Chapter XIV 



265 



The Utricularia Trap 



be useless if it were set in this way. In the trap it must be able to 

 resist the water pressure to which it is normally subjected, until a 

 trigger action is applied, when it must then be weak enough to fold 

 up, allowing the entrance of a water column. And it must of course 

 not allow the leaking in of water when the trap is set. How these de- 

 mands are met may be understood by examining the structures in- 

 volved. For models of the U. vulgaris trap, see p. 266. 



The door is divided into a relatively thin anterior half, and a 

 thick posterior half {31 — 3 etc.). The former includes the areas 

 of the upper hinge and the middle area. The two cell courses of 

 the upper hinge have the structure seen before, with deeply constricted 

 walls in the inner course {31 — 9). Those of the middle area, not a 

 region of sharp bending, have about equal thickness, with no con- 

 strictions. There is no central hinge and below the mid-point of the 



Fig. 9. — Diagrams of the entrances of Utricularia vulgaris (i, 2) and U. capensis (3 riglit) 

 or caerulea (3 left ) embodying the different mechanical conditions in these two types of 

 trap; pd, general direction of thrust of the door, and, pt, of the threshold; r, relaxed position 

 of the door; a, point of impact of prey; PH20, pressure of water against the door; iz, slope 

 of inner zone of the pavement epithelium; mz, slope of middle zone of same; d, thrust of 

 the door; Ih, thrust of the lateral hinge; c, composition of these thrusts. 



door lie the middle piece and the lateral hinges. Their total thickness 

 is usually greater than elsewhere, and in the lateral hinges the outer 

 course cells are thin and the inner thick and are constricted with many 

 constrictions {30 — 6-8). As these merge into the middle piece the 

 two cell courses become nearly equal in thickness and the walls are 

 thick. The threshold is broad and semicylindrical in form (jo — 5-8). 

 The outer third as seen in sagittal section (the "doorstep") bears 

 glandular hairs, the middle third of pavement cells supplying a volu- 

 minous velum of balloon cells, and the inner third being dense pave- 

 ment {2j — 2). The whole is surrounded by the massive trap walls, 

 giving firm support. The lower part of the door, when closed, rests 

 cramped into the relatively narrow arc of the threshold, exerting a 

 firm pressure by its middle piece {30 — 8). In the set posture the 

 upper part of the door assumes a convex form, thereby increasing the 

 pressure of the door selvage on the pavement, widening the angle 

 between the two {30 — 3). Just after release, the door, now in the 

 relaxed posture, has its upper part convex. It is watertight in this 

 condition. As water is withdrawn the upper half of the door becomes 



