SPECIAL AND GENERAL 147 



their release are prepared by the activity of the organism, either by so 

 raising the strains, loosening the tissues or weakening the cell-walls, that 

 the existent strain, or a slight mechanical excitation, serves to produce 

 the sudden dehiscence. The plant prepares in the same way for the abscission 

 of leaves, flowers, and fruits either by the provision of special abscission 

 layers, in which the cells readily separate, or by the death of intervening 

 tracts of tissue. 



As soon as the required instability has been produced, mechanical 

 agencies of external or of internal origin may release the dehiscing 

 mechanism. Changes of turgor or of the tissue-strains may act in this way, 

 whether produced by transpiration or by some indirect stimulatory reaction 

 of light, heat, or of chemical substances. In some cases a localized stimulus 

 may act at a distance. Thus Darwin 1 has shown that a touch upon the 

 antenna-like prolongation of the rostellum of the orchid Catasetum causes 

 the pollinia to be shot forth by the release of pre-existent strains. Direct 

 contact with the pollinia is ineffective, so that presumably the antenna 

 receives a contact or seismonic stimulus and transmits an excitation to the 

 pollinium, causing the hindrance to movement to be removed. 



Apart from the above movements which take place in plants fully 

 supplied with water, movements and change of shape may be produced 

 by a fall or loss of turgor due to excessive transpiration or plasmolysis. 

 Phenomena of this kind, though physical in origin, are nevertheless of 

 considerable biological importance, as, for instance, when the drooping 

 of flaccid insolated leaves aids in shielding them from an excessive loss of 

 water. From a mechanical standpoint it is naturally immaterial whether 

 the loss of turgor is due to death, transpiration, or plasmolysis. Delicate 

 tissues shrivel when very much water is removed from them, but it is only 

 when all the free water has been displaced that further drying removes the 

 water of imbibition and produces changes of shape in the cell-walls which 

 may lead to hygroscopic movements and curvatures. Movements of this 

 kind take place in dead as well as in living tissues, although turgor can 

 only be restored in cells which have not been killed by drying. 



Movements diie to turgor or to the tissue-strains dependent on turgor. 

 An instance of sudden movement without any tearing of the tissues is 

 afforded by the stamens of Parietaria, Urtica, Pilea, Spinacia, Atriplex 

 and a few other plants. The stamens of Urtica are inwardly curved and 

 fixed between the perianth and the ovary, or when the latter is absent 

 they are pressed against one another. As development progresses strains 

 arise which mainly find expression in the compression of the inner under 

 side of the filament. When this is sufficient to overcome the mechanical 



1 Darwin, The various contrivances by which Orchids are fertilized ; Haberlandt, Sinnesorgane 

 im Pflanzenreich, 1901, p. 62. 



L 2 



