1883-84.] EdinburgJi Naticralists' Field Club. 157 



thus entrapped by a leaf, the peptic glands on the upper surface of 

 the lobes pour out an acid secretion immediately the animal matter 

 comes in contact with them. Moist nitrogenous substances, when 

 placed on the glands of a leaf, even although the sensitive hairs are 

 not touched, not only cause the glands to secrete, but the lobes slowly 

 close. 



PiNGUICULA. 



The species of this genus are distributed principally over the north- 

 temperate regions, and are mostly inhabitants of moist mountainous 

 places. In P. vulgaris (Common Butterwort, — so called from its 

 power of coagulating milk) the leaves are oblong, sessile, of a pale- 

 green colour, and form a spreading rosette, from the centre of which 

 the flower-stalks spring. Their margins are slightly incurved, and 

 their upper surfaces are thickly beset with stalked and sessile glands : 

 these secrete a viscid, colourless fluid, which can be drawn out into 

 long threads. When the glands are excited by the pressure of an 

 object, the margins curve inwards ; but drops of water, or mere irrita- 

 tion of the surface without continuous pressure, produce no move- 

 ment. Darwin found that the pressure of fragments of glass produced 

 incurvation as soon as nitrogenous matter, but in a less degree. A 

 motor impulse does not appear to be conveyed to the other glands 

 when any individual one is irritated ; for although the excited gland 

 may secrete copiously, the others remain passive. Darwin found that 

 although fragments of glass produced incurvation, they caused little 

 or no secretion ; but when a solution of carbonate of ammonia was 

 applied, there was increase of the secretion, but no movement. It 

 would thus appear that the secretion and movement take place 

 independently of each other. The shortest time in which Darwin 

 observed plainly marked incurvation was 2 h. 17 m. ; and the long- 

 est time during which a leaf remained incurved was less than 48 h. 

 In the majority of cases they had re-expanded in 24 h. The use 

 of this incurvation is apparent, as insects are washed into the incurved 

 margins by rain, and are thus, by the rolling in of the margins upon 

 them, brought into contact with a greater number of glands, which 

 are thus induced to secrete more freely. When Darwin placed large 

 pieces of meat on the leaves, he observed that they were not em- 

 braced, but were pushed in by the incurving margins towards the 

 centre of the leaf, in some cases as much as xV of f^n inch. The use 

 of this pushing he conjectured was to bring large insects into contact 

 with as many glands as possible. The incurving of the margins also 

 serves another purpose. When many glands are induced to secrete, 

 the secretion trickles down and is caught in the incurved margins, so 

 that insects are more quickly and completely dissolved there than on 

 any other part of the leaf. Insoluble substances, such as bits of glass, 

 have little or no power of causing secretion from the glands ; but 



