RESPONSE OF ZEPHYRANTHES 217 



flowers of Crocus vermis and Tidipa Gesneriana, specially 

 investigated by Pfeffer. He found that the flowers opened 

 under the action of a rise of temperature, and closed under 

 the action of a fall. He also established the important fact 

 that the opening and closing of flowers is a phenomenon of 

 differential growth under variation of temperature. The 

 dorsiventral perianth-leaf of the flower is affected unequally 

 at its two sides by rise or fall of temperature. In opening 

 under rise of temperature, the upper side grows relatively 

 the faster, the opposite effect being induced during fall of 

 temperature. I shall show that thermonastic movements 

 are not of one but of two distinct types : (i) the positive, in 

 which, as in Crocus, rise of temperature induces a movement 

 of opening ; and (2) the negative, where rise of temperature 

 induces a movement of closure, as in a number of the 

 Water-Lilies (Nymphaea) which grow in India. 



Positive Thermonastic Response of Zephyranthes 



I first give an account of the movement of the petal of 

 Zephyranthes, the reaction of which to variation of tempera- 

 ture is similar to that of Crocus. Viewed from above, the 

 inner side of the petal of a flower is the upper side ; under 

 rise of temperature the flow^er opens evidently by the 

 enhanced rate of growth of the relatively more active upper 

 side, as shown by the following experiments. 



Experiment 123. Effect of variation of temperature. — • 

 In obtaining a record, all the floral leaves except one were 

 removed, this particular petal being attached to the record- 

 ing lever. There was an up-movement, or a movement of 

 closure, under fall of temperature, while rise of temperature 

 induced a movement of opening. The up-movement is 

 recorded by a down-curve and vice versa (fig. 124). 



Experiment 124. Responsive movement under radiation. 

 I next determined the effect of radiation on the movement 

 of the petal. It has been shown that the effects of radiation, 

 whether visible or invisible, are similar, the effect of the 



