Chapter XIV — 263 — The Utricularia Trap 



thelium, the middle zone of which lies just within this edge. The 

 outer zone, which carries the velum, faces outwardly {36 — 8, 9). The 

 inner region bears glandular hairs of various forms, at first with 

 conical capitals, then with bifids. Quadrifids of large size occupy the 

 interior wall surface. The door lies almost at right angles to the plane 

 of the threshold, result of the forward bending of the rostrum. The 

 action of the door has been already described (p. 257). Histologically 

 the door presents a unique feature in the very great depth of the inner 

 course cells in the upper hinge region, the door gradually tapering in 

 thickness toward the edge. Of this we may say that these thick cells 

 can exert a strong tangential thrust so as to press the door selvage 

 firmly against the pavement, the outer zone of which bears the velum, 

 seen in living material at Perth, W. Australia. The door selvage is not 

 thickened. Its cells are of equal thickness in both courses, and there is 

 no obvious middle piece. This means that the door selvage must bend 

 over the pavement, not impinge edgewise on it. The tripping mechan- 

 ism consists of short, bent, glandular hairs, 30-40 in number, scattered 

 on the surface of the door below the middle point {36 — 5). 



The dimorphism in the traps of P. latifolia has been indicated. 

 There are two sizes of traps. In the larger, the threshold behind the 

 pavement bristles with a dense fringe of conical glands of graduated 

 sizes, described by Lang. Inside this pale stand some bifid glands. 

 In the small sized trap there are no conical glands. In their place 

 there are glands with single-celled capitals of the form of the bifids and 

 quadrifids. Inside the traps are bifids {36 — 8, 9). 



The walls consist of four courses of cells, the two epiderms and 

 two courses of parenchyma. The epiderms vary in thickness. The 

 outer is thickest in the middle of the sides, and the inner thickest at 

 the angles, here forming a hinge structure. 



The total thickness of the three walls, which have four courses of 

 cells throughout, is always greatest at the middle of their faces, pro- 

 ducing a hinge effect at the angles. Further, the outer epiderniis is 

 always thin at the angles and progressively thicker toward the middle 

 of the faces, while the inner is thick at the angles and th n elsewhere, 

 the more readily allowing compression on the inside of the angles and 

 on the outside of the faces. It is evident from mere inspection that 

 these massive walls must exert a big pull when the trap is exhausted 



of water (3^ — 7)- 



In closing this account one cannot but wonder at the astonishing 

 variety of trap structure. It is not less astonishing that there is no 

 evidence that one form of trap is superior to another in action. The 

 fact of variety is one with the same phenomenon observed when we 

 survey attentively some other unit of structure. It seems as though 

 nature, or to deify her fruitfulness, Nature, is not nor ever has been 

 content to make some one thing, however satisfactory, and to let it go at 

 that. She must show that she is not bound to the details of a pattern 

 that, in this case, she can make a whole shelf full of different kinds of 

 traps, as if to puzzle you to pick the best. 



Digestion. — Goebel remarked the great difficulty, because of their 

 small size, of studying the traps ot Utricularia to determine the pres- 

 ence or absence of digestive activity. It had of course been quite 



