6 4 



MOVEMENTS OF CURVATURE 



By means of rubbing with a wet rod covered with gelatine, and with an 

 ordinary wooden one, it can easily be determined whether an organ shows 

 contact or seismonic irritability, for we are dealing here with as distinct 

 types of irritability as in the geotropism or heliotropism of a root or stem. 

 The distinction would still be justified even if subsequently the two forms 

 of irritability should be found to be closely related, for we are dealing 

 here with collective terms for types of response varying as regards their 

 character and mode of production. Since, however, the distinction is 

 primarily based upon the perception or non-perception of the exciting 

 agency, it is immaterial whether the response is rapid or slow, and whether 

 it takes the form of a curvature, of a secondary thickening, or of a pro- 

 duction of haustoria or other attaching organs. 



Although the details of the mode of perception are still unexplained, 



it is impossible to deny 

 that the sensation of con- 

 tact is produced under 

 similar conditions in both 

 plants and animals l . In 

 both cases a stimulation 

 is only exercised when 

 unequal pressure is ex- 

 erted at different points, 

 so that local variations of 

 pressure are produced. 

 It is not the statical 

 pressure but the rubbing 

 against the solid body 

 which acts as a stimulus, 

 but changing local variations of pressure produced without lateral move- 

 ment may also act as an excitation, as in the hooks of tropical climbers, 

 or when a weighted cork stuck full of pins is allowed to rest upon the 

 skin and its centre of gravity laterally displaced. A tickling sensation 



FlG. 24. Leaves of Dtonaea muscipula, A unstimulated and showing 

 the three sensitive hairs on each leaf-lobe, B stimulated leaf which has 

 closed and captured an earwig. 



1 Cf. Pfeffer, 1. c., p. 499. On the sensation of contact in man cf. Tigerstedt, Physiologie d. 

 Menschen, 1898, Bd. II, p. 71 ; Frey u. Kiesow, Zeit-chr. f. Psychologic und Physiol. d. Sinnes- 

 organe, 1899, Bd. XX, p. 126. In plants direct contact with the cell-wall is necessary, and hence no 

 stimulus is exercised when direct contact is prevented by the interposition of a layer of gelatine or 

 mucilage. Cf. Pfeffer, 1. c., p. 513. 



[The anatomical studies of Haberlandt (Sinnesorgane im Pflanzenreich, 1901, p. 117) have 

 brought nothing essentially new to light. The statement (1. c., p. 122) that only a tangential 

 stretching of the ectoplasmic membrane of the protoplasm is capable of producing an excitation is 

 not supported by the facts. Thus sudden and pronounced curvatures produced by the aid of gelatine- 

 covered rods do not exercise any stimulating action on tendrils, whereas the gentle movement of 

 a thread weighing 0-00025 of a milligram does so and can obviously produce only a minimal amount 

 of tangential stretching. The fact that sharp local inward bending of the outwardly curving epi- 

 dermal walls may produce a stimulatory response has already been pointed out by Pfeffer.] 



