Page:Researches on Irritability of Plants.djvu/69

46 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

Mimosa depends, undergoes great variation. The sensitiveness 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 insensitive 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