Chapter XIV — 243 — The Utricularia Trap 



the limits here imposed it will be practically impossible to show in 

 detail the contributions of the several investigators to our knowl- 

 edge in this field, and it must suffice to indicate critical observa- 

 tions. It may as well be said that the study of the anatomy of the 

 trap is by no means easy, if we desire to have exact knowledge of 

 the emplacement of the various parts. This is because on cutting 

 the trap, the tissue tensions are disturbed and the parts (especially 

 the door) disarranged; and it is necessary to know the exact rela- 

 tion between the valves (door and velum) and the threshold. This 

 cannot be finally determined by the study of the traps which have 

 been cut, though useful evidence can be got this way, but only by 

 the examination of the entire, healthy organ. An accurate descrip- 

 tion must be based on living turgid material, and errors have been 

 made by placing faith on paraffin sections. Again, the presence of 

 mucilage makes the trap slippery, and the knife, which must be very 

 keen, readily slips, so that to make a true sagittal section is diffi- 

 cult and this has led to mistakes. The much used figure first pub- 

 lished by GoEBEL in his 1891 paper is wrong for this reason, and the 

 figure used in a recent (German) edition of the Bonn textbook is 

 equally wrong. 



Accounts dealing with our knowledge of the anatomy of the trap 

 (U. vulgaris and closely allied forms) are those of Benjamin (1848), 

 CoHN (1875), Darwin (1875), Hovelacque (1888), Dean (1890), 



GOEBEL (1891), MeIERHOFER (1902), LUETZELBURG (1910), EkAM- 

 BARAM (1916), FRANfA (1922), MeRL (1922), CzAJA (1922), WlTHY- 



COMBE (1924), Lloyd (1929), Kruck (1931) and Nold (1934)- Dur- 

 ing the prevalence of the earlier view that the role of the trap was 

 wholly passive, the results of investigation fell far short of adequacy 

 in the presentation of details of structure later found to be im- 

 portant. This period ended with Luetzelburg in 1910. With 

 Brocher's discovery in 191 1 attention was concerned more and more 

 with these details, though not always with sufficiently critical ob- 

 servation, and sometimes with the entire lack of it. This appHes 

 particularly to the entrance structures, more so to the door, of which 

 the special features began to be appreciated only with Withycombe 

 and Merl. 



The general features of the trap have already been described. 

 Broadly speaking two regions are to be considered alone and in re- 

 lation with each other, the walls and the entrance mechanism. The 

 appendages (antennae etc.) are of less importance and will be de- 

 scribed in a comparative study of the various types of traps. 



The walls. — ■ In the species before us the walls are composed of 

 two courses of cells, the outer and inner, both clothed with a thin 

 cuticle on their exposed surfaces. In general the outer course cells 

 are smaller in surface extent than the inner, in the ratio of about 

 three to two, linear dimensions. The relative thickness of the two 

 courses varies. Along the profile of the trap, the inner cells are the 

 deeper, but this relation is reversed on the sides of the trap, where 

 the outer cells are deeper. This is connected with the movement 

 of the walls from convex to concave, the outer cells suffering increas- 

 ing compression during the excretion of water. Nor is the total 



