Chapter XII — 203 — Dionaea and Aldrovanda 



in Aldrovanda, for the purposes of comparing it with that of Dionaea 

 (there is a definite analogy between them), Ashida draws the following 

 parallel between them. The motile zone of Aldrovanda is but three 

 cells in thickness; that of Dionaea composed of several to many cells 

 in thickness, according to age of plant. 



Aldrovanda Dionaea 



The outer epidermis 

 The outer epidermis The mass of parenchyma beneath 



(either one or both) 



The middle course of parenchyma The parenchyma as a whole 



The inner epidermis 

 The inner epidermis The parenchyma beneath 



(either one or both) 



The actual leaf movement embraces two phases of motion which 

 Ashida calls (a) the shutting movement, to the "shut" stage, when 

 the rims of the lobes just meet; {19 — 21b) and (b) the narrowing 

 movement leading to the "narrowed stage" {ig — 2id, 22). In re- 

 covery the opening passes through the "rebulging movement" from 

 the narrowed stage to the merely shut stage and the reopening move- 

 ment, completing the opening. To avoid confusion these terms will 



be used. 



In the shutting movement the margins of the thick region ap- 

 proach sufficiently to bring the margins of the thin regions together. 

 This follows on the application of a weak stimulus, but proceeds no 

 further. In time reopening occurs. If, however, the stimulus is suf- 

 ficiently strong, this posture is passed through, the edges of the thick 

 region approach mutually still further, the thin regions press on each 

 other mutually, and the free-side lobe buckles, dishing itself in against 

 the more rigid bristle-side lobe. Ashida maintains that these two 

 movements are not simply a continuation the one of the other, as will 

 be seen. 



The rapid shutting movement is caused by the loss of turgor by 

 the inner epidermis. This allows the other two layers to expand and 

 the curvature ensues mechanically. The membranes of the outer epi- 

 dermis are stretched irreversibly. The undulations described by 

 Ashida disappear, having previously been maintained by the outward 

 pressure of the inner epidermis. The posture thus attained now 

 changes to that of the narrowed posture by the narrowing movement. 

 This is a slow movement, accomplished by the slow elongation of 

 the outer epidermal cells, that is, by growth. Resulting is the mutual 

 appression of the two thin regions, during which water must escape 

 from the inclosed space. Ashida tried to determine by means of 

 colored fluid where the water escapes but did not get any very convinc- 

 ing evidence. It has been suggested above that the escape is between 

 the non-valvular parts of the lips at the forward and rear ends of 

 the margins. Ashida, by means of ingenious optical apparatus, was 

 able to record photographically the advance of the whole movement. 

 The shutting movement is very rapid, occupying about one fiftieth of 

 a second, following on a latent period of 0.09 seconds. This rapid 

 movement involves the expulsion from the trap of water, the pres- 



