of the process by which these are brought about. In muscle it is supposed that during the act of contraction there is a transfer and redistribution of fluid material.[1] In the case of Mimosa there is known to be an escape of fluid from the excited cells; there is a diminution of turgor. It is supposed that this may in some unknown way be connected with a diminution of pressure within the cell.[2]
In the case of the stamens of Cynereæ, Pfeffer[3] observed a contraction under excitation of as much as 30 per cent. of the original length. There is an escape of water from the cells into intercellular spaces. The mode in which the fall of turgor takes place is uncertain, and various suppositions have been made to account for it. It has been thought that the escape of fluid is brought about by the elastic cell wall which forces liquid out of the cell, when the protoplasm lining it has become permeable under excitation. There may in addition be an active contraction of protoplasm which might force the liquid out of the cell-vacuole. This latter supposition is regarded by many as improbable, though the observations of Schütt and Benecke indicate that under stimulation the protoplasm of a diatom contracts away from the cell wall. Similar withdrawal of protoplasm
- ↑ 'Schäfer, working on the highly differentiated wing-muscle of the wasp, concludes that each sarcomere contains a darker substance near the centre, divided into two parts by Hensen's disc. At each end of the sarcomere the contents are clear and hyaline. In the act of contraction the clear material flows, according to Schäfer, into tubular pores, in the central dark material.'—Starling: Elements of Human Physiology, 8th edition, p. 91.
- ↑ 'When the pressure in the cell decreases, we naturally assume this to be due to decreasing osmotic pressure, a decrease which may well amount to 212 to 5 atmospheres, and may be due either to the transformation of osmotically active substances into bodies with larger molecules, or to alterations in the permeability of the plasma, and an excretion of materials from the cell. As evidence of excretion of material we may quote the fact that Pfeffer observed crystals of unknown nature appearing on evaporation of the liquid expressed from the intercellular spaces. Still there are several reasons for doubting this conclusion. It is a remarkable fact that plasmolytic research affords no evidence of any decrease in osmotic pressure."— Jost: Plant Physiology, English edition, 1907, p. 515.
- ↑ Cf. Pfeffer: Physiology of Plants, vol. iii., English edition, p. 75.
