Francis E. Lloyd —74— Carnivorous Plants 



and placing an insect (a gnat) on an area of the wall which had been 

 dried. A slight amount of secretion then occurs which is insufficient to 

 act and readtly dries up. But if an insect wet with pitcher fluid is used, 

 an ample secretion from the gland ensues and the insect is digested in 

 the time indicated above. It would appear according to Fenner's in- 

 terpretation that the pitcher fluid acts as a stimulant to secretion. In 

 this way the body of a smaU insect comes into contact with a more 

 vigorous secretion. The greater activity, therefore, is not within the 

 depths of the pitcher fluid but in the films of fluid by which the bodies 

 of the insect adhere to the glands. Into this position they come nat- 

 urally enough since they float towards the walls, and the fluid level, 

 by shaking (as by the wind), is moved so that insects stick on the 



walls above it. 



The collection of Nepenthes accumulated at the University of 

 Pennsylvania by Professor Macfarlane, furnished an abundant amount 

 of material for the study of proteolysis by Hepburn (191 9), who car- 

 ried out his experiments with unopened pitchers, and opened pitchers 

 from which insects were excluded by means of cotton wool plugs. Some 

 of these were stimulated by the introduction of glass beads after shak- 

 ing. A distinction between "stimulated" and "unstimulated" pitchers 

 became evident: Their fluid was found to differ in its activity. Bac- 

 teria were carefully excluded by means of active bactericides, and all 

 experiments were controlled. With various substrates (ovalbumin, 

 fibrin, ovomucoid, Heyden's nutrient and Witte peptone) and by 

 means of formol titration (Sorensen) he found that the fluid from 

 stimulated pitchers digested all of them; but not that of unstimulated. 

 In the presence of very dilute HCl edestin was also acted upon by 

 fluid of stimulated but not by that of unstimulated pitchers. Carmine 

 fibrin in the presence of acid was digested by both, but not by that of 

 unstimulated pitchers in the absence of acid. Protean (from the globu- 

 lin of the seed of castor bean, Ricinus communis) and ricin were 

 attacked by the fluid of both stimulated and unstimulated pitchers if 

 in the presence of very dilute acid. With sufficiently long exposure, 

 glycyltryptophane was "apparently" hydrolysed by the fluid of 

 stimulated pitchers. It appeared that the fluid of stimulated pitchers 

 possessed proteolytic power in the absence of acid (as weU as with 

 acid) while that of unstimulated pitchers always required the ad- 

 dition of acid. It is not clear how the stimulation acts: whether by a 

 change of acidity creating a favorable medium for an enzyme already 

 present, or by the activation of a zymogen already present or by an 

 increase in the secretion of the protease of the glands. 



In 1932 Stern and Stern reopened the question. They chose a 

 series of substrates (gelatin, casein, edestin, ovalbumin and serum 

 protein), and tested the effect of the pitcher secretion on them through- 

 out the whole physiological range of pH, and found that they obtained 

 two maxima, one at pH 4.7 and 7.0 for gelatin, pH 3 and 8 for edes- 

 tin, pH 4 and 8 for ovalbumin. Serum protein was not measurably 

 attacked between pH 1.5 and 8.4. The behavior of casein is anoma- 

 lous. The curve shows two maxima, at pH 3 and 5.5, with a deep dip 

 between, due probably to the flocking of the protein at the isoelectric 

 point and the binding of the enzyme. The tryptic optimum was not 



