296 CONCLUDING REMARKS [Chap. XV. 



impulse to the exterior tentacles much more quickly than 

 when they are mechanically irritated; whilst with Dionsca 

 the absorption of nitrogenous matter causes the lobes to 

 press together with extreme slowness, whilst a touch excites 

 rapid movement. Somewhat analogous cases may be ob- 

 served, as I have shown in another work, with the tendrils 

 of various plants; some being most excited by contact with 

 fine fibres, others by contact with bristles, others with a flat 

 or a creviced surface. The sensitive organs of Drosera and 

 Dionffia are also specialised, so as not to be uselessly affected 

 by the weight or impact of drops of rain, or by blasts of air. 

 This may be accounted for by supposing that these plants 

 and their progenitors have grown accustomed to the repeated 

 action of rain and wind, so that no molecular change is thus 

 induced; whilst they have been rendered more sensitive by 

 means of natural selection to the rarer impact or pressure 

 of solid bodies. Although the absorption by the glands 

 of Drosera of various fluids excites movement, there is a 

 great difference in the action of allied fluids; for instance, 

 between certain vegetable acids, and between citrate and 

 phosphate of ammonia. The specialised nature and perfec- 

 tion of the sensitiveness in these two plants is all the more 

 astonishing as no one supposes that they possess nerves; 

 and by testing Drosera with several substances which act 

 I>owerfully on the nervous system of animals, it does not 

 appear that they include any diffused matter analogous to 

 nerve-tissue. 



Although the cells of Drosera and Dionsea are quite as 

 sensitive to certain stimulants as are the tissues which sur- 

 round the terminations of the nerves in the higher animals, 

 yet these plants are inferior even to animals low down in the 

 scale, in not being affected except by stimulants in contact 

 with their sensitive parts. They would, however, probably 

 be affected by radiant heat; for warm water excites ener- 

 getic movement. When a gland of Drosera, or one of the 

 filaments of Dioniea, is excited, the motor impulse radiates 

 in all directions, and is not, as in the case of animals, di- 

 rected towards special points or organs. This holds good 

 even in the case of Drosera when some exciting substance has 

 been placed at two points on the disc, and when the tentacles 

 all round are inflected with marvellous precision towards the 



