42 



THE AMERICAN NATURALIST [Vol. XL 



conditions the cactus may absorb water much faster than it gives 

 it up by transpiration. 



It should be noted that the specimens of cactus which were 

 experimented upon both in the field and in the laboratory did 

 not have an adequate water supply at their disposal previous to 

 the times of the experiments. As a general thing not until some 

 time has passed after the water has been absorbed, does new 

 growth appear with its embryonic structure and its evanescent 

 leaves and then only does the rate of transpiration become greatly 

 increased- During the periods of drought the plants make but 

 little new tissue. These peculiarities of Opuntia versicolor were 

 observed repeatedly and will be presented in the succeeding 

 resume of representative experiments. 



The transpiration of Opuntia versicolor,— an entire plant, — 

 was as follows: — 



March 25 . . . . 51.0 milligrams in one hour 

 March 26 .... 63.0 ^ " " " " 

 April 25 ... . 19.9 " " " " 

 June 30 ... . 27.5 " " " " 

 July 4 .... 26.1 " " " " 

 During the period from March to July the rainfall was unusu- 

 ally small and the cactus had an insufficient supply of water. 

 The rates of March, April, and June, therefore, represent the 

 transpiratory activities of the plant in times of drought. At 

 various times in the midst of the dry seasons Fouquieria, Covil- 

 lea, and other plants had been irrigated and the effects on their 

 transpiration were recorded (Cannon, :05b). To learn how an 

 increase in the water supply of Opuntia versicolor would influence 

 its rate, as well as to learn how the rate under such circumstances 

 would compare with that during dry conditions, it, also, was irri- 

 gated. On June 27, which was a time of drought, nine gallons 

 of water were poured slowly on the ground at the base of the 

 cactus, but it did not show by an accelerated rate (see the rate 

 of July 4, above) that it had absorbed any of the water. That it 

 had really done so, however, was indicated by the fact that the 

 plant had become rigid by the increased turgescence of its tissues. 



