4 RESEARCHES ON IRRITABILITY OF PLANTS 



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 sup- 

 posed that this may in some unknown way be connected 

 with a diminution of pressure within the cell. 3 



In the case of the stamens of Cynerece, 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 supposi- 

 tions 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 Schiitt and Benecke indicate 

 that under stimulation the protoplasm of a diatom contracts 

 away from the cell wall. Similar withdrawal of protoplasm 



1 ' Schafer, 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 Schafer, 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 2 £ 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 plasmo- 

 lytic research affords no evidence of any decrease in osmotic pressure." — 

 Jost: Plant Physiology, English edition, 1907, p. 515. 



a Cf. Pfeffer : Physiology of Plants, vol. iii., English edition, p. 75. 



