MOVEMENTS PRODUCED BY CONTACT-STIMULATION 



to this zone or, if all regions are capable of receiving a stimulus, little or no 

 power of transmitting stimuli is possessed by the protoplasm. The hyphae 

 of this fungus have indeed no perceptible contact-irritability, and the same 

 applies to Mucor mucedo and M. stolonifer, whose sporangiophores behave 

 similarly to those of Phycomyces. On the other hand, the conidiophores of 

 Aspergilhis and Penicillium, as well as pollen-tubes and apparently also root- 

 hairs, seem to be devoid of this form of irritability J , for the partial enclosure 

 of particles of soil by the root-hairs appears to be produced in a purely 

 mechanical manner. 



In all the above-mentioned plants the reaction only takes place at the 

 point stimulated, whereas the leaf-tentacles of various species of Drosera 

 afford good instances of the transmission of stimuli from the receptive to 

 the responding regions 2 . Contact and also chemical stimuli are only 

 perceived by the head of the tentacle, whereas 

 the curvature occurs at the base and median portion 

 of the stalk. When an insect alights on the leaf and 

 adheres to it, both kinds of stimuli co-operate, but 

 similar results are produced when either acts 

 separately. Since, however, the chemical stimuli 

 are more active, a partial recovery from the original 

 curvature occurs more readily during prolonged 

 contact than during the continued application of 

 a chemical stimulus 3 . Thus the presence of a frag- 

 ment of glass on the tentacles is only able to keep 

 them fully curved for a few hours to a day, whereas 

 the body of an insect may cause them to remain 

 curved for one or more weeks, that is until all the 

 soluble proteids have been digested and absorbed 

 so that further chemical stimulation ceases 4 . 



Darwin showed that a curvature was only produced when the head of 

 the tentacle was mechanically or chemically stimulated, and not when the 

 stimuli were directly applied to the stalk or to the lamina of the leaf. 

 Hence when the head of a tentacle is cut off the latter can be excited 

 to a curvature by the transmission of a stimulus from a neighbouring 

 tentacle, but not by direct excitation. The effect of strong chemical and 



FIG. 27. Leaf of Drosera 

 rotundifolia (magnified). The 

 tentacles on the left are curving 

 as the result of stimulation. 



1 Kny, Sitzungsb. d. bot. Vereins v. Brandenburg, 12. Juni, 1881 ; Dietz, Unters. a. d. bot. 

 Inst. zu Tubingen, 1888, Bd. II, p. 482 ; Miyoshi, Flora, 1894, p. 86. 



2 For details see Darwin, Insectivorous Plants, 1875 ; Pfeffer, Unters. a. d. bot. Inst. zu 

 Tiibingen, 1885, Bd. i, p. 511. For anatomical details see Haberlandt, Physiol. Anat., 2. Aufl., 

 1896, p. 397; Rosenberg, Physiol.-Cytol. Unters. iiber Drosera rotundiflora, 1899, p. 42 ; Haber- 

 landt, Sinnesorgane im Pflanzenreich, 1901, p. 94. 



3 Cf. Pfeffer, Unters. a. d. bot. Inst. zu Tiibingen, 1885, Bd. I, p. 514. 



4 Darwin, 1. c., pp. 13, 21, 92 seq., 22, 117; Goebel, Pflanzenbiol. Schilderungen, 1893, Bd. II, 

 p. 203. 



G 2 



