542 LECTURE XXXVI. 



movements at all times of the day under the influence of changes of tempera- 

 ture but the more energetic the longer the interval since the last movement was 

 accomplished. This is still more conspicuous in the case of Nymphaa alba, Oxalis 

 rosea and 0. valdiviensis, Mesembryanthemum, and the motile flowers of Compositse. 

 These close in the evening, and then even increments of temperature to the extent of 

 10-28° C. scarcely effect their opening; in the morning, on the contrary, a 

 rise of temperature causes these flowers to open even when it is dark. 



The reader may easily see for himself these effects of variations of tem-. 

 perature by taking Crocuses and Tulips cultivated in flower-pots, in the Spring 

 when the weather is cool and the flowers outside are closed, and placing 

 them simply in a warm room, where they will often open after a few minutes. 

 The observation may be made even more simply: if a closed Tulip-flower 

 is placed with its stalk in warm water at 20-25° it opens visibly, and this in 

 fact was the experiment by which Hofmeister first detected the influence of 

 variations of temperature on the opening and closing of flowers. In the case 

 of the Tulip and Crocus, Pfeffer was able by warming and cooling the flowers 

 to make them open and close eight times in one day; but even in these cases 

 the opening was more energetic if the flowers had been closed for some time, 

 and conversely. Particularly sensitive Crocus flowers may be made to open and close 

 completely by a variation of 5°C. in eight minutes; with variations of I2-20°C. this 

 resulted in as little as three minutes. Pfeffer found, moreover, that Crocus flowers 

 are sensitive to even ^° C, and those of the Tulip to variations of 2° C. 



Those flowers which are sensitive to variations of temperature are more- 

 over sensitive to variations of light. Nevertheless the sensitiveness is greater 

 for the one or the other according to the species of plant : the flowers of 

 the Crocus and Tulip which are so very sensitive to variations of temperature, 

 become closed when suddenly placed in darkness, and opened on being illuminated, 

 and this indeed with an energy which is capable of overcoming the effect of opposite 

 though feebler stimuli due to temperature. On the other hand greater variations in 

 temperature are again able to reverse the opening or closure effected by light 

 and darkness. In Oxalis, NymphcBa alba. Taraxacum, &c., on the contrary, the 

 closure in the evening cannot be arrested by raising the temperature, and just 

 as little can lowering the temperature in the morning prevent the opening. If 

 however these flowers are kept closed during the day, they can be opened in 

 the evening by raising the temperature, and so on. 



From this different susceptibility for variations of light on the one hand, and 

 for variations of temperature on the other, we obtain the simplest explanation of 

 the fact that some flowers in the open exhibit a marked daily periodicity, while others 

 close and open at any time of the day on sudden changes in the weather ; evidently 

 the former are sensitive for the daily variation of light more than for variations of 

 temperature, the latter, on the contrary, are very susceptible to sudden warming and 

 cooling. 



Finally it has yet to be mentioned that some flowers close when the light is 

 too strong, as well as when the temperature is too high, thus resembling periodically 

 motile foliage-leaves — with which, however, this only happens when the light is too 

 intense. Pfeffer observed this in . the flowers of Oxalis valdiviensis. Calendula, 



