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 



