Newcombe, Gravitation sensiviteness not confined to apex of root, 105 



The most of the responding roots bent at an angle of a- 

 bout 20°, tho a few were as low as 10 °, and one was 40 

 and one 80°. 



Cucurbita pepo. Seedlings of this species show even more 

 remarkable bebavior on the centrifuge than those of Vicia faba. 

 When 24 seedlings had 2.5 mm removed fromthe root-tip, and were 

 revolved at 8 g. temperature 24 °, period 7 hours, every root was 

 fonnd bent ontward; the angles ranged from 10° to 75°, 13 of the 

 roots showing angles of 45° or over, and only 3 having angles 

 less than 30°. 



An experiment made with 12 seedlings from whose roots 

 3 mm were removed, and the seedlings then revolved at 4 g, 

 temperature 24°, period 5.5 hours, showed 10 roots curved out- 

 ward at angles varying from 15° to 40°. and the other 2 

 roots straight. 



Under the same conditions as the foregoing, except that 

 the speed was eqnal to 8 g and the period was 6 hours, 26 roots 

 from which 3.5 mm had been amputated, showed 23 outward curves 

 at angles ranging from 10 ° to 45 ° — only 3 angles were less 

 than 30 ° — while the other 3 roots were straight, Plate III shows 

 one set of these seedlings. 



Twelve seedlings with 4 mm of the root-tip removed, revolved 

 at 8 g, temperature 24°, period 6 hours, gave 9 roots curved 

 outward at angles from 15 ° to 40 ° — 5 being over 20 ° — and the 

 other three roots straight. 



The foregoing experiments show that the roots of all 7 species 

 of plants used, except those of Ricinus communis, make bends on 

 the centrifuge when not more than 2 mm of the root-tip are removed. 

 Three of the seven — Zea mais, Pisum sativum and Lupinus albus — 

 show a good proportion of curves when 2.5 mm of the root-tip are re- 

 moved; and Vicia faba and Cucurbita pepo will curve on the centrifuge 

 when 4 mm have been amputated. 



The first question to determine is whether these curves are 

 plastic. All writers who have followed Darwin's lead have con- 

 cluded that the sensory tissue is confined to the apical one to two 

 millimeters. Since now all of the 7 species used in this work, ex- 

 cept one, show curves when 2 mm of the tip are excised, one must 

 believe that the curves produced in these experiments are plastic, or 

 he must admit the extension of the sensory tissue thru the most of 

 the elongating zone. 



Since all the devices hitherto used to demostrate the re- 

 striction of geotropic sensitiveness to the apical one or two 

 millimeters of the root have, in the first part of this pa- 

 per, been shown to fail of their purpose, the question of 

 plasticity of the root on the centrifuge may be examined with- 

 out bias. 



There are four results which have been obtained with 

 roots on the centrifuge which teil against the notion of plas- 

 ticity. 



