46 RESEARCHES ON IRRITABILITY OF PLANTS 



responsive down movement, though here recovery is very 

 protracted. But we have seen that the recovery of Neptunia 

 is also a very slow process. 



The responsive movement of Mimosa is due, as has 

 been noted, to the unequal excitabilities of the upper and 

 lower halves of the pulvinus. The excitability of the tissue 

 is again modified by the state of turgor. In Mimosa there 

 is induced a periodic variation in the relative turgescence 

 of the two halves of the pulvinus. On account of this the 

 differential excitability, on which the motile response of 



Fig. 1 8. — Series of responses of leaf of Desmodium gyrans, 

 under electrical stimulation. 



Mimosa depends, undergoes great variation. The sensitive- 

 ness of this plant is in consequence often found to disappear 

 completely at certain hours of the day. I shall, moreover, 

 show in Chapter VII that the leaf of Mimosa becomes in- 

 sensitive when its pulvinus absorbs an excess of water. Thus 

 the mechanical movement of the sensitive plants on which 

 depended the assumption that * ordinary ' plants were 

 insensitive, rests on a basis which is very unreliable. 



Responsive movements may, on the other hand, be 

 demonstrated in ordinary plants by the employment of a 

 suitable contrivance. In a radial organ diffuse stimulation 

 induces equal contractions on all sides, which balance each 



