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 40/1000 of an inch, so that the two extreme dots were 80/1000 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 1 to 2/1000 of an inch, and the two further dots by 3 to 4/1000 of an inch, than they were before; so that the two extreme [page 318] dots now stood about 5/1000 of an inch (.127 mm.) nearer together than before.  If we suppose the whole upper surface of the lobe, which was 400/1000 of an inch in breadth, to have contracted in the same proportion, the total contraction will have amounted to about 25/1000 or 1/40 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.

Finally, with respect to the movement of the leaves, the wonderful discovery made by Dr. Burdon Sanderson* is now universally known; namely that there exists a normal electrical current in the blade and footstalk; and that when the leaves are irritated, the current is disturbed in the same manner as takes place during the contraction of the muscle of an animal.

The Re-expansion of the Leaves.—­This is effected at an insensibly slow rate, whether or not any object is enclosed.  One lobe can re-expand by itself, as occurred with the torpid leaf of which one lobe alone had closed.  We have also seen in the experiments with cheese and albumen that the two ends of the same lobe can re-expand to a certain extent independently of each other.  But in all ordinary cases both lobes open at the same time.  The re-expansion is not determined by the sensitive filaments; all three filaments on one lobe were cut off close to their bases; and the three

* Proc.  Royal Soc.’ vol. xxi. p. 495; and lecture at the Royal Institution, June 5, 1874, given in ‘Nature,’ 1874, pp. 105 and 127.

  Nuttall, in his ‘Gen. American Plants,’ p. 277 (note), says that,
  whilst collecting this plant in its native home, “I had occasion to
observe that a detached leaf would make repeated efforts towards disclosing itself to the influence of the sun; these attempts consisted in an undulatory motion of the marginal ciliae, accompanied by a partial opening and succeeding collapse of the lamina, which at length terminated in a complete expansion and in the destruction of sensibility.”  I am indebted to Prof.  Oliver for this reference; but I do not understand what took place. [page 319]