STRUCTURE OF PITCHERS 273 



of nectar. It can find no foothold, and slips down to the 

 glandular zone or into the fluid, The mode of action 

 of this " sHpping " zone is remarkable, and has recently- 

 been experimentally studied by Knoll (1914a). He used 

 wingless ants, which possess curved claws with which they 

 walk on rough surfaces, and adhesive cushions with which 

 they can climb even on a perfectly smooth glass plate. The 

 claws are useless on the smooth surface ; even the projecting 

 cells, as they point downwards, afford no grip. With the 

 adhesive papillae the ants can walk properly on a smooth 

 wax surface, but the wax on the Nepenthes cells forms a 

 coating of minute scales ; these adhere to the papillae and 

 slip off the surface. The papillae covered with slippery 

 wax scales are useless, unless the ant has an opportunity 

 of cleaning them, which, on the vertical wall of the pitcher, 

 it has not. The trap is deadly. 



As has been mentioned, pitchers are formed in some 

 species on leaves which have not functioned as tendrils. 

 This tendency reaches its climax in those cases where they 

 are produced on the surface of the ground or almost buried 

 in the soil, on leaves which are otherwise scarcely developed. 

 Such pitchers are efficient traps for small animals creeping 

 about the soil surface — worms, leeches, centipedes, and so on. 



The liquid in the pitcher, before it has received any 

 prey, is neutral in reaction and contains no digestive enzymes. 

 The presence of an insect, or of fibrin or albumen artificially 

 introduced, stimulates the glands to an energetic secretion 

 of acid — perhaps formic acid — and of proteolytic enzymes. 

 Digestion of fragments of fibrin or albumen takes place 

 within an hour. It seems likely that the enzyme is peptic, 

 breaking down the protein to peptone which is quickly 

 absorbed ; Vines (1905) found that peptones were broken 

 down to amino acids by the action of erepsin (cp. Hepburn, 

 1919). The liquid appears to have antiseptic properties, 

 perhaps in consequence of its acidity, which prevent the 

 development of the bacteria of decay. 



Of great interest is the fact that in these pitchers, the 

 function of which is the digestion of organic food, there is 



T 



