IRRITABILITY. 545 



chemical stimuli. Not all motile leaves, however, are thus 

 sensitive, as is clearly seen in the case of the lateral leaflets 

 of Desmodium. The effect of such a stimulus is to cause 

 the leaves or leaflets to take up a position closely resembling 

 the nocturnal position, though, as we shall see, the mechanism 

 in the two cases is different. Some organs which do not 

 exhibit spontaneous movement, and are not affected by 

 variations in the intensity of light, are sensitive to stimuli 

 of this kind : instances of this are afforded by the stamens 

 of the Cynareae, of Berberis, of the Cistinese, of Sparmannia, 

 the style of Goldfussia anisopkylla, the lobes of the stigma 

 of Mimulus, Martynia, Bignonia, and others, the tentacles 

 on the leaf of Drosera, the blade of the leaf of Dionaea, 

 of Aldrovanda, and of Pinguicula. The effect of stimulation 

 on the stamens of the Cynareae is to cause them to shorten 

 (Pfeffer) : the stamens of Berberis rise up from their nearly 

 horizontal position so that the anther comes into contact 

 with the stigma : the effect of a stimulus on those of Spar- 

 mannia and of the Cistineae is, on the contrary to cause them 

 to bend outwards and downwards towards the petals : in 

 both cases the stamens slowly regain their original position 

 (Morren). Stimulation causes the curved style of Gold- 

 fussia anisophylla to straighten itself, or even to bend over 

 in the opposite direction (Morren). The stigmas of Mimulus, 

 Martynia, Bignonia, etc., are bilabiate, and the effect of 

 stimulation is that the two lips close together. The marginal 

 tentacles of Drosera curve inwards towards the centre of 

 the leaf on being stimulated : the two halves of the leaf-blade 

 of Dioncea muscipula fold upwards and meet, as do also those 

 of the leaf of A Idrovanda vesiculosa : the margin of the leaf 

 of Pinguicula curves inwards. 



The nature and the degree of the sensitiveness are by no 

 means the same in all these cases. In most Oxalidaceae and 

 Leguminosae movement can only be induced by violent 

 shaking, whereas, in some, such as Mimosa pudica and Oxalis 

 sensitiva, a touch suffices. A very slight touch also suffices to 

 induce movement in the sensitive floral organs, and in the leaves 

 of Dionaea and Aldrovanda. In the other insectivorous plants 



v - 35 



