Fig. 12. Response of the main pulvinus of Mimosa pudica.
The most striking and familiar example of response is afforded by the main pulvinus of Mimosa pudica of which a record is given in Fig. 12. It is generally assumed that sensibility is confined to the lower half of the organ. It will be shown in a subsequent Paper that this is not the case. The upper half of the pulvinus is also sensitive though in a feeble degree, its excitability being about 80 times less than that of the lower half. On diffuse stimulation the predominant contraction of the lower half causes the fall of the leaf, the antagonistic reaction of the upper half being, in practice, negligible. In order to avoid unnecessary repetition, I shall ignore the feeble antagonistic reaction of the less excitable half of the organ, and shall use the word ‘contraction’ for ‘relatively greater contraction.’
It is interesting in this connection to refer to the response of the leaf of Water Mimosa (Neptunia oleracea). Here the reaction is very sluggish in comparison with that of Mimosa pudica. A tabular statement of contractile response of various radial, anisotropic and pulvinated organs will show a continuity in the contractile reaction; the difference exhibited is a question of degree and not of kind.
TABLE 1—PERIODS OF MAXIMUM CONTRACTION AND OF RECOVERY OF DIFFERENT PLANTS.
| Specimen | Period of maximum contraction | Period of recovery. |
| Radial organ: Tendril of Passiflora | 100 seconds | 4 minutes. |
| Anisotropic organ: Hooked tendril of Passiflora | 120 " | 13 " |
| Pulvinated organ: Pulvinus of Neptunia Oleracea | 180 " | 57 " |
| Pulvinus of Mimosa pudica | 3 " | 16 " |
As regards the excitatory fall of the leaf of Mimosa pudica, Pfeffer and Haberlandt are of opinion that this is due to the sudden diminution of turgor in the excited lower half of the pulvinus. The weight of the leaf, no longer supported by the distended lower cells, causes it to fall. This is accentuated by the expansion of the upper half of the pulvinus which is normally in a state of compression. According to this view the excitatory fall of the leaf is a passive, rather than an active, movement. I have, however, found that in determining the rapidity of the fall of Mimosa leaf the factors of expansive force of the upper half of the pulvinus and the weight of the leaf are negligible compared to the active force of contraction exerted by the lower half of the pulvinus ([p. 87]).
With regard to the fall of turgor, it is not definitely known whether excitation causes a sudden diminution in the osmotic strength of the cell-sap or an increase in the permeability of the ectoplast to the osmotic constituents of the cell. Pfeffer favours the former view, while others support the theory of variation of permeability.[F]