Cnar. IL] THE EFFECTS OF REPEATED TOUCHES. 31 
after the secretion has been all washed away by heavy rain; 
and this often occurs, though the secretion is so viscid that it 
can be removed with difficulty merely by waving the leaves 
in water. If the falling drops of water are small, they 
adhere to the secretion, the weight of which must be increased 
in a much greater degree, as before remarked, than by the 
addition of minute particles of solid matter; yet the drops 
never cause the tentacles to become inflected. It would 
obviously have been a great evil to the plant (as in the case 
of occasional touches) if the tentacles were excited to bend 
by every shower of rain; but this evil has been avoided by 
the glands either having become through habit insensible to 
the blows and prolonged pressure of drops of water, or to 
their having been originally rendered sensitive solely to the 
contact of solid bodies.* We shall hereafter see that the 
filaments on the leaves of Dionza are likewise insensible to 
the impact of fluids, though exquisitely sensitive to momen- 
tary touches from any solid body. 
When the pedicel of a tentacle is cut off by a sharp pair of 
scissors quite close beneath the gland, the tentacle generally 
becomes inflected. J tried this experiment repeatedly, as I 
was much surprised at the fact, for all other parts of the 
pedicels are insensible to any stimulus. These headless 
tentacles after a time re-expand; but I shall return to this 
subject. On the other hand, I occasionally succeeded in 
crushing a gland between a pair of pincers, but this caused 
no inflection. In this latter case the tentacles seem paralysed, 
as likewise follows from the action of too strong solutions of 
certain salts, and of too great heat, whilst weaker solutions 
of the same salts and a more gentle heat cause movement. 
We shall also see in future chapters that various other fluids, 
some vapours, and oxygen (after the plant has been for some 
time excluded from its action), all induce inflection, and this 
likewise results from an induced galvanic current.t 
* [Pfeffer’s experiments, given move; but that, if similar needles in 
above (p. 22), explain the failure of 
rain to cause movement.—F. D.] 
+ My son Francis, guided by the 
observations of Dr. Burdon Sanderson 
on Dionea, finds that, if two needles 
are inserted into the blade of a leaf 
of Drosera, the tentacles do not 
connection with the secondary coil of 
a Du Bois induction apparatus are 
inserted, the tentacles curve inwards 
in the course of a few minutes. My 
son hopes soon to publish an account 
of his observations. 
