50 THE CARBOHYDRATE ECONOMY OF CACTI. 



of hour-degree units inclosed by the air-temperature 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 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 curves of the air- 

 temperatures differ by 138.4 hour-degree units. 



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

 fn 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 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 Pf effer states that ' Prolonged exposure to a temperature of from 

 45 C. to 46 C. kills most Phanerogams ' (Pfeffer's Plant Physiology, vol 11, 

 p. 226)." 



It is evident from these results that the conditions of temperature are 

 accentuated within the plant during the arid fore-summer with its clear 

 days of intensive insolation. During the seasons of rain, there is, of course, 

 relatively much less direct insolation on account of cloudy and overcast 

 weather and this would result in correspondingly lower temperatures of the 

 plant. 



To summarize, then, the period December to March is characterized by 

 the winter rains and low temperatures ; the period April through June by 

 great desiccation and high temperatures ; in July and August, the humid 

 mid-summer, there occur heavy rains and slightly lower temperatures, 

 while in part of September, October, and November, the arid autumn, there 

 is little significant rainfall and the temperature, while still high in the 

 beginning of the period, moderates in October and November. The limits 

 of these periods may, of course, extend in either direction and vary some- 

 what in different years. 



In this section are given the results of a study of the seasonal variation 

 in carbohydrate-content of Opuntia phwacantha with a view to establishing 

 the nature and rate of its metabolic rearrangements and disintegration as 

 affected by the climatic conditions just discussed. For each series of 

 analyses the joints of the same age were taken from a single large, healthy 



