Chapter XII — 197 — Dionaea and Aldrovanda 



The trap stands at the apex of the petiole, the midrib, carrying 

 a single annular vessel with an ample phloem, being continuous from 

 one to the other. But it always stands asymmetrically, resulting 

 from a twist in the stalk, in such a manner that the mutually appressed 

 lobes in the young trap are turned with their free margins to the 

 left (as viewed from above) through an angle of about 90 degrees. 

 In addition to this torsion, the trap is bent backwards (i.e., to the 

 right as seen from above) through an angle of 30 to 40 degrees (77 — 

 5). Monti evidently refers to this posture when he said, "In bar- 

 bularum medio folliculus oblique appenditur." It is thus brought 

 about that, when the traps are open, their openings face outwardly 

 away from the stem, instead of tangentially. The course of devel- 

 opment of the leaf is here worth a glance {ig — 6). In its earliest 

 stage, the whole leaf consists of a mere conical protuberance from 

 the stem apex. Soon it becomes apparent that the basal half is broad- 

 ening to form the fiat petiole, while the now more cyhndrical end 

 is to become the trap. Very soon this begins to show torsion which 

 progresses until, when the leaf is approaching maturity, the trap 

 comes to lie in its definitive position. In the meantime the bristles 

 have developed, first the outer followed by the inner. At an early 

 stage it can be clearly seen, as it was by Caspary, that the trap and 

 inner bristles do not he in the same plane. In maturity, the bristles 

 project much beyond the trap and so produce the bristly appearance 

 of the plant. 



The position of the mature traps resulting from torsion and bend- 

 ing may be regarded as a distinct adaptation, since their mouths, 

 when open, are all placed so as to avoid obstruction from neighboring 

 leaves, which in view of their numbers and crowding, is obviously 

 advantageous for the easy approach of prey. 



In describing the action of the trap, whose pecuharities of posture, 

 much less pronounced but present in Dionaea, have just been de- 

 scribed, a special terminology is required, proposed by Ashida. It 

 is clear that if the trap is twisted 90 degrees to the left (in the sense 

 above indicated) the one side or lobe of the trap must come to lie 

 against the bristles {ig — 5). This Ashida calls the bristle-side lobe. 

 The other lobe is the free-side lobe. The importance of this distinc- 

 tion lies in the fact that both lobes are concavo-convex and lie dished 

 the one into the other (ig — 7). That is, the outer surface of the 

 bristle-side lobe and the inner surface of the free-side lobe are convex, 

 the other two concave. Since this has been brought about in the 

 course of development, the two lobes acquire a different set, the 

 effect of which will be clear when the action of the trap is described. 

 The trap has a unique structure, which we shall now describe. 

 Morphologically it is a leaf blade, each half being nearly semi-circular, 

 the circle being subjected to some degree of skewing. Each, of course, is 

 attached to the midrib, which is the thickest portion. Fenner, 

 CzAjA and others have called the midrib the "hinge," but as the 

 proximal parts of the lobes do not move at any time, this is a mis- 

 nomer. It is true that textbook figures taken from earlier authors 

 would indicate the contrary, but they are certainly wrong, as Ashida 

 has clearly shown. 



