Chapter XII — 199 — Dionaea and Aldrovanda 



into more carefully, further details of anatomy will be examined. 

 The thick region, as above said, is composed of three cell layers, the 

 two thin epiderms sandwiching a middle course of large thin-walled 

 cells of cyhndrical form. In these three courses the cells are elon- 

 gated at right angles to the midrib, and have straight walls, excepting 

 that the outer epidermis beyond the motile zone, to be delimited 

 later, has wavy-walled cells. In passing over into the thin region 

 of the lobe, the middle course of cells ceases entirely, so that there 

 remain only the two epiderms juxtaposed {ig — 19). In the inner 

 zone of this region the cells are elongated and have straight walls, 

 but there is a gradual transition to irregularity, when the walls be- 

 come wavy. The thinnest part of the valve is toward the outer 

 edge, where it is reduced to 0.5 mm. and is here only one cell in 

 thickness. This is accounted for by the fact that, as Cohn and 

 Caspary observed, the cells of the two epiderms become mutually in- 

 tercalated, the cells of the inner course protruding between those of the 

 outer course to occupy part of the general outer surface and vice versa 

 {ig — 12, 13, 17). The margin of the thin region is reflexed to form 

 the valve and has a greater thickness, namely about i mm. Along 

 the edge of the valve stands a row of sharp, stiff, unicellular hairs 

 which, when the trap is closed, intercross to prevent any escape 

 of prey between the valves (Cohn) {ig — 18), recalling the analogous 

 arrangement in Dionaea, but in a reverse sense. This the trichomes 

 accomplish more by numbers than by strength, which is indeed not 

 great, as Darwin observed. His doubt on the usefulness of the de- 

 vice is, however, scarcely justified. 



The cells of the two epiderms of the thick regions differ in size. 

 The outer epidermis per unit of measurement is composed of more 

 and therefore more slender cells, than the inner, in the ratio of about 

 7:5. The cells of the middle course are longest near but not next 

 the midrib. The structure of this is seen in ig — 10, 11, 23. 



The inner and outer surfaces of the trap are supplied with a variety 

 of trichomes with various functions. On the outer surface there are 

 squat, two-armed hairs {ig — 16) similar to those found over the 

 general plant surface. Their capital cells are devoid of cuticle, and 

 they secrete mucilage. On the inner face of the lobes are to be found 

 three kinds of hairs. On the surface of the distal zone of the thin 

 portion there are four-armed hairs resembling superficially the "quad- 

 rifid hairs" (so called by Darwin) in the interior of the traps of many 

 species of Utricularia {ig — 15). Aside from having four arms, which 

 lie prostrate against the surface of the leaf, they are otherwise of 

 the same structure as the two-armed hairs, and like them are devoid 

 of cuticle, and secrete mucilage (Goebel). They are distributed in 

 a broad zone lying adjacent to the valve (ig — ^8). The innermore 

 region is devoid of them. On the inner surface of the thicker region 

 of the lobe occur bun-shaped glands which may be regarded as di- 

 gestive and absorptive in function {ig — 14). They arise from two 

 (Fenner) epidermal cells, on which stands a short stalk of four cells 

 which expand into balloon shaped upper ends, clothed with a dozen 

 or more cells to form the capital. They too lie in a zone of much 

 density toward the outer margin of the thick region, and are few 



