STIMULATION IN THE LEAVES OF DIONAEA 
69 
ing object is then withdrawn the two lobes approach each other 
rapidly, the leaf returning to its original condition only after several 
hours, usually by the following day. When the closure occurs in this 
way the marginal bristles of the two lobes become interlaced, while 
the lobes themselves bulge out widely from each other with their 
ventral surfaces convex. In the second type of movement, when the 
leaf closes over an insect, the two lobes approach each other in such 
a way that the ventral surfaces often become concave. This brings 
the upper surfaces into closer contact with the insect and leaves a 
smaller opening between the two lobes than is the case with the first 
type of closure. In the present discussion only the first type of move- 
ment will be considered. 
In order to determine what region of the leaf is most active in 
causing the closing movement, both the dorsal and ventral surfaces 
were similarly marked with rows of dots made with India ink and 
running both parallel to and perpendicular to the midrib. The dis- 
tances between the dots having been microscopically measured, the 
leaves were stimulated and the measurements were repeated after 
closing and again after reopening. In order to measure these distances 
on the dorsal surface of one lobe, it was necessary to remove part of 
the opposite lobe, which was accomplished by removing an oblong 
piece with a sharp scalpel, several days being allowed for the leaf to 
recover before the beginning of the experiment itself. Such measure- 
ments were made on both young and mature leaves of various sizes, 
but always with similar results. 
In Table I. are given the measurements made on the ventral 
surfaces of five different leaves; the numerals of the experiment 
numbers refer to the leaf and the letters refer to the successive spaces 
between dots, from the midrib outward to the margin. The values 
themselves are merely relative. In the case of No. i the leaf was 
caused to close three successive times and was allowed to reopen 
after each closure. From these measurements it appears at once 
that the distances between the transversely arranged dots on the 
ventral surface increase considerably when the leaf closes and change 
com.paratively little during reopening. 
To study the transverse expansion of the lower leaf surface thus 
indicated as accompanying closure, the percentage of this expansion 
was calculated for each distance, on the basis of the corresponding 
measurement obtained when the leaf was in the original open condi- 
