Chap. XIII.] TRANSMISSION OF MOTOR IMPULSE. 257 



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 rHrr 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 

 iHv of an inch apart, so that a small portion of the upper 

 surface of the midrib had contracted in a transverse line 

 tAtt 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 narrow 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 

 rJ^ of an inch, so that the two extreme dots were tWs- 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 TiHi^ of an inch, and the two further dots by f wJ of an 

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

 now stood about tAt of an inch (.127 mm.) nearer together 

 than before. If we suppose the whole surface of the lobe, 

 which was tV^ of an inch in breadth, to have contracted in 

 the same proportion, the total contraction will have amount- 

 ed to about -jihi or ^ of an inch (.635 mm.) ; but whether 

 this is sufficient to account for the slight inward curvature 

 of the whole lobe, I am unable to say." 



" [Rntnlln hns discussed the are made on the lower or ex- 



mcchanlRin of closure In Dlonaea ternal snrfnce of the leaf, and 



In his InterpstlnB essay In 'Flora,' the distance between them is 



1877. He n^rees In general with found to Increase when the leaf 



the statements above given, but closes. When the loaf opens the 



as in the case of Drosera, so distance does not perfectly re- 



here he believes that the move- turn to its former dimensions, 



ments are associated with a suinll and thus shows n certain aniount 



amount of actual growth. Marks of permanent growth has taken 



