatures of the nights. 



In table 12 are given the maximum and minimum temperatures of the 

 locality in which the plants investigated grew. 



TABLE 12. 



More instructive than the air temperatures are the temperatures of the 

 cactus joints themselves. Some results on this point have been obtained by 

 McGee, 1 and while these are not complete throughout the year, an idea can 

 be obtained therefrom of the relation of air temperatures to that prevail- 

 ing within the isolated plant. McGee summarizes his results as follows : 



" (1) Joints of Opuntia blakeana 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. 



" (6) The temperatures of the joints in a meridional position rise sharply 

 after sunrise, reaching a maximum about 11 a. m. They then slowly drop until 

 12 h 30 m 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 

 line 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 units 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 



1 MCGEE, J. M. The effect of position upon temperature and dry weight of joints of 



Opuntia. Carnegie Inst. Wash. Year Book, 15, 73, 1916. 

 4 



