Chapter XII — 189 — Dionaea and Aldrovanda 



which the lobes are closely apposed, pressing together, their mar- 

 ginal regions being curved outwards. Ashida in his studies of Al- 

 drovanda has called these the "shutting" and the "narrowing" phases 

 of closure. We are now to consider the first of these, to which more 

 attention has been paid than to the latter. Darwin investigated 

 the mechanism of closure by making marks on the upper surface of 

 a lobe in the transverse sense before stimulation, viewing the same 

 through a window cut in the opposite lobe. When closure had been 

 effected, the marks were found to be closer together and he concluded 

 from this that closure is accompanied by a transverse contraction 

 of the more superficial cells of the whole upper surface and sub-surface. 

 He thought also that the tissues above the midvein took part with a 

 hinge-like action. 



This contraction was attributed by Munk (1876) to the loss of 

 turgor by more sensitive superficial tissues ("parenchyma") lying 

 beneath the upper epidermis, accompanied by the active expansion 

 of the tissues of the lower layers of parenchyma near the under epi- 

 dermis. De Candolle, from anatomical study, seems to have held 

 essentially the same view. 



Burdon-Sanderson, seeking for a "resistance" which has to be 

 removed in responding to stimulation, could find it only in the turgor 

 of the leaf. "In the case of cells which are excitable the immediate 

 effect of excitation is suddenly to diminish the power (of turgescence) 

 and thereby produce a diminution of the volume of the cells which 

 is equal to that of the water (probably holding diffusible bodies in 

 solution) which is discharged into the intercellular spaces." It was 

 already known from the work of Bruecke, cited by Munk, that 

 the only mechanism of the actual movements of the sensitive plant 

 (Mimosa) was such a diminution of turgor in the sensitive region of 

 the pulvinus. 



Batalin (1877) re-examined the matter and, confirming Darwin's 

 observations, extended the account to include subsequent opening. 

 Using the same method as Darwin, namely, measurements of changes 

 between ink marks during closure, Batalin came to agree with him 

 that there is a real contraction of the upper side of the lobes and a 

 concomitant expansion of the lower, both longitudinal and transverse. 

 He takes issue with Darw^in (and Ziegenspeck, 1925), however, 

 holding that the midvein takes no part, or at least a very small and 

 unobservable part. When the trap remains closed, as it does for a 

 week or ten days (or even longer) if it has been fed a Hving insect, 

 it enters at once into a second phase of movement. The lobes begin 

 to compress together mutually, so that in a half-hour (as I have 

 observed) much of their inner surfaces are in actual contact, leaving 

 however a space above the midvein. The compression is such that 

 the margins of the lobes are turned outwards and the cilia come to 

 lie more nearly parallel to the general plane of the lobes (iS — 4b). 

 As Batalin observes, the pressure exerted is enough to crush a soft- 

 bodied insect. Darwin thought that this compression is owing to 

 the absorption of animal matter. Batalin said that it is caused by 

 the reduction of the expansion of the lower surface, for he determined 

 that during the slow compression of the lobes together after the in- 



