106 N"ewcombe, Gravitation sensitiveness not confined to apex of root. 



1) As seen in my own experiments, the secondary roots of 

 Zea mais on the centrifuge, at a speed of 8 g, assume their 

 specific angle, not growing in the direction of the radii, where 

 plasticity would carry them. 



2) In nearly every one of the experiments recorded in this pa- 

 per, some roots have continned to grow straight, while others 

 have curved outward. Why should not all curve, if the curves 

 are merely plastic? 



3) The roots of Ricinus communis, as recorded above, fail to 

 bend on the centrifuge even when as little as 1.5 mm are am- 

 putated. Yet the roots of Ricinus are most likely as plastic as 

 those of Cucurbita or Vicia faba, which curve after 4 mm 

 have been removed. 



4) Unless jWiesner's results can be proved erroneous, he 

 has shown the untenableness of the notion of plastic 

 curves even at a speed of 41 g, by finding uninjured roots 

 remaining unbent when revolved in a Chamber of carbon 

 dioxide. 



The önly evidence in favor of plastic curving is that offered 

 by Brunchorst, when he states that by Alling his revolving Cham- 

 ber with damp sawdust, no curves came on the centrifuge. He 

 himself states that the sawdust was wont to become too dry, and 

 my own experience proves that not only does the sawdust become 

 dry in rapid revolution, but it packs hard against the peripheral 

 walls of the revolving dish, and the water passes mostly to 

 the periphery, 



But after all this has been said, the reader will still 

 feel uncertain of the indirect evidence offered and will look 

 for dir e et evidence. This is given by the following ex- 

 periments. 



The floor of one of the damp Chambers hitherto used was 

 covered with thick sheet cork. Nine seedlings of Vicia faba 

 had 3 mm excised from their tips and were then fastened 

 in the damp Chamber in the usual way. On the radially out- 

 ward side of each root, in contact with the terminal 5 to 

 8 mm of the root, there was secured by pinning to the cork floor 

 a strip of cork 15 mm long. This strip of cork would Support 

 the root, and prevent plastic bending when the centrifuge was 

 revolved. 



The revolution employed was at a speed nqual to 8 g for 

 each root, the temperature was 24.5°, and the period 7.5 

 hours. At the end of the experiment, 7 of the 9 roots were found 

 bent outward at angles ranging from 15 ° to 70 °, The other 2 roots 

 were straight, The 7 roots had curved by lifting their posterior part 

 from the cork bar as the tip pressed against the cork, as illu- 

 strated in Fig. 5. 



At the time of the examination just recorded, the cork bars 

 were removed from the sides of the roots, and the preparation 

 was continued in revolution at the rate of one revolution in 3 minutes, 



