140 IRRITABILITY. 



Section. 9. Irritability. 



193. The vitality of plants is often exhibited by various 

 spontaneous motions; by the sensible effects produced by the 

 actions of external agents, all of which phenomena are attribu- 

 ted to irriiahHity, 



Of the former of these phenomena, the most common 

 is what is generally called the sleep of plants. In plants 

 with compound leaves, the leaflets often close on the ap- 

 proach of darkness, and expand again on the return of 

 day. Many flowers also undergo the same changes. Some 

 flowers, however, are enable to sustain the liijht for the 

 whole day, and close their flowers under the direct rays of 

 the sun. In some cases also, the calyx and floral leaves em- 

 brace the flower, seemingly for the purpose of protecting it 

 from the action of the cold and moisture of the night. Most 

 of the preceding phenomena are, no doubt, due to the action 

 of the light, since they may be made to take place by artificial 

 arrangements, for the production of light and darkness. — 

 Lamp light will make some plants unfold their petals, which 

 have been closed for the night. 



194. In some cases there are constant movements of 

 leaves or petals. We have upon record, remarkable exam- 

 pies of this kind. In the Megaclinium falcatum^ the labellum 

 is in constant motion. In the Pterostylis, there is a kind of 

 convulsive action of the labellum. The filaments of the Os- 

 cillatorias are continually writhing like worms in pain. The 

 Hedysarum gyrans is the most remarkable instance of this char- 

 acter. This plant has ternate leaves, the terminal leaflet, 

 which is larger than those at the side, does not move except 

 to sleep ; but the lateral, especially in warm weather, are in 

 continual motion, both day and night, even when the terminal 

 leaflet is asleep. External stimuli produce no effect. The 

 motions are very irregular, the leaflets rise jv fall^ more or 

 less quickly, and retain their position for uncertain periods. 

 Cold water poured upon it stops the motion, but it is immedi- 

 ately renewed by warm vapor. 



195. Movements produced by the action of external agents 

 are various. The common sensitive plant, offers a familiar 

 example; by touching one of the leaflets, the whole closes, and 

 the petiole bends downwards to the stem. The touching the 

 base of the stamens of the Cassia, causes it to fly up against* 

 the pistil. The Dionea Muiscipula, is a case very much in 

 point, but not very common. The lamina of the leaf is sur- 

 rounded by long stiff bristles, and if the upper surface of the 



