EFFECT OF AMPUTATION OF UPPER HALF OF PULVINUS. _Experiment 24._--The determination of the _rôle_ played by different parts of the pulvinus in response and recovery is of much theoretical importance. Our knowledge on this subject is unfortunately very scanty. The generally accepted view is that on excitation “the actual downward curvature of the pulvinus is partly due to a contraction of the walls of the motor cells consequent upon the decrease of turgor, but is accentuated by expansion of the insensitive adaxial half of the pulvinus--which was strongly compressed in the unstimulated condition of the organ--and also by the weight of the leaf.”[H] According to Pfeffer, after excitation of the organ, “the original condition of turgor is gradually reproduced in the lower half of the pulvinus, which expands, raising the leaf and producing compression of the upper half of the pulvinus, which aids in the rapid curvature of the stimulated pulvinus.”[I] [H] Haberlandt, ‘Physiological Plant Anatomy,’ 1914, p. 570. English Translation, Macmillan & Co. [I] PFEFFER--‘Physiology of Plants,’ vol. 3, pp. 75 and 76. English Translation, Clarendon Press. It was held, then, that the rapidity of the fall of leaf under stimulus is materially aided (1) by the expansion of the upper half of the pulvinus, which is normally in a state of compression, and (2) by the weight of the leaf. So much for theory. The experimental evidence available regarding the relative importance of the upper and lower halves of the pulvinus is not very conclusive. Lindsay attempted to decide the question by his amputation experiments. He showed that when the upper half was removed the leaf carried out the response, but rigor set in when the lower half was amputated. Pfeffer’s experiments on the subject, however, contradicted the above results. He found that “after the upper half of the pulvinus was carefully removed, no movement was produced by stimulation, whereas when the lower half is absent a weakened power of movement is retained.” Pfeffer, however, adds, “since the operation undoubtedly affects the irritability, it is impossible to determine from such experiments the exact part played by the active contraction of the lower half of the pulvinus.”[I]