Newcombe, Gravitation sensitiveness not confined to apex of root. 109 



diminishes rapidly posteriorly. where its effect is overcome by 

 autotropism; or (2) the gravitation-sensitiveness is more equal 

 thruout the elongating zone, but autotropism is stronger in 

 the posterior part of the elongating zone than in the ante- 

 rior part. 



The behavior of Ricinus communis, both when the seed- 

 lings are laid horizontally at rest, and when those with beheaded 

 roots are revolved on the centrifuge, is different from that of 

 other species. The roots of Ricinus laid horizontally have never, 

 in my experiments, shown a straightening behind the declining 

 tip, but all have niade a sharp geotropic cm-ve downward, and 

 continued in that direction; and on the centrifuge, with as little 

 as 1.5 mm of the tip amputated, they have shown no curves when 

 revolved at 8 g. 



This behaviour may be accounted for by any of three 

 hypotheses: 1) The geotropic sensitiveness of the root may 

 be confined to the apical one and one-half millimeters, and 

 autotropism may be weak; 2) geotropic sensitiveness may ex- 

 tend thru the elongating zone, but be much stronger in the 

 apical one and one-half millimeters, and autotropism be weak; 

 3) geotropic sensitiveness may be more evenly distributed thru 

 the elongating zone, and the roots may be highly sensitive to 

 wounding, by which beheaded roots on the centrifuge are made 

 non-responsive. 



Nothing comes out more clearly in these experiments than 

 the fact that roots, growing under the normal Stimulus of gravitation, 

 aswell as beheaded roots on the centrifuge, behave very differently; 

 and this difference manifests itself, not only between the different 

 species, but also between the individuals of the same species. To 

 substantiate this statement, I need recall merely the frequent 

 failure of roots of Lupmus albus, Phaseolus midtiflorus, and Cucur- 

 bita pepo to bend more than 45 ° downward af ter lying horizontally 

 in a damp-chamber for 24 hours, and the difference in the curving 

 on the centrifuge between such species as Phaseolus midtiflorus and 

 Cucurbita pepo. 



What these differences in behavior depend upon is difficult 

 to say. Wiesner related the reponse on the centrifuge to the 

 length of the elongating zone, saying the longer the elongating 

 zone, the greater the length of tip possible of renioval without 

 destroying the geotropic sensitiveness. My experiments do not 

 confirni this view. Phaseolus midtiflorus and Cucurbita pepo, 

 for instance, have the same length of elongating zone, usually 

 7 mm, but occasionally 9 mm. Yet when 2.5 mm were am- 

 putated from the root-tips and both species were revolved on 

 the centrifuge at 8 g, Phaseolus formed curves in only one-- 

 fourth of its roots, while Cucurbita formed curves in every 

 one of its roots. A similar comparison of Ricinus communis 

 and Pisum sativum shows elongating zones of about the same 

 length, that of Ricinus sometimes appearing one millimeter 

 longer. But the roots of Pisum on the centrifuge after remo- 



