376 PINGUICULA VULGARIS. CHAP. XV I 



strength acts powerfully on Drosera, and it is just possible that 

 tho 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 weak 

 solution of carbonate of ammonia. A stronger solu- 

 tion 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 nitro- 

 genous 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 glan* 

 dular hairs have no power of movement. I observed 

 on several occasions that the surface of the leaf be- 

 came 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 move- 

 ment 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 



