12 MOVEMENT 



this view leaves out of consideration the special forms of irritability which the plant 

 has developed for particular purposes. In the case of either a growing plant or 

 a motile zoospore, a curvature or change of direction is due to an external or internal 

 stimulus modifying the previous activity, but in the nodes of grasses when laid 

 horizontal the external stimulus of gravity first awakens growth and then determines 

 its direction. Aitiogenic and autogenic curvatures, although they may co-operate, 

 do not always occur together. Hence a plant showing active circumnutation may 

 only respond to external stimuli by a feeble curvature, while an active power of 

 response may be accompanied by very slight circumnutation. There are, indeed, 

 plants in which aitiogenic movements are carried out in a different manner to 

 autogenic ones. 



SECTION 3. The Mechanism of Movement. 



Amoeboid movement and the locomotion of zoospores are effected in 

 a different way to the growth curvatures resulting from modifications 

 of nutation, and these again are of different origin to the temporary 

 movements resulting from changes of turgidity coupled with the elastic 

 contraction and expansion of the cell-walls. 



All active nutation curvature is the result of unequal growth on the 

 two sides of the cell or curving organ. If the more active growth occurs 

 first on one side and then on the other, the apex will move to and fro more 

 or less regularly, but if the zone of more active growth travels round the 

 growing region, the apex will describe an ascending spiral in space. The 

 latter is especially well shown in the case of climbing plants and these 

 may twine around a support with or without torsion of the stem 1 . 



Most plants only carry out movements of nutation, and in such cases 

 the power of curvature is lost with the cessation of growth, but is regained 

 with the resumption of growth, as in the geotropically stimulated nodes of 

 grasses. The absence of curvature may also be due to the fact that the 

 energy of growth is unable to overcome the mechanical rigidity of the 

 organ affected. The woody stems of Conifers, for instance, may be able 

 to curve as the result of cambial activity up to their second or even third 

 year, but not beyond this 2 . Similarly the curvatures shown when a 

 herbaceous stem is split longitudinally give evidence of tissue-strains, 



of the leaflets of Mimosa and of the tentacles of Drosera could be regarded as modified circum- 

 nutation. Cf. also Wiesner, Bewegungsvermcigen der Pflanzen, 1881, p. 202. 



1 Nageli und Schwendener, Mikroskop, a. Aufl., 1877, p. 416 ; Schwendener und Krabbe, 

 Abhandlg. d. Berl. Akad., 1892, p. 56; Kolkwitz, Ber. d. bot. Ges., 1895, p. 495 ; and the literature 

 quoted in these works. 



3 [Errera (Proc. British Ass., 1904) states that the trunks of tall adult trees may curve geo- 

 tropically at their bases. The curvatures observed were, however, undoubtedly produced when young, 

 for to bend an old stem upwards at its base, the developing wood-elements would have to overcome 

 a mechanical moment representing in them pressures of many hundred or thousand atmospheres.] 



