Chapter XIV — 247 — The Utricularia Trap 



triangular, the base forming the free surface, the apex continuous with 

 the wall. The free surface is sHghtly convex, with broken curves 

 indicating three regions, an outer, continuous with the cheeks, carrying 

 scattered stalked glandular hairs; a middle, clothed with a layer of 

 densely crowded glands, called the pavement epithelium by Goebel, 

 and an inner of epidermal cells, forming a shelf projecting into the 

 interior of the trap. 



The outer region is part of the vestibule, and we may think of it as 

 a doorstep. The inner region is merely a part of the inner wall sur- 

 face, but re-entrant. The middle region is of critical importance. We 

 shall use Goebel's name for it, recognizing however that the surface is 

 not epithelial but consists of closely set glandular cells which arise from 

 the epithelium below. It is a pavement of packed tiles, each tile being 

 the capital of a glandular hair. We pause here to recall the structure 

 of the glandular hairs in Utricularia. Arising from a basal epidermal 

 cell, each consists of a middle cell (Goebel), strongly cuticularized, 

 short and discoid in shape, supporting a glandular capital of one, but 

 more usually two cells, sometimes four (quadrifids), uncuticularized 

 (BiJSGEN 1888). The middle cells may be supported on a shorter or 

 longer tubular extension of the epidermis cell wall, as is the case of the 

 hairs surrounding the entrance. Those of the pavement are similar 

 to the glandular hairs of the general outer plant surface, but differ in 

 having capitals elongated, at right angles to the axis of the entrance, 

 so that, on looking down on the threshold, the pavement appears to be 

 made of closely packed sausages. The capitals may be one or two 

 celled. Each gland arises from a laterally compressed epidermal cell, 

 so narrow that the middle cell lies tightly against the neighboring 

 ones. The terminal cells are similarly tightly packed, forming the 

 visible pavement (Gislen 191 7) (C/. various figures on 25 to zg). 



Like the glandular hairs in general, the pavement glands loosen 

 and shed their cuticles, but most curiously in a single piece, except in 

 the inner zone {2g — 4). To describe this behavior we have to recog- 

 nize three zones of the pavement epithelium, outer, middle and inner. 

 In the outer zone, the cuticles of its glands enlarge into balloons, but 

 remain attached mutually and to the glands which bear them. In the 

 middle zone, broadest at the ends, the cuticles remain mutually at- 

 tached, but are freed from the capitals which produced them, and 

 from the inner zone glands, but remain attached to the ballooned 

 cuticles of the outer zone. The glands of the inner zone behave 

 individually, their cuticles enlarging and bursting. There is formed 

 in this way a membrane, which I call the velum, consisting of two 

 parts, a cushion of cuticular balloons running from one end of the 

 pavement to the other on the outer zone, and attached to it a thin 

 membrane, bearing the markings of the capitals which produced it, the 

 two together forming a valve which, stretching from one side of the 

 pavement to the other, overlies the door edge (25 — 4-8). The inner 

 zone is lenticular, broadest at the middle, and scarcely reaching the 

 outer ends of the pavement. Its glands are larger and not very 

 tightly packed. The middle zone, entirely free of cuticles, presents a 

 soft yielding surface into which the door edge can sink somewhat 

 under pressure. 



