CHAP XV. ON TIIE DROSERACE^E. 365 



The specialised nature of the sensitiveness possessed 

 by Drosera and Dionaea, and by certain other plants, 

 well deserves attention. A gland of Drosera may be 

 forcibly hit once, twice, or even thrice, without any 

 effect being produced, whilst the continued pressure 

 of an extremely minute particle excites movement. 

 On the other hand, a particle many times heavier 

 may be gently laid on one of the filaments of 

 Dionoea with no effect; but if touched only once by 

 the slow movement of a delicate hair, the lobes close ; 

 and this difference in the nature of the sensitiveness of 

 these two plants stands in manifest adaptation to their 

 manner of capturing insects. So does the fact, that 

 when the central glands of Drosera absorb nitro- 

 genous matter, they transmit a motor impulse to the 

 exterior tentacles much more quickly than when they 

 are mechanically irritated ; whilst with Dionaea the 

 absorption of nitrogeneous matter causes the lobes 

 to press together with extreme slowness, whilst a 

 touch excites rapid movement. Somewhat analogous 

 cases may be observed, 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 Dionaea 

 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 sup- 

 posing 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 move- 



