Chap. XIIL TRANSMISSION OF MOTOR IMPULSE. 317 



surfaces. It appears therefore that the cells must con- 

 tract. The chief seat of the moyement is evidently 

 in the thick mass of cells which oyerlies the central 

 bundle of vessels in the midrib. To ascertain whether 

 this part contracts, a leaf was fastened on the stage of 

 the microscope in such a manner that the two lobes 

 could not become quite shut, and having made two 

 minute black dots on the midrib, in a transverse line 

 and a little towards one side, they were found by the 

 micrometer to be ^H^ - of an inch apart. One of the 

 filaments was then touched and the lobes closed ; but 

 as they were prevented from meeting, I could still see 

 the two dots, which now were -rkwo of ^^ i^^ch apart, 

 so that a small portion of the upper surface of the 

 midrib had contracted in a transverse line two of an 

 inch (-0508 mm.). 



We know that the lobes, whilst closing, become 

 slightly incurved throughout their whole breadth. 

 This movement appears to be due to the contraction 

 of the superficial layers of cells over the whole upper 

 surface. In order to observe their contraction, a nar- 

 row strip was cut out of one lobe at right angles to 

 the .midrib, so that the surface of the opposite lobe 

 could be seen in this part when the leaf was shut. 

 After the leaf had recovered from the operation and 

 had re-expanded, three minute black dots were made 

 on the surface opposite to the slit or window, in a line 

 at right angles to the midrib. The distance between 

 the dots was found to be two of an inch, so that the 

 two extreme dots were two- of an inch apart. One of 

 the filaments was now touched and the leaf closed. 

 On again measuring the distances between the dots, 

 the two next to the midrib were nearer together by 

 \ o"o of an inch, and the two further dots by -f^ of 

 an inch, than they were before ; so that the two extreme 



