128 IRRITABILITY. 



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. 



236. In some cases there are constant movements of leaves 

 or petals. We have upon record remarkable examples of this 

 kind. In the Megacliniiun falcatum the labellum is in constant 

 motion. In the Pterostylis there is a kind of convulsive action 

 of the label lura. The filaments of the Oscillatorias are con- 

 tinually writhing like worms in pain. The Hedysarum gyrans 

 is the most remarkable instance of this character. 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 or i-dW more or less quickly, and retain their position 

 for uncertain periods. Cold water poured upon it stops the 

 motion, but it is immediately renewed by w^arm vapor. 



237. The spores of some cryptogamic plants exhibit motions, 

 which are said to depend on hair-like processes or cilia. In 

 certain cells, also, of the same class of plants, bodies are met 

 with, called Phytozoa — plant-animals, which exhibit movements. 



In the higher orders, also, movements have been observed in 

 the fovilla, or contents of the pollen grains, when moistened with 

 w^ater. These have been consi<iered the result of irritability, but 

 they are now generally regarded as mere mechanical movements, 

 which may be exhibited by many forms of matter, as minute 

 grains of Gamboge and other substances. 



238. Movements produced by the action of external agents 

 are various. The common sensitive plant offers a familiar ex- 

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

 petiole bends downward to the stem. The touching the base 

 of the stamens of the Cassia causes it to fly up against the pistil. 

 The Dionea muscipula is a case very much in point, but not very 

 common. The lamina of the leaf is surrounded by long stiff" 

 bristles, and if the upper surface of the leaf is touched, the sides 

 collapse, the bristles passing each other like the teeth of a steel- 

 trap, thus effectually holding any insect that may light upon its 

 surface, and the more the insect struggles for liberty, the more 

 closely the leaf contracts. 



239. The eflfect of poison on plants is exhibited by move- 

 ments indicating their action. 



236. What cases of constant movement ? — 237. What of some spores ? 

 What of the contents of pollen grains? — 238. Wliat example of movementa 

 occasioned by external agents ?— 239. What effect of poisons ? 



