4 THE WATER-BALANCE OF SUCCULENT PLANTS. 



The temperature of the soil at 12 to 15 cm. shows a daily variation be- 

 tween 45° and 57° F. in February, near the end of the winter wet season, 

 which gradually rises until at the end of the arid foresummer, late in 

 June, the daily range is from 87° to 104° F. At a depth of_25 cm. the tem- 

 perature is more equable, ranging from 55° to 60° F. in February and 

 between 86° and 88° F. late in June. The equable-temperature room of the 

 Desert Laboratory varies between 56° and 58° F. in February and 74° F. 

 in June and July, representing the conditions at a depth of about 2.5 and 3 

 meters in the open. 



Air-temperatures vary from between 28" and 75° F. in February to 

 between 57° and 114° F. late in June. The integration of the temperature 

 record shows that the total number of hour-degrees heat -units (Fahrenheit) 

 amounts to 326,385. 



The relative humidity ranges from saturation in July and August, to as 

 low as 10 and 12 per cent in May and June, falls again in September and 

 October, and rises with the winter rains. 



The succulents described in this paper absorb some solution from the 

 soil during the winter wet season despite the low temperatiires and the 

 fact that the greater part of the root-system is decorticated and brownish. 

 This will be evident upon an examination of the measurements recorded. 

 The temperature exposures of March induce root-development in the 

 sahuaro, and flower-buds begin to swell on the terminal portions of the 

 trunks and branches formed during the previous season of growth. Fruits 

 are matured and ripened about the time of the beginning of the summer 

 rains, and another addition to the water-balance takes place which is ac- 

 companied by a growth-extension of the trunks and branches. These 

 become distended to the utmost, and as the rains cease a gradual loss begins 

 which is only checked by the beginning of the moist season of the winter. 



Both growth and flower-formation occur in the prickly pears chiefly 

 during the dry foresummer, the flatfish joints becoming much shrunken 

 during the latter part of June and late in November. Growth and repro- 

 duction take place in the bisnaga during the summer rainy season, and the 

 shrinkage of the short, thick trunks is not so noticeable to the eye as in 

 the other types mentioned. (MacDougal, D. T., The Course of the Vege- 

 tation in Southern Arizona, Plant World, xi, pp. 189, 217, 261 (1908); 

 Across Papagueria, Plant World, xi, pp. 93, 123 (1908); Botanical Fea- 

 tures of North American Deserts, Publication 99, Carnegie Institution of 

 Washington (1908).) 



