Newcombe, Gravitation sensitiveness not confined to apex of root. 99 



sition when responding rheotropically to a stream of water, con- 

 tinue on in the same direction for days after the flow of water 

 had ceased, producing a form like Fig. 4, 



It is not unusual to find individual seedlings of Vicia faba 

 and Lupinus albus when suspended horizontally in a damp Cham- 

 ber bending 2 or 3 mm of the tip obliquely downward, but 

 straightening again in the elongating zone so that the course of 

 the root continues horizontal or between the horizontal and the 

 vertically downward position, thus producing a form quite similar 

 to Czapek's preparation as shown in Fig. 3. Czapek obtained 

 the straightening of bent roots by revolving the bent roots on the 

 klinostat. But the cases just cited show that the straightening 

 will often occur when the füll Stimulus of gravitation is applied to 

 a horizontal root at rest. It is evident therefore that the ten- 

 dency of a root to respond geotropically is opposed* s by 

 its own autotropism. In the case of Vicia faba aiid Lupinus 

 albus the autotropism of a horizontally lying root is sometimes 

 able to prevent a complete response to gravitation. 



Instead of ascribing, as Czapek does, the continued horizon- 

 tal growth of a root, in the form shown in Fig. 3, to the absence 

 of geotropic sensitiveness in the part posterior to the 2 mm of 

 the apex, is it not just as reasonable to ascribe the straightening 

 to autotropism combined with a greater sensitiveness to gravitation 

 in the apex than in the straightening part? 



If we assume that the apical 2 mm are much more sensi- 

 tive to gravitation than the part behind, then the root shown in 

 Fig. 1 must bend into the form of Fig. 2; and the seedling shown 

 in Fig. 3 cannot bend its post-apical part downward, for in so 

 doing it would throw its apex out of the vertical, out of the po- 

 sition of equilibrium. 



Thus it is seen that Czapek's experiment with the glass-caps 

 has not and cannot prove the absence of geotropic sensitiveness 

 in the part of the root posterior to the apical one or two milli- 

 meters. 



The argument against the methods and conclusions of Czapek 

 applies with just as much force to those in his second paper 1 ) 

 and to those recently published by F. Darwin 2 ), Massart 3 ), An- 

 drews 4 ) and Cholodnyj 5 ). 



Thus there is presented here an hypothesis which will account 

 for the results obtained by Czapek, and which implies the possession 



*) Über den Nachweis der geotropischen Sensibilität der Wurzelspitze. 

 (Jahrb. wiss. Botanik. XXXV. 1900. 313.) 



2 ) On a method of investigating the gravitational sensitiveness of the 

 root-tip. (Journ. Linn. Soc. XXXV. 1902. 266.) 



3 ) Sur rirritabilite" des plantes superieures. (Mem. couron. par l'Acad. 

 de Belgique. Bruxelles. 1902. — Review in Bot. Zeit. 61. 1903. Abt. II. 23.) 



4 ) A natural proof that the root-tip alone is sensitive to the gravitation 

 Stimulus. (Proc. Indiana Acad. Sei. 1905. 189.) 



5 ) Zur Frage über die Verteilung der geotropischen Sensibilität in der 

 Wurzel. (Schriften des Naturforschervereins in Kiew. 1906. — Review in Bot. 

 Zeit. 65. 1907. Abt. II. 189.) 



7* 



