Francis E. Lloyd —108— Carnivorous Plants 



gestion he invariably placed the substrate to be acted on "on the 

 leaf", and I think it is quite evident from the context that Darwin 

 did not think of the margin of the leaf as having a localized digestive 



action. 



Pmguicula was first studied and shown to be carnivorous by Dar- 

 win. "I was led to investigate the habits of this plant by being told 

 by Mr. W. Marshall that on the mountains of Cumberland many 

 insects adhere to the leaves" {Insectivorous Plants, p. 297). He noted 

 the presence of two kinds of glands, sessile and stalked, later studied 

 carefully by Fenner. Having studied Drosera extensively Darwin 

 first looked for and discovered movements of the leaves. In a se- 

 ries of 17 experiments small flies, or portions of larger flies, smaller 

 and larger fragments of meat, meat juice stabilized in small bits of 

 sponge, even fragments of glass were placed in various positions in 

 rows parallel to the margin, near the apex, and along the midrib, and 

 he found curvatures of the leaf margin to occur within periods of a few 

 (2-4) hours, to increase for some hours and finally to disappear. The 

 apex of the leaf never shows motion, this being confined to the margins. 

 He found evidence leading him to believe that the stimulus could be 

 transmitted to a distance of about 6 mm. (his exp. 13). A weak so- 

 lution of ammonium carbonate caused marked incurvation of the leaf 

 margin in 3.5 hrs., a stronger solution (i to 218 H2O) causing no move- 

 ment, probably due to damage. Mechanical irritation of the leaf 

 surface either before or after the apphcation of meat juice, thus im- 

 itating the actions of dying prey, did not hasten or increase the re- 

 sponse. The effect produced by fragments of glass was as rapid as 

 that following the application of nitrogenous substances, but the de- 

 gree of curvature was less. The substances used other than glass in- 

 cited a more or less copious flow of secretion. 



Darwin commented on the brevity of the response action, there 

 being a complete restoration of form within 24 hours. He was thus 

 prompted to doubt the usefulness of the behavior, but ventured the 

 idea that the infolded margin could prevent the washing away of prey, 

 as in fact was observed by a friend of Darwin in Wales. If the prey 

 is large the infolding leaf margin pushed it further toward the middle 

 of the midrib, thus bringing it into contact with more glands, an effect 

 comparable to the action of the tentacles in Drosera. The margins of 

 the leaf are always curved up, and this Darwin thought to help to 

 conserve the fluids from loss, keeping them on the leaf surface to be 

 absorbed. Goebel could not substantiate Darwin's conclusions about 

 the sensitivity of the Pmguicula leaf, his experimental results being 

 mostly negative. On the other hand, Fenner, one of Goebel's 

 students, did find sHght movements on the application of fragments 

 of glass, followed by quick recovery. The secretion of mucilage is 

 thereby excited. When an insect falls on or near the leaf margin, an 

 abundant secretion foflows, overwhelming it. This escape of fluids 

 from the leaf alters the tensions and this results in the inrolhng of 

 the leaf margin which does not occur in older mature leaves. When the 

 insects sink down to the leaf surface and come into contact with the 

 sessile glands (75 — 11), an acid secretion of greater viscosity and con- 

 taining a digestive enzyme escapes from these. Goebel had shown 



