66 MOVEMENTS OF CURVATURE 



apparently be perceived in the internal living cells as well as in the 

 peripheral ones, and possibly many plants may exist in which the epidermal 

 cells are quite insensitive to seismonic stimulation. Even in the case 

 of tendrils it is uncertain whether the subjacent cortex is able to perceive 

 contact-stimuli as well as the epidermis, or whether the latter only has this 

 special form of irritability. A contact-stimulus may easily be localized in 

 the epidermal cells as regards its application, but a blow or shaking almost 

 unavoidably affects the cortical cells as well as the epidermis. In every 

 case the change of pressure must be rapid even though transitory, for 

 statical pressure as well as gradual changes of pressure or tension are 

 inoperative as stimuli. In this respect seismonic irritability agrees with 

 contact-irritability, which requires for its excitation special pressure relation- 

 ships. Hence it is hardly surprising that tendrils should not be stimulated 

 by the strongest bending or twisting, so long as the latter fail to produce 

 the localized pressure-gradients required for excitation. 



Both seismonic and contact irritability may, like geotropism and helio- 

 tropism, be developed in the same organ, and this may possibly be the case 



in the leaf of Dionaea mnscipula. 

 The flaccidity and the transitory 

 disturbances of growth produced by 

 mechanical agencies may be re- 

 garded as the result of seismonic 

 stimulation, and in this sense this 

 r special form of irritability is pos- 



FIG. 25. Epidermal cells from a longitudinal section of 



the tendril of Cucumis sativtts, showing the pits in the SCSSed to 3. limited deTee by all 

 outer walls. ' 



growing organs including tendrils. 



It is difficult to decide whether Mimosa pudica has a feeble contact- 

 irritability, since every mechanical agency of any intensity excites the usual 

 seismonic response. 



Mechanical agencies probably awaken more or less feeble reactions 

 in all plants, and it has already been mentioned that in addition to the 

 special seismonic irritability, other forms of sensitivity to mechanical 

 agencies may be developed. Indeed, all stimulation resulting from 

 movements of water, or from other forms of movement in the plant, 

 may be termed mechanical, while if geotropic irritability is awakened 

 by the changes in the position of the denser particles of the cell it becomes 

 closely related in character to a form of internal contact-irritability. The 

 manner in which currents of water exercise a rheotropic stimulus is quite 

 uncertain, but it also is probably akin to a form of contact-stimulation. 



as a stimulus to the leaf of Dionaea mnscipula, [The streaming-cells of Chara and Kitclla possess 

 very pronounced seismonic irritability, although here the response is not a movement but a cessation 

 of movement. Less pronounced seismonic irritability is shown by streaming-cells in general. Cf. 

 Ewart, Protoplasmic Streaming in Plants, 1903, p. 72.] 



