38 THE PLA]N^T WOELD. 



having all passed from the reflexed into a rotate position. At 

 11:00 a. m. the withering had proceeded to an evident extent. 

 August 18tli had been a very wet day, and for twenty-four hours 

 thereafter there was a high relative humidity. In spite of this 

 high humidity, however, the new flowers of the 19th opened at 

 2 :30 to 3 :00 p. m., no earlier than usual. It happened that it 

 rained at 3 :45, and thus the flowers were wet, but no hastening 

 nor delay of the opening was observed. I had occasion during 

 several nights to study the effect of these rains and consequent 

 humidity, and always found this delay in closure and withering. 

 This observation is of interest, as it indicates that the closure 

 may be due in part to active transpiration, and the delay above 

 described to the reduction of transpiration by the rain or humid- 

 ity. It is recognized as generally true that the rate of transpira- 

 tion in even delicate flowers is very much less than in the leaves 

 of the plants which bear them, and this can readily be believed 

 to be true of many desert plants, the flowers of which remain in 

 an unwilted condition under the hottest sun, when the leaves are 

 distinctly wilted or even withered. Son;ie flowers, removed from 

 the stem, have been observed to remain turgid a remarkably long 

 time. I have, however, made no exact experiment. It would 

 seem probable that in the case of some, if not all, cactus flowers 

 the rate of transpiration is greater, as compared with many flow- 

 ers, than might be expected, and not at all low. 



]*^ot only is the fading of the flower delayed l\v high humid- 

 ity, but, as I have had recently to notice, the quite cool nights 

 which may occur in the early part of September may also produce 

 the same eifect, and it would seem, for the same reasons, namely, 

 by suppressing transpiration. Thus, on September 12, at 6 :00 

 a. m., some flowers on one plant which was still active in their 

 production, though the season Avas drawing to a close, were still 

 open. This was the first time I had observed this since the cessa- 

 tion of the rains. The previous night was exceptionally cool, 

 the mercury going down to 14.5° C. The sun rose at 5 :00, and 

 the warmth, or perhaps I should say the heat, soon effected the 

 withering. I may here remark that one may easily be deceived, 



