IRRITABILITY. 549 



half-an-hour in air at 45 C. Thirdly, in the case of motile 

 organs containing chlorophyll, such as foliage-leaves, exposure 

 to light is essential. From the observations of Sachs, and 

 Bert, it appears that the leaves of Mimosa cease to be irritable 

 when kept in darkness for two or three days, the leaflets 

 losing it earlier than the petioles ; irritable stamens, on the 

 contrary retain their irritability in darkness. The irritability 

 of motile leaves, like that of growing leaves (p. 380), is clearly 

 dependent upon phototonus. It appears, from Bert's observa- 

 tions on the Sensitive Plant, that the rays of low refrangibility 

 (red-yellow) are those which are most favourable to the photo- 

 tonic condition, whereas green light causes a loss of irritability 

 almost as rapidly as darkness. He exposed plants to white, 

 red, yellow, green, violet, and blue light, and he found that at 

 the end of twelve days the plants in the green light had 

 entirely lost their sensitiveness, whereas all the others were 

 highly sensitive and remained so for many weeks. With 

 regard to the relative value of the other colours Bert's results 

 are not definite inasmuch as the glasses which he used were 

 by no means monochromatic. Lastly, a supply of water is 

 necessary. Sachs observed that a Mimosa left unwatered for 

 a considerable time lost its irritability, and Darwin mentions 

 that the leaves of a plant of Porlieria hygrometrica left un- 

 watered ceased to perform their daily periodic movements, 

 the leaflets remaining partially or completely closed during 

 the day. 



Irritability is also temporarily destroyed when the motile 

 organs are exposed to the action of chloroform or ether, and 

 this appears to be true, so far as the observations go, of other 

 anaesthetics. This has been observed in the case of Mimosa 

 by Bert and by Pfeffer, and in the case of the stamens of 

 Berberis, Mahonia, the Cynareae and Cistineae, the stigmas of 

 Mimulus, Martynia, etc., by Heckel. Heckel also ascertained 

 that a solution of morphia (ro per cent.) acts on the stamens 

 of Berberis. After exposure to ether Darwin found the leaf 

 of Dioncea muscipula did not move when the sensitive hairs 

 were touched, but it did so when the tip of it was cut off. 

 It appears that the sensitiveness of the leaf was much 



