206 DROSERA ROTUNDIFOLIA. [Cuar. X. 
tentacles tends to travel laterally and towards the centre of 
the leaf, but not centrifugally, is by no means clear. 
Mechanism of the Movements, and Nature of the Motor 
Impulse.—W hatever may be the means of movement, the 
exterior tentacles, considering their delicacy, are inflected 
with much force. <A bristle, held so that a length of 1 inch 
projected from a handle, yielded when I tried to lift with it 
an inflected tentacle, which was somewhat thinner than the 
bristle. The amount or extent, also, of the movement is 
great. Fully expanded tentacles in becoming inflected 
sweep through an angle of 180°; and if they are beforehand 
reflexed, as often occurs, the angle is considerably greater. 
It is probably the superficial cells at the bending place 
which chiefly or exclusively contract; for the interior cells 
have very delicate walls, and are so few in number that 
they could hardly cause a tentacle to bend with precision 
toa definite point. Though I carefully looked, I could never 
detect any wrinkling of the surface at the bending place, 
even in the case of a tentacle abnormally curved into a com- 
plete circle, under circumstances hereafter to be mentioned. 
All the cells are not acted on, though the motor impulse 
passes through them. When the gland of one of the long 
exterior tentacles is excited, the upper cells are not in the 
least affected ; about half-way down there is a slight bending, 
but the chief movement is confined to a short space near the 
base; and no vart of the inner tentacle bends except the 
basal portion. With respect to the blade of the leaf, the 
motor impulse may be transmitted through many cells, from 
the centre to the circumference, without their being in the 
least affected, or they may be strongly acted on and the 
blade greatly inflected. In the latter case the movement 
seems to depend partly on the strength of the stimulus, and 
partly on its nature, as when leaves are immersed in certain 
fluids. 
The power of movement which various plants possess, 
when irritated, has been attributed by high authorities to 
the rapid passage of fluid out of certain cells, which, from 
their previous state of tension immediately contract.* 
Whether or not this is the primary cause of such movements, 
fluid must pass out of closed cells when they contract or are 
" * Sachs, ‘ Traitéde Bot.’ 3rd edit. 1874, p. 1038. This view was, I believe, 
first suggested by Lamarck. 
