IRRITABILITY. 117 



like a mantle, immediately after the overcloudng of the 

 whole horizon. It then rained the whole time until late 

 at night, and the plant remained in its true nocturnal 

 condition until the following morning. Some Mimosse 

 closed their leaves the same day at a quarter past five, 

 and Morren observed, that during the annular eclipse 

 which occurred in 1836, the leaves of many irritable 

 plants closed almost completely together. 



166. Decandolle found that he could bring about the 

 nocturnal conditions of the leaves of some plants in the 

 day- time, and the diurnal conditions at night, by alternate 

 exposure of the plants to the influence of the light of 

 Argand lamps, but that in other cases he could not. 



167. The times at which flowers close in the evening, 

 are various in different species, and also at certain times 

 of the year. The Daisy is said to close in summer at 

 5 P. M., in spring at 3 p. M. A regular closing and 

 opening at certain hours, is to be seen only in young 

 flowers ; some flowers, after closing in the evening, will 

 not open again at their accustomed time, if the sun does 

 not shine, and especially if it is cold and rainy. 



168. Some plants open their petals or produce their 

 flowers only at night, which, by morning, wither away. 

 The flower of Cereus grandiflorus begins to expand at 

 about 6 or 7 P. M., is fully blown at midnight, and 

 ceases to flourish towards 3 or 4 A. M. ; other plants 

 shut their flowers before an impending storm, and others 

 close them whilst the sun is shining upon them. 



169. Under this head are also to be classed those mo- 

 tions induced in the Sensitive plant, and Venus's Fly- 

 trap, etc., by the contact of external agents ; the Sensi- 

 tive plant requires a temperature of 77 Fah., and plenty 

 of moisture, in order to exist in a very irritable state, 

 and which is said to be so considerable in its native 

 country, that heavy walking over the ground in the 

 neighbourhood, is often sufficient to cause a closure of 

 its leaves. 



1 70. Of those under the third head, we may notice the 



