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Show 376 PINGUICULA VULGARIS. CHAP. XVI. strength acts powerfully on Drosera, and it i~ just possible that the solution was too strong. I regret that I d1d not try a woakm· solution. Exr1eriment 17.-As the pressure from l•its of glass causes incurvation I scratched the margins of two leaves for some minutes wit'h a blunt needle, but no effect was produced. The surface of a leaf beneath a drop of a strong infusion of raw meat was also rubbed for 10. m. with the end of a bristle so as to imitate the struggles of a captured insect; but thi~ part of the margin did not bend sooner than the other parts with undisturbed drops of the infusion. We learn from the foregoing experiments that the n1argins of the leaves curl inwards when excited by the 1nere pressure of objects not yielding any soluble matter, by objects yielding such matter, and by some fluids-namely an infusion of raw meat and a weak solution of carbonate of ammonia. A stronger solution of two grains of this salt to an ounce of water, though exciting copious secretion, paralyses the leaf. Drops of water and of a solution of sugar or gum did n.ot cause any movement. Scratching the surface of the leaf for some minutes produced no effect. Therefore, as far as we at present know, only two causes- namely slight continued pressure and the absorption of nitrogenous matter-excite movement. It is only the margins of the leaf which bend, for the apex never curves towards the base. The pedicels of the glandular hairs have no power of movement. I observed on several occasions that the surface of the leaf became slightly concave where bits of meat or large flies had long lain, but this may have been due to injury from over-stimulation. The shortest time in which plainly marked movement was observed was 2 hrs. 17 m., and this occurred when either nitrogenous substances or fluids were placed on the leaves; but I believe that in some cases CHAP. XVI. MOVEMENTS OF THE LEAVES. 377 there was a trace of movement in 1 hr. or 1 hr. 30 m. The pressure from fragments of glass excites mov - ment almost as quickly as the absorption of nitrogenous matter, but the degree of incurvation thus caused is much less. After a leaf has become well incurved and has again expanded, it will not soon answer to a fresh stimulus. The margin was affected longitudinally, upwards or downwards, for a distance of ·13 of an inch (3·302 mm.) from an excited point, but for a distance of ·46 of an inch between two excited points, and transversely for a distance of ·2 of an inch (5·08 mm.). The n1otor impulse is not accompanied, as in the case of Drosera, by any influence causing increased secretion ; for when a single gland was strongly stimulated and secreted copiously, the surrounding glands were not in the least affected. The incurvation of the margin is independent of increased secretion, for fragments of glass cause little or no secretion, and yet excite movement; whereas a strong solution of carbonate of ammonia quickly excites copious secretion, but no movement. One of the most curious facts with respect to the movement of the leaves is the short ti1ne during which they remain incurved, although the exciting object is left on them. In the majority of cases there was wellmarked re-expansion within 24 hrs. from the time when even large pieces of 1neat, &c., were placed on the leaves, and in all cases within 48 hrs. In one instance the margin of a leaf remained for 32 hrs. closely inflected round thin :fibres of meat; in another instance, when a bit. of sponge, soaked in a strong infusion of raw meat, had been applied to a leaf, the margin began to unfold in 35 hrs. Fragments of glass keep the margin in curved for a shorter time than . do nitrogenous bodies; for in the former case there was |
