Chapter XIV —249— The Utricularia Trap 



around from one end of the threshold to the other in a semicircular 

 sweep. Here the door curves inwardly at first, to form the upper hinge, 

 below this outwardly to form the bulging middle area. The curves of 

 the upper hinge are most pronounced in front, and are reduced at the 

 sides. The outward spring of the door depends on the physical prop- 

 erties of the upper hinge chiefly. The lateral hinges resist this pull, 

 but can bend passively. 



If now we examine the histology of the door we find (29—1, 2), 

 on inspecting a section in any direction, that in general the outer 

 course of cells is thin, the inner thick, in the ratio of about one to 

 three, differing from place to place. In the central hinge they are of 

 nearly equal thickness, as also in the middle piece. The cells of the 

 outer course are all flat, their anticlinal walls zigzag (Bijsgen) and 

 these walls are strongly supported by buttresses at their angles. These 

 prevent their collapse under bending. This layer must put up with as 

 much bending as the outer, but passively. Many of these cells bear 

 glandular hairs {24—1; 26 — 2), some with pyriform, some with 

 spherical capitals. Those with pyriform capitals are scattered over the 

 upper part of the door surface, and are shorter stalked as one ap- 

 proaches the middle point of the door. Near the door edge, arranged 

 in a crescent parallel with it, is a row of glands with short stalks and 

 globose capitals, a quite large one at the middle point. Just above the 

 level at which this stands there are hairs of different structure. These 

 are stiff, tapering, sharply pointed bristles, four in number, standing at 

 the angles of a trapezoid, in an oblique posture, extending upward, 

 then curving delicately (25 — 3, 5). Each is composed of three to 

 five cells, the basal the shortest, the terminal the longest. Ekam- 

 BARAM described short hinge cells, like those in the sensitive hairs of 

 Aldrovanda, but this is a mistake. They anchor in the outer cell layer, 

 by a broad base, as correctly shown by Meierhofer, without any 

 bulbous insertion as Merl showed {26 — 4). These four (or in 

 U. flexuosa six, Ekambaram) hairs constitute the tripping mecha- 

 nism of the trap. A touch of these in any direction but, according 

 to CzAjA, best from above down, causes some distortion of the middle 

 piece of the door selvage. This distorts the door edge from its equal 

 seat, upsetting the dehcate equilibrium, and permitting the water 

 pressing against the door to push it in, assuming the trap to be 

 properly set. As we shall see, the tripping mechanism shows a wide 

 variety of form in the genus. That just described is found only in the 

 vulgaris type and in Biovularia, though in the latter the door has a 

 different plan of structure. 



When facing the door, the outer course of cells presents a plan as 

 follows. In the region of the upper hinge, the cells are isodiametric, 

 very wavy walled, with strong buttresses (Cohn 1875). Their walls 

 lie athwart those of the inner course cells. The same is true of the 

 middle area. In the central hinge they are very small, corresponding 

 in size to the cells of the inner course. The same is true of the middle 

 piece, where the cells are very small and their walls are strongly 

 fortified with broad and thick buttresses. These cells, however, while 

 small when seen en face, are deep, and equally deep with those of the 

 inner course. Along the selvage to the outer hmits of the lateral 



