Plants That Feed on Insects. 23 
zigzag line of vessels runs round the whole circumference 
of the leaf, while in the mid-rib all the vessels are in 
close contiguity, so that all parts of the leaf seem to be 
brought into some degree of communication. The presence 
of vessels, however, is not necessary for the transmission of 
the motor impulse, for it is transmitted from the apices of 
the sensitive filaments, which are hardly one-tenth of an 
inch in length, into which no vessels are seen to enter. Slits 
made close to the bases of the filaments, parallel to the mid- 
rib, and thus directly across the course of the vessels, some- 
times on the inner and sometimes on the outer sides of the 
filaments, do not interfere with the transmission of the motor 
impulse along the vessels, and conclusively show that there 
is no necessity for a direct line of communication from the 
filament, which is touched towards the mid-rib and opposite 
lobe, or towards the outer parts of the same lobe. With 
respect to the movement of the leaves, the wonderful discov- 
ery made by Dr. Burdon Sanderson, and published in 1874, 
offers an easy explanation. There is, says this distinguished 
authority, a normal electrical current in the blade and foot- 
stalk, which, when the leaves are irritated, is disturbed in the 
same manner as is the muscle of an animal when contraction 
takes place. 
After contraction has endured for a greater or less time, 
dependent upon circumstances which we do not well under- 
stand, re-expansion of the leaves is effected at an insensibly 
slow rate, whether or not any object is enclosed, both lobes 
opening in all ordinary cases at the same time, although 
each lobe may act to a certain extent independently of the 
other. The re-expansion is not determined by the sensitive 
filaments, for these may be cut off close to their bases, or 
be entirely removed, and re-expansion occur in the usual 
manner. It is believed that the several layers of cells form- 
ing the lower surface of the leaf are always in a state of 
tension, and that it is owing to this mechanical state, aided 
probably by fresh fluid being drawn into the cells, that the 
