1 68 The Sense-Organs of Plants. [Sess. 



appearance at the beginning of the experiment ; fig. 2 repre- 

 sents the seedlings when the curvature has been completed in 

 one case. A somewhat analogous method has been successfully 

 applied by F. Darwin (' Ann. of Bot./ xiii. 567) in demonstrat- 

 ing the localisation of gravi-perception in the cotyledons of 

 seedlings of Setaria and Sorghum. 



The intracellular mechanism of gravi-perception has been 

 described by Haberlandt ('Ber. d. Bot. Gesell./ 1900, p. 18), 

 and formulated in the statolith theory. A group of cells (stato- 

 cytes) in the root apex contains movable starch-grains. When 

 the root is vertical these grains lie on the basal wall of the 

 cell ; on the position of the root being changed to the hori- 

 zontal, the starch-grains fall into a new position on the now 

 lower side of the cell, where they impact the protoplasm, thus 

 producing an internal contact-stimulus which, transmitted to 

 the motor region, initiates the geotropic curvature. 



The following has been found a useful method for demon- 

 strating geotropic phenomena. Mustard-seeds are planted on 

 a flower-pot saucer standing on its edge. The saucer is kept 

 moist, covered with an inverted pot, and placed in the dark. 

 After a few days the seeds have germinated, and appear as 

 represented in Plate XI., fig. 1. The saucer may now be rotated 

 through any angle, so that the roots lie horizontal, oblique, &c. 

 Plate XL, fig. 2, shows what happens when the saucer is 

 turned through 180°, and illustrates the positive geotropism 

 of the root and the apo-geotropism of the shoot. Darwin's 

 decapitation experiment may also be formed with the saucer 

 turned through 90°, so that the roots are horizontal. The 

 results of such an experiment are shown in Plate XI I. , fig. 1. 

 The upper root was decapitated after the saucer was rotated, 

 the lower before : the former has curved, the latter not, — 

 because its sense-organ, the root-tip, was removed before the 

 rotation, so that it was unable to perceive the stimulus when 

 applied by the horizontal position. 



Photo-irritability. — The localisation of photo-irritability was 

 another of Darwin's discoveries {I. c, p. 468 et seq.) The cap- 

 ping experiments by which he proved the tips of seedlings of 

 grasses to be photo-perceptive are familiar. Eothert (Cohn's 

 ' Beit, zur Biol.,' 1896, 7. 3), F. Darwin, and many others have 

 worked in this field, and have amplified and extended our 



