GENERAL CONCLUSIONS. 



The results of observations upon the greneral climatic conditions preva- 

 lent in the Tucson region and the facts broug-ht to lig'ht by an inspection 

 of the data obtained by weig-hing and measurement of native succulents 

 present some features of unusual interest. These are briefly set forth in 

 the following- paragfraphs. 



The annual cycle includes two seasons in which the soil-moisture con- 

 tent is hig-h and the relative humidity is at its maximum. During- the 

 summer rainy season the temperature may reach 115° F., while in the 

 winter wet season the temperatures are low, but rarely remain below the 

 freezing'-point for more than a few hours. Alternating- with these seasons 

 in which moisture is more abundant is the dry foresummer of April, May, 

 and June and the arid aftersummer of August, September, and October, 

 in which the temperatures are high and the relative humidity very low, 

 sometimes falling to 8 and 10 per cent in June. 



The rate of the transpiration is larg-ely determined by the evaporating 

 capacity of the air and operates to make a notable depletion of the water- 

 balance in the plants under natural conditions during the dry foresummer 

 and aftersummer. The influence of other conditions and of separate agen- 

 cies may be detected, however. First is to be noted the indi-viduality of 

 the reactions found in the alternations in form and volume, which is well 

 illustrated by a comparison of the loss of weight shown by echinocacti 

 Nos. 7 and 8. The former, weighing 36 kg., lost weight at a rate of 10 to 

 14 grams daily in the month preceding January 9, 1909, while No. 8, nuder 

 the same conditions of illumination, temperature, relative humidity, and 

 wind action, showed a rate of 5 to 27 grams daily, although its weight was 

 but 29 kg. 



Both plants were in an inclosed room. Nos. 9 and 10 (echinocacti) were 

 exposed to open-air conditions and the first weighing, 17 kg., showed a 

 rate of loss of 36 grams daily for the month ending December 8, 1908, and 

 No. 10, weighing 15 kg., showed a rate of 61 grams during the same period; 

 No. 9 lost at the rate of 38 grams daily during the six months ending May 

 12, 1909, and No. 10 at the rate of 30 grams in the same period. The 

 conditions were more favorable for rapid transpiration in this plant than 

 in No. 9. The disparity would doubtless have been even greater under 

 equivalent conditions. 



Of the pair in the inclosed room, the larger lost most water during 

 December, but the smaller showed a rate, low at first, but which increased 

 until it was nearly double that of the plant which had a total weight one- 

 fourth greater. The pair of plants in the open displayed a reverse rela- 

 72 



