548 LECTURE XX. 



each of the other pairs of leaflets of the same pinna will fold 

 up in succession: if the stimulus is sufficiently strong its 

 effect may extend to other pinnae causing their leaflets to 

 fold up, or even to the main petiole which then sinks down- 

 wards. Stimulation of one leaf, if sufficiently powerful, will 

 cause movement in another. In the case of Drosera, Darwin 

 ascertained that stimulation of the central tentacles of a leaf 

 eventually causes the inflexion of the marginal tentacles, and 

 that the stimulus travels more readily longitudinally than 

 transversely through the leaf. Morren observed also in Spar- 

 mannia africana that stimulation of one stamen causes the 

 others to move. In Dionaea the stimulation of one half of 

 the lamina causes both halves to close. 



In some cases the irritability to contact is especially 

 localised in particular parts of the organ. Thus, in Mimosa 

 pudica, no movement ensues if the upper side of the pulvinus 

 of the primary petioles is touched, but only when the sensi- 

 tive hairs on the under side of the pulvini are touched ; and, 

 in the leaflets, it is the upper surface which is sensitive ; in 

 more general terms, that side is sensitive towards which 

 the movement consequent on stimulation takes place. In 

 Drosera the irritability of the tentacles is localised in the 

 terminal gland; tentacles deprived of their glands are not 

 irritable, as Darwin ascertained. In Dionaea movement only 

 ensues when the irritable hairs on the upper surface are 

 touched. 



We have now to consider the conditions of irritability. In 

 the first place, a supply of free oxygen is essential. Dutrochet 

 observed that the leaves of Mimosa pudica lose their irrita- 

 bility in vacuo, an observation which Kabsch confirmed and 

 extended to the stamens of Berberis and Helianthemum. 

 Kabsch also ascertained that these stamens lose their irrita- 

 bility in an atmosphere of nitrogen, hydrogen, or carbon 

 dioxide. Darwin observed that an atmosphere of carbon 

 dioxide had a paralysing effect on the tentacles of Drosera. 

 Secondly, the temperature must be suitable. According to 

 Sachs the leaves of Mimosa lose their irritability when kept 

 for some hours in air at a temperature of I5C., and within 



