4 i8 APPENDIX 



irritability diminishes, but finds that the same applies to the heliotropic irritability, 

 and concludes that the general decrease of irritability is due to the direct action 

 of the high temperature. 



He also finds that shaking favours geotropic responses as found by Haberlandt, 

 but does not appreciably affect heliotropic reactions. This affords no proof of the 

 starch statolith theory, since not only will all other dense particles be affected, but 

 also intermittent variations of hydrostatic pressure in the cell will be produced by 

 every up and down movement. Jost found that roots subjected to centrifugal forces 

 of 0-02 to 0-05 g. showed geotropic curvatures before any appreciable unilateral 

 accumulation of starch had taken place, and found movable starch in tertiary non- 

 geotropic roots. Darwin and Pertz (Proc. Royal Soc., 1904, Vol. LXXIII, p. 477) have 

 shown, however, that these roots become geotropic when the others are removed, 

 and they were unable to find any geotropic response without a movement of the 

 starch-grains, probably because less sensitive plants and longer exposures were 

 used. 



Nemec (Beih. z. bot. Centralbl., 1904, Bd. xvn,p. 45) states that after the removal 

 of the starch-bearing columella of Lupinus roots, a geotropic curvature takes twenty 

 hours, and by this time movable starch-grains have reappeared. 



The geotropic flowers of Clivia nobilis have motile starch-grains, which are 

 absent from the non-geotropic flowers of Clivia miniata. Many non-geotropic 

 organs have motile starch-grains, however. 



Fitting (Jahrb. f. wiss. Bot., 1905, p. 331) has shown that the collection and 

 movement of the starch-grains are of no importance in geotropic perception, for the 

 response may be as rapid when the starch-grains do not move as when they do, 

 independently of whether they are regularly or irregularly distributed. 



In a strong magnetic field the diamagnetic starch-grains would tend to be 

 repelled from the neighbourhood of either pole-piece, and the paramagnetic con- 

 stituents of the protoplasm to be attracted. Hence if the starch statolith theory were 

 correct, roots of Pisum placed horizontally above and below one pole of an 

 extremely powerful electromagnet should show stronger and more rapid curvatures 

 in the lower than in the upper series. The reverse is, however, the case, according 

 to the observations of Bayliss and Ewart, so that the gravitational and magnetic 

 forces appear to directly stimulate the protoplasm. The exposures were for periods 

 of four to six hours in a room heated to 23 C. The possibility of a thermotropic 

 action of the heated magnet upon the radicles was avoided by enclosing them in 

 moist cotton wool, or by interposing layers of wet blotting-paper. Short exposures 

 appear merely to exercise a disturbing action on the roots, and even with 

 prolonged exposures in so intense a field as that used for these experiments, the 

 magnetic action is feeble as compared with that of gravity. Vertically placed roots 

 showed no perceptible tendency to curve towards or away from either pole of the 

 magnet, either during the exposure or when subsequently rotated on a klinostat. 



The Localization of the Geotropic Irritability of the 'Root-tip. Piccard (Jahrb. f. 

 wiss. Bot., 1904, Bd. XL, p. 94) rotated kidney-bean roots 30 to 40 times per second, 

 arranged obliquely to the axis of rotation which passed just behind the sensitive 

 apex (Fig. 70). Hence the apical and growing zones were subjected to opposed 



