Cuar. XV. ON THE DROSERACES. 365 
The specialised nature of the sensitiveness possessed 
by Drosera and Dionza, 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 
Dionza 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 Dionwa 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 Dionza 
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- 
