478 _ The Botanical Gazette. [December, 
nocturnal positions, since the angles vary with different indi- 
viduals, and with the same individual at different times. All 
that was sought was the general extent and direction of the 
movements. 
I pass over the account of earlier investigations which forms 
the historical introduction to my paper and set forth the 
Present state of knowledge as to the physiology of the 
motor organs. 
Pfeffer? found that the rise of the leaves of Phaseolus, when 
assuming the night position, was brought about by the in- 
crease of the turgidity of the cells of the lower half of the 
pulvinus and a diminution of the turgor of the cells in the 
upper half; the reverse occurs when assuming the day posi- 
tion. From this it is evident that changes of an opposite 
nature take place simultaneously in the cells of the two op- 
posed halves of the pulvinus. 
There are three ways in which the change of the hydro- 
static pressure might occur: (1) by a change in the osmotically 
active substances of the cell sap; (2) by a change in the 
elasticity of the cell-walls; (3) by a change in the resistance 
of the protoplasm to the escape of cell sap. The first two 
are shown by Vines? to be out of the question, and the third 
remains as the explanation of the changes in turgidity. With 
illumination comes the restoration of the protoplasm to its 
original condition, and the absorption of water into the cell. 
The change in the protoplasm being entirely invisible, it 
may be inferred that there occurs arear ent of the molec- 
ular structure or possibly the breaking down of the proto- 
plasmic molecule. In the latter case the restoration of the 
original condition would simply be the building up of the 
protoplasmic molecule. Briefly, the mechanism of the MOVE: 
ment may be stated as variations in the degree of turgidity of 
the opposed halves of the motor organs, in accordance witl 
changes in the molecular structure of the protoplasm. The 
molecular changes are brought about under the influence of 
the alternation of light and darkness. f 
In connection with this subject I must mention the work o 
Haberlandt* on the conduction of ‘‘stimuli” in Mimosa 
* Pfeffer; Die periodischen Bewegungen der Blattorgane 166. 1875. 
* Vines; Physiology of Plants 62-64. 1886. ; 
4 Haberlandt; Das reizleitende Gewebe-system der Sinnpflanze. eee 
/ 
