Experiment 15.—­A row of drops of a solution of one part of carbonate of ammonia to 875 of water (1 gr. to 2 oz.) was placed on the margin of a leaf.  In 1 hr. there was apparently some slight incurvation, and this was well-marked in 3 hrs. 30 m.  After 24 hrs. the margin was almost completely re-expanded.

Experiment 16.—­A row of large drops of a solution of one part of phosphate of ammonia to 4375 of water (1 gr. to 10 oz.) was placed along the margin of a leaf.  No effect was produced, and after 8 hrs. fresh drops were added along the same margin without the least effect.  We know that a solution of this [page 376] strength acts powerfully on Drosera, and it is just possible that the solution was too strong.  I regret that I did not try a weaker solution.

Experiment 17.—­As the pressure from bits of glass causes incurvation, I scratched the margins of two leaves for some minutes with 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 this 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 margins of the leaves curl inwards when excited by the mere 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 week 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 not 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 [page 377] there was a trace of movement in 1 hr. or 1 hr. 30 m.  The pressure from fragments of glass excites movement 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