394 The Movements of Plants 



sensitive cotyledon is intact. In view of this result it was impossible 

 to rely on the amputations performed on roots as above described 



At this juncture a new and brilliant method originated in Pfeffer's 

 laboratory 1 . Pfeffer and Czapek showed that it is possible to bend 

 the root of a lupine so that, for instance, the supposed sense-organ at 

 the tip is vertical while the motile region is horizontal. If the motile 

 region is directly sensitive to gravity the root ought to curve down- 

 wards, but this did not occur : on the contrary it continued to grow 

 horizontally. This is precisely what should happen if Darwin's theory 

 is the right one : for if the tip is kept vertical, the sense-organ is in 

 its normal position and receives no stimulus from gravitation, and 

 therefore can obviously transmit none to the region of curvature. 

 Unfortunately this method did not convince the botanical world 

 because some of those who repeated Czapek's experiment failed to 

 get his results. 



Czapek 2 has devised another interesting method which throws 

 light on the problem. He shows that roots, which have been placed 

 in a horizontal position and have therefore been geotropically stimu- 

 lated, can be distinguished by a chemical test from vertical, i.e. un- 

 stimulated roots. The chemical change in the root can be detected 

 before any curvature has occurred and must therefore be a symptom 

 of stimulation, not of movement. It is particularly interesting to 

 find that the change in the root, on which Czapek's test depends, 

 takes place in the tip, i.e. in the region which Darwin held to be the 

 centre for gravitational sensitiveness. 



In 1899 I devised a method 3 by which I sought to prove that the 

 cotyledon of Setaria is not only the organ for light-perception, but 

 also for gravitation. If a seedling is supported horizontally by 

 pushing the apical part (cotyledon) into a horizontal tube, the coty- 

 ledon will, according to my supposition, be stimulated gravitationally 

 and a stimulus will be transmitted to the basal part of the stem 

 (hypocotyl) causing it to bend. But this curvature merely raises 

 the basal end of the seedling, the sensitive cotyledon remains hori- 

 zontal, imprisoned in its tube ; it will therefore be continually 

 stimulated and will continue to transmit influences to the bending 

 region, which should therefore curl up into a helix or corkscrew-like 

 form, and this is precisely what occurred. 



I have referred to this work principally because the same method 

 was applied to roots by Massart 4 and myself 5 with a similar though 



1 See Pfeffer, Annals of Botany, vin. 1894, p. 317, and Czapek, Priugsheiin's Jahrb. 

 xxvii. 1895, p. 243. 



2 Berichte d. Deutsch. bot. Ges. xv. 1897, p. 516, and numerous subsequent papers. 

 English readers should consult Czapek iu the Annals of Botany, xix. 1 1 J05, p. 75. 



3 F. Darwin, Annals of Botany, xm. 1899, p. 567. 



4 Massart, Mtm. Conronnes Acad. R. Belg. LXII. 1902. 



5 F. Darwin, Linnean Soc. Journ. xxxv. 1902, p. 266. 



