Xewcombe, Gravitation sensitiveness not confined to apex of root. 109 
diminislies rapiclly posteriori};, wliere its effect is overcome by 
autotropism; or (2) the gTavitation-sensitiveness is niore eqnal 
thruout tbe elongating zone, but autotropism is stronger in 
tbe posterior part of tbe elongating zone tban in tbe ante¬ 
rior part. 
Tbe bebavior of Ricinus communis, botb wben tbe seed- 
lings are laid borizontally at rest, and wben tbose witb bebeaded 
roots are revolved on tbe centrifnge, is different from tbat of 
otber species. Tbe roots of Ricinus laid borizontally bave never. 
in my experiments. sbown a straigbtening bebind tbe declining 
tip, but all bave inade a sbarp geotroj)ic cnrve downward, and 
continued in tbat direction; and on tbe centrifnge, witb as little 
as 1.5 mm of tbe tip ampntated, tbey bave sbown no cnrves wben 
revolved at 8 g. 
, Tbis bebavionr may be acconnted for by any of tbree 
bypotbeses: 1) Tbe geotropic sensitiveness of tbe root may 
be confined to tbe apical one and one-balf millimeters, and 
autotropism may be weak; 2) geotropic sensitiveness may ex- 
tend tbru tbe elongating zone, but be mucb stronger in tbe 
apical one and one-baff millimeters, and autotropism be weak; 
3) geotropic sensitiveness may be more evenly distributed tbru 
tbe elongating zone, and tbe roots may be bigbly sensitive to 
wounding, by wbicb bebeaded roots on tbe centrifnge are made 
non-responsive. 
Notbing comes out more clearly in tbese experiments tban 
tbe fact tbat roots, gTowing under tbe normal Stimulus of gravitation, 
as well as bebeaded roots on tbe centrifuge, bebave very diff erently; 
and tbis difference manifests itself, not only between tbe different 
species, but also between tbe individuals of tbe same species. To 
substantiate tbis Statement, I need recall merely tbe frequent 
failure of roots ot Lupimts cdbus, Phaseolus multiflorus, and Cucur¬ 
bita pepo to bend more tban 45 ^ downward after lying borizontally 
in a damp-cbamber for 24 bours, and tbe difference in tbe curving 
on tbe centrifuge between such species as Phaseolus multiflorus and 
C\(curbita pepo. 
Wbat tbese differences in bebavior depend upon is difficult 
to say. Wiesner related tbe reponse on tbe centrifuge to tbe 
lengtb of tbe elongating zone, saying tbe longer tbe elongating 
zone, tbe greater tbe lengtb of tip possible of removal witbout 
destroying tbe geotropic sensitiveness. My experiments do not 
confirm tbis view. Phaseolus multiflorus and Cucurbita pepo, 
for instance, bave tbe same lengtb of elongating zone, usuaUy 
7 mm, but occasionally 9 mm. Yet wben 2.5 mm were am- 
putated from tbe root-tips and botb species were revolved on 
tbe centrifuge at 8 g, Phaseolus formed cnrves in only one- 
fourtb of its roots, wbile Cucurbita formed cnrves in every 
one of its roots. A similar comparison of Ricinus communis 
and Pisum sativum sbows elongating zones of about tbe same 
lengtb, tbat of Ricinus sometimes appearing one millinieter 
longer. But tbe roots of Pisum on tbe centrifuge after remo- 
