Chapter XII — 193 — Dionaea and Aldrovanda 



What is the nature of stimulation is certainly not known. Haber- 

 LANDT regarded the constricted cells at the base of a sensitive hair 

 as sense organs which are activated by compression. Brown found 

 that the sensitive hairs do not respond to decompression procured 

 by two successive movements of a hair which had previously been 

 kept in an extreme bent position. Propagation of the stimulus cannot 

 be dependent upon the vascular tissues, since they are absent from 

 the trigger hairs; and in Aldrovanda, which has no vascular tissues 

 except the single strand along the midvein, it is even more obvious that 

 the path of movement must be found in the parenchyma, but whether 

 of the epidermis alone or of the internal tissues also, is not yet known. 



That response, an event following on stimulation, is accompanied 

 by electrical disturbances Burdon-Sanderson showed, and the char- 

 acter of these permitted him to Hken them to those which occur 

 during muscular contraction, though this is not the same as iden- 

 tifying the contraction, asserted by Darwin and Batalin, of the 

 upper surface with muscular contraction (F. Darwin, 1875), espe- 

 cially when now such contraction has been questioned (Broavn). 

 The molecular transposition measured by Burdon-Sanderson might 

 indeed be the expression of sap movements, and such sap movements 

 need not be great quantitatively to upset an equilibrium and might 

 constitute a trigger action to start the mechanism a-going. 



Whatever the tensions in the open trap lobe may be, they must 

 be duplicated in the similar trap of Aldrovanda, and when we look 

 at this beyond, it is a help to comprehend what happens in Dionaea 

 when such tensions are reUeved. 



For Dionaea we may at present say: — 



i) During the open condition there are tensions present which 

 are so distributed that they maintain the trap in an open position, 

 the lobes standing at an angle as great as 80 deg. (Darwin). 



2) When stimulated the lobes close, the ciHa becoming interlaced 

 like the fingers of clasped hands. The lobes remain concavo-convex, 

 inclosing a wide space between them. During this closure the outer 

 face of the lobe expands, the inner remains unaltered, or at least 

 it does not contract. If the stimulus is prolonged by chemical stim- 

 ulation (as when an insect has been introduced), the lobes continue 

 toward a greater mutual compression and thus obliterate to some 

 measure the inclosed space (the "narrowing" of Ashida). Batalin 

 thought that this is due to a subsequent contraction of the lower 

 face of this lobe. It might be due to a passive extension of the upper 

 face resulting from rapid exudation of secretion, depleting the tissues 

 of water. The edges of the lobes, which do not actively participate 

 in the movements, become bent outwards and the cilia now extend 

 less transversely, so that the two sets become more nearly parallel. 



3) With the cessation of secretion and absorption, the lobes re- 

 open, this being the result of increased growth of the upper faces, 

 the expansion of the lower faces being maintained. 



The mechanically stimulated trap closes, and reopens without nar- 

 rowing in about 24 hours, when it will respond again. But repeated 

 daily responses are followed by decreasing sensitivity, probably due to 

 the completion of growth. 



