TTewcombe, Gravitatiou seiisiviteness not confined to apex of root. 105 
The most of the responding roots bent at an angle of a- 
bont 20®, tho a few were as low as 10®, and one was 40o 
and one 80®. 
Cucurhita pepo. Seedlings of this species show even more 
reinarkable behavior on the centrifuge than those of Vicia faba. 
When 24 seedlings had 2.5 mm remoyed fromthe root-tip, and were 
revolved at 8 g, temperatnre 24®, period 7 honrs, every root was 
found bent ontward; the angles ranged from 10® to 75®, 13 ofthe 
roots showing angles of 45® or over. and only 3 havina- angles 
less than 30®. 
An experinient made with 12 seedlings from whose roots 
3 mm were removed, and the seedlings then revolved at 4 g, 
temperatnre 24®, period 5.5 honrs, showed 10 roots cnrved ont¬ 
ward at angles varying from 15® to 40®, and the other 2 
roots straight. 
ünder the same conditions as the foregoing, except that 
the speed was eqnal to 8 g and the period was 6 honrs, 26 roots 
from which 3.5 mm had been ampntated, showed 23 ontward cnrves 
at angles ranging from 10 ® to 45 ® — only 3 angles were less 
than 30 ® — while the other 3 roots were straisht. Plate III shows 
one set of these seedlings. 
Twelve seedlings with 4 mm of the root-tip removed, revolved 
at 8 g, temperatiu’e 24®, period 6 honrs, gave 9 roots cnrved 
ontward at angles from 15® to40® — 5 being over 20® — and the 
other three roots straioht. 
The foregoing experiments show that the roots of all 7 species 
of plants nsed, except those of Bicinus communis, make bends on 
the centrifnge when not more than 2 mm of the root-tip are removed. 
Three of the seven — Zea mais, Pisnm sativum and Lupinus albus — 
show a good proportion of cnrves when 2.5 mm of the root-tip are re¬ 
moved; and Vicia faba and Cucurhita pepo will cnrve on the centrifnge 
when 4 mm have been ampntated. 
The first qnestion to determine is whether these cniwes are 
plastic. All writers who have followed Darwin’s lead have con- 
clnded that the sensory tissne is confined to the apical one to two 
miUimeters. Since now all of the 7 species nsed in this work, ex¬ 
cept one, show cnrves when 2 mm of the tip are excised, one must 
beheve that the cnrves prodnced in these experiments are plastic, or 
he mnst admit the extension of the sensory tissne thrn the most of 
the elongating zone. 
Since all the devices hitherto nsed 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 fall of their pnrpose, the cinestion of 
plasticity of the root on the centrifnge may be examined with- 
ont bias. 
There are fonr resnlts which have been obtained with 
roots on the centrifnge which teil against the notion of plas^ 
ticity. 
