IRRITABILITY TO CONTACT AND TO MECHANICAL SHOCKS 71 



oscillations during movement is partly the result of the nature of the 

 motor-mechanism, and is partly due to the induction of opposed reactions 

 by the realized movement. The extent of the maximal movement in 

 Mimosa is determined by the diminution of the energy of contraction 

 and the increase of the mechanical resistance as the curvature progresses. 

 Similarly, in tendrils under sub-maximal stimulation, the curvature ceases 

 as soon as the stimulation is balanced by the counter-actions, although 

 an additional curvature is possible when the stimulus is increased. 



Few detailed observations upon the progress of movement have been made. 

 Bert l found, for instance, that the end of the primary petiole of Mimosa had sunk 

 22 mm. 7 seconds after stimulation, but that on the commencement of the return 

 movement it rose 4 mm. in the first minute, 4-5 mm. in the second, 3 mm. in each of 

 the third, fourth, and fifth minutes, 2 mm. in the sixth, i mm. in the eighth, and 

 0-5 mm. in the ninth minute. 



Uses. The importance of the movements of tendrils for purposes of attachment 

 does not need to be emphasized. The movements of stamens and stigmas induced 

 by seismonic stimuli are usually for the purpose of ensuring the transference or 

 reception of pollen, while in carnivorous plants the responses to seismonic, chemical 

 and contact-stimuli are especially connected with the capture and digestion of insects. 

 The extremely readily induced movements of Mimosa pudica and similar plants pro- 

 bably aid in keeping off large browsing animals such as goats and camels, and may 

 also be of use in warding off the attacks of injurious insects. One can often see how 

 goats, after the first tug at a bush of Mimosa, seek less bewildering pasturage, and 

 how a surprised fly hastens from a leaf on which his descent has excited a move- 

 ment 2 . The folded leaflets and drooping leaves of Mimosa pudica are less readily 

 injured by rain and hail, while the re-expansion on continued stimulation aids in 

 avoiding a prolonged derangement of the functional activity of the leaf. 



It is uncertain whether the slow response of the leaflets of Oxalis to mechanical 

 stimuli has any biological utility, for the leaves are not more readily injured by 

 mechanical agencies than other non-irritable ones. 



SECTION 16. Movements produced by Mechanical Stimuli. 



The mechanism of movement has been studied most in the cases 

 of the stamens of Cynareae and the pulvini of Mimosa, and as far as we 

 know similar mechanisms are employed in other motile organs which 



1 Bert, Mem. de 1'Acad. de Bordeaux, 1870, T. vn, p. 41. A similar progress was observed 

 by Cohn (Abhdlg. d. schles. Ges. f. vaterl. Cultur, 1861, Heft I, p. 13) in the stamens of Cynareae, 

 and by Burdon- Sanderson (Proc. of the Royal Society, 1877, Vol< xxv P 4*6; Pnil< Tr ans., 1882, 

 p. 48 of the reprint) in the leaf of Dionaea muscipula. 



3 See Johow, Kosmos, 1884, Bd. II, p. 124; G. Haberlandt, Tropenreise, 1893, p. 36; Ewart, 

 Annals of Botany, 1897, Vol. xi, p. 339 (Protective movements of leaflets); Burgerstein, Wiener 

 illustrirte Gartenzeitung, Marz 1898. 



