28 MOVEMENTS OF CURVATURE 



motile organism diffuses the action of gravity. Similarly the feeble nutation 

 movements of most stems continue on a rotating klinostat \ whereas the 

 pronounced circumnutation of the shoots of twiners 2 , of tendrils 3 , and of 

 the stolons of Mucor stolonifer* cease sooner or later when the action 

 of gravity is eliminated, feeble and irregular nutation movements taking 

 their place. 



Baranetzsky showed that when the stem of a climber is slowly rotated horizontally, 

 the curved growing zone straightens, and its circumnutation ceases. Gravity here is 

 utilized by the plant as a directive stimulus, the power of reaction progressively and 

 periodically altering on the different sides, so that the growing zone bends alternately 

 to all quarters of the compass as it performs its circumnutation. This power of 

 producing an autogenic change of irritability is a special adaptation, for the apices 

 of non-climbers do not exhibit any such pronounced circumnutation when bent from 

 the perpendicular 5 , but usually show slight pendulum movements, owing to variations 

 in the growth of opposite sides. If the plant is very slowly revolved horizontally on 

 a klinostat, the growing apex circumnutates as the result of the curvatures produced 

 by gravity, and light will act in exactly the same way. This, however, results from 

 the orthotropism of the plant which causes the repeated elimination of the pro- 

 gressively changing geotropic or heliotropic curvature. 



It is not certain whether autogenic changes of irritability take place when the 

 plant is rotated on a klinostat, or whether the geotropic curvature of the apex is 

 essential for their initiation. A fact worthy of note in this connexion is that a shoot 

 of Cuscuta not only ceases to circumnutate when rotated horizontally on a klinostat, 

 but also loses its irritability to contact 6 . In many cases also the direction of circum- 

 nutation may change periodically, while a large number of plants only show circumnuta- 

 tion under special conditions, as for instance when the stems of Tropaeolum majus or 

 Polygonum Fagopyrum are etiolated 7 . It is, however, uncertain whether this is due 

 to the greater power of response of the thin-walled cells of the etiolated tissues, or 

 whether an inherent tendency to circumnutation has been excited or allowed to 

 become manifest. 



The apices of twining stems primarily curve out of the vertical as the result of 

 their klinogeotropism, whereas when longer their own weight produces a certain 



1 Fritzsche, Ueber d. Beeinflussung d. Circnmnutation durch verschiedene Factoren, 1899, p. 1 6. 



2 Baranetzsky, Die kreisformige Nutation u. d. Winden d. Stengel, 1883, p. 24; Ambronn, 

 Mechanik d. Windens, 1884, Th. i, p. 6; Wortmann, Bot. Ztg., 1886, p. 314. Cf. II, 84. 



3 Wortmann, Bot. Ztg., 1887, pp. 86, 97. Darwin (Climbing Plants, 1875, p. 131) observed 

 that the circumnutation of a tendril of Echinocystis lobata almost ceased when it was bent down- 

 wards, but recommenced when it was placed in a horizontal position. 



* Fritzsche, I.e., p. 21. 



5 Id., I.e., p. 20. Baranetzsky (I.e., p. 14) states that the previously erect stem of a twiner 

 begins to circumnutate when bent out of the perpendicular. 



6 Peirce, Annals of Botany, 1894, Vol. vm, pp. 86, 116. Ordinary tendrils remain irritable to 

 contact after prolonged rotation on a klinostat. 



7 Noll, Bot. Ztg., 1885, p. 664. Cf. also M. Scholtz, Cohn's Beitrage z. Biologic, 1892, Bd. v, 

 P- 393- 



