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



vaj of 2 mm from the apex, gave curves in two thirds of their 

 number, while under the same circumstances, the roots of Ricinus 

 gave no curves. 



Summary. 



1) All the attempts thus far made to demonstrate the Limi- 

 tation of gravitation-sensitiveness to the apical two millimeters of 

 the root have failed, for the phenomena accord equally well with 

 the hypothesis of the extension of sensitiveness thru the elongating 

 zone, but diminishing from the apex backward; or the phenomena 

 accord with the hypothesis of a more equable sensitiveness thru 

 the elongating zone, and a stronger autotropism in the posterior 

 than in the anterior part. 



2) Experiments on the centrifuge with beheaded roots show 

 that gravitation sensitiveness is present, in most of the species em- 

 ployed, more than 2.5 mm posterior to thetip, and in some species 

 more than 4 mm distant from the tip. 



3) The kind of geotropic curve which an orthotropic root 

 will make when displaced from its position of equilibrium de- 

 pends upon the relation of its geotropism to its autotropism. 

 In frequent cases, the autotropism of the root prevents the 

 horizontally placed seedling from sending its roots vertically 

 downward. 



4) The length of the elongating zone cannot be seen 

 to have any relation to the extent of the geotropically sen- 

 sitive zone. 



University of Michigan, Ann Arbor, (U. S. A.). 



Nov. 30, 1907. 



Explanation of plate. 



The lower figure is made from a photograph of a preparation of Cucur- 

 bita pepo, 3.5 mm of the root-tips having been removed, and the preparation 

 subsequently revolved on the centrifuge at 8 g for 6 hours. All the roots that 

 have grown show outward curves. 



The upper figure is a similar preparation of Vicia faba. The roots had 

 3.5 mm removed, and the preparation^was subsequently revolved on the 3entri- 

 fuge at 8 g for 6 hours. Here there are 6 pronounced outward curves, one 

 inward, one obliquely outward, and 4 roots nearly straight. 



