MOVEMENTS PRODUCED BY MECHANICAL STIMULI 81 



The whole of the inner side of the leaf is irritable, but the three large hairs 

 found on the upper side of each half of the leaf are especially sensitive (cf. Fig. 24, 

 p. 64). This is, according to Munk, simply because, owing to the structural arrange- 

 ments, pressure applied to the hair is transmitted with increased intensity by leverage 

 to the irritable parenchyma cells at the base of the hair. The cells at the tip of 

 the hair appear to be insensitive, since they can be cut away without producing an 

 excitation \ but the cells at the base of the hair appear to become flaccid when the 

 leaf is stimulated. This is of importance in that it aids in the bending of the basal 

 joint of each hair, enabling it to lie flat against the leaf when the latter closes. 



The leaf is not sufficiently sensitive to be excited by the impact of a single rain- 

 drop 2 , but responds to that of a jet of water, and also when the irritable hairs are 

 touched with a moistened gelatine-covered rod 3 . It is not, however, certain whether 

 the leaf possesses contact-irritability in addition to seismonic irritability. 



The mechanism of movement is apparently similar in the leaf of Aldrovanda 

 vesiculosa to that in Dwnaea, and the hairs on the inner surface of the leaf appear to 

 produce an excitation with especial readiness when touched. The leaves of this plant 

 only open when the temperature is fairly high, and very feeble contact is then suffi- 

 cient to excite them *. 



Stamens. All members of the Cynareae appear to possess more or less irritable 

 filaments, and the same is also the case with a few species from the other sub-orders 

 of the Compositae, such as Cichorium intybus and Telekia spectosa. On the other 

 hand, all stamens whose cell- walls are readily extensible are not capable of perceptible 

 irritable movements 5 . 



The mechanism of movement of the stamens of Berberis * and of Mahonia 

 appears to be similar in character to that of the pulvinus of Mimosa. 



The movements of the stamens of other plants 7 which cause them to approach 

 or recede from the stigma appear to indicate a power of response to seismonic 

 stimuli 8 . Apparently, it is owing to the anatomical structure and distribution of the 

 irritable tissues that the stamens of Helianthemum and of other Cistaceae, as well as of 

 Mesembryanthemum, always move in the same direction wherever they may be touched, 



1 Munk, 1. c., p. 103. 



3 Darwin, 1. c., p. 273. 



8 Pfeffer, Unters. a. d. hot. Inst. zu TUbingen, 1885, Bd. I, p. 518. 



* For details see Stein, Bot. Ztg., 1874, p. 389; Cohn, Beitragez. Biol., 1875, I, Heft 3, p. 71 ; 

 Darwin, Insectivorous Plants ; Goebel, Pflanzenbiol. Schilderungen, 1893, Bd. II, p. 70 ; Haberlandt, 

 Physiol. Pflanzenanat., 2. Aufl., 1896, p. 480; Biol. Centralbl., 1901, Bd. XXI, p. 375; Sinnesorgane 

 im Pflanzenreich, 1901, p. 103. 



5 Cf. Pfeffer, Physiol. Unters., 1873, pp. 107, 151. A detailed enumeration is given by Hansgirg, 

 Physiol. u. Phycophytol. Unters., 1893, p. 141 ; Neue Unters. lib. d. Gamo-u. Karpotropismus, 1896, 

 p. 106 (reprint from Sitzungsb. d. bohna. Ges. d. Wiss.). 



6 Pfeffer, Physiol. Unters., 1873, pp. 127, 158. At a later date (Zur Kenntniss d. Plasmahaut u. 

 d. Vacuolen, 1890, p. 326, footnote) Pfeffer showed that intercellular spaces are normally present in 

 the active tissues. A summary of the literature is given by Usteri, Bot. Centralbl., 1900, Bd. LXXXIV, 

 p. 228. According to Haberlandt (I.e. 1901, p. 24), the papillose part of the inner surface of the 

 stamens of Berberis and Mahonia is especially irritable. 



7 Facts and literature are given by Hansgirg, 1. c., 1893 and 1896 ; Beihefte zum bot. Centralbl., 

 1902, Bd. XII, p. 273 ; Haberlandt, 1. c., 1901, pp. 17, 21, 32, 46, 51. 



8 Cf. also Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1885, Bd. I, p. 518. 



PFEFFER. HI Q. 



