Chapter XII 



DIONAEA MUSCIPULA AND ALDROVANDA 



VESICULOSA 



Dionaea: general description. — Early discovery. — Original description by Ellis. — 

 Work of Curtis (1834), Oudemans (1859), Caxby (1868), Darwin, Goebel. — Mor- 

 phology (Seed and seedling. Structure of mature leaf: trap. Lobes, glands, sensitive hairs. 

 Internal structure). — Physiology. — Aldrovanda: general description. — Discovery, dis- 

 tribution. — Morphology (Seed. Germination. Mature leaf. Posture of the trap). — 

 Physiology. 



These two monotypic genera are members of the family Droser- 

 aceae, and while the former, Dionaea, is well known, it being widely 

 grown in greenhouses, Aldrovanda is well known chiefly to such bot- 

 anists as have a special interest in these curious plants. Dionaea 

 has a very restricted geographical range, Aldrovanda a very wide 

 one. Though the method of trapping animals is identical, the one 

 is a land plant, and Aldrovanda a submersed water plant. We con- 

 sider these separately. 



Dionaea muscipula Ellis, Venus' fly trap: — This is a small 

 plant (77 — 2), consisting of a rosette of leaves three to six inches 

 across arising from a rootstock growing more or less horizontally. 

 The rootstock is apparent even in the young seedling (Smith 193 i). 

 Long scapes are sent up bearing several flowers in a short cyme with 

 two to fourteen flowers. These are of pentamerous structure, five 

 small elliptical sepals alternate with five white cuneate and somewhat 

 oblique petals, usually fifteen stamens. The leaf consists of two re- 

 gions, a basal "footstalk" as Darwin called it, articulated by means 

 of a short cylindrical portion (midrib) with the blade which is a trap. 

 The footstalk is a more or less expanded leaf-like structure, either 

 broadly obcordate to narrowly obcordate in form, depending on ex- 

 posure to fight and the presence of surrounding vegetation. The 

 upper part of the leaf, a "striking and noteworthy" trap, to quote 

 Goebel, consists of two dished lobes of trapezoidal form. The outer 

 margins are "ciliated," that is, are provided with a row of coarse 

 projections, prongs or teeth. Ellis (1770) spoke of the arrange- 

 ment as "a miniature form of a rat-trap," and Curtis (1834) com- 

 pared it to "two upper eyefids joined at their bases." Springing 

 from the upper surface of the two lobes there are six slender, sensi- 

 tive hairs, three on each side placed in triangular position (in ex- 

 ceptional cases a smaller or larger number has been noted {iS — i). 

 The rest of the surface is covered rather densely with two kinds of 

 sessile glands, most of which under usual circumstances are colored 

 with brilliant red pigment, giving a bright red tinge to the surface. 

 When with suitable temperatures the sensitive hairs are moved, the 

 two lobes swing swiftly on their common axis, and the finger-like 

 cilia intercross to form a barred cage. Darwin interpreted this ini- 

 tial posture as an arrangement to allow small insects, relatively value- 



I'-. 



