720 Hiley . — On the Value of Different Degrees of 
similar discrepancies were probably due to unhealthy conditions of growth 
or to unintentional stimulation previous to rotation on the wheel. Wigand 
succeeded in making radicles grow nearly horizontally on a wheel, with 
a vertical axis, rotating at high speed. It may be mentioned here that 
Giltay (’10) undertook to show that radicles did actually take up the 
position of the resultant of centrifugal force and gravity when growing 
on a horizontal centrifugal wheel, for Wigand’s results had never been 
experimentally improved upon. As a result of 368 tests, Giltay found 
them fall 2*1° below the calculated direction of the resultant ; this inaccuracy 
is slight enough to be accounted for by variations in the speed of rotation. 
Sachs (Y 4 ) worked on the effect of centrifugal force on secondary 
roots ; he showed that if the radicle of a bean be amputated below the part 
where secondary roots have been given off, then these secondary roots 
respond positively to the force. 
Elfving (' 80 ) rotated vertically placed radicles on a horizontal wheel so 
that the radicles bent outwards at an angle to the vertical ; and the 
greater the centrifugal force, the greater was this angle of inclination. 
Schwarz (’ 81 ) showed that sporangiophores of Mucor are responsive to 
centrifugal force and bend towards the centre of rotation. 
Czapek (’ 95 ) was the first to attempt to work out the relation between 
the speed or magnitude of response and the magnitude of the force. His 
method was to determine the reaction time (Latenzzeit) of radicles stimu- 
lated by centrifugal forces of different magnitudes. He experimented on 
radicles of Vida Faba and Lupinus albns (which gave results identical with 
each other), subjected to forces varying from 0*0005 mg. to 38 mg. To the 
former stimulus no reaction was visible after eight hours. Reaction to 
o-ooi mg. took place in six hours and the reaction time gradually diminished 
to | hour for 35 to 38 mg. 
From this time forward the literature of Geotropism is dominated 
by the conception of ‘ presentation time i. e. the shortest period of 
exposure to a stimulus that will produce a noticeable after-effect when the 
object is placed after stimulation either in its normal position or on a hori- 
zontal clinostat. This presentation time is a critical time which can 
be determined experimentally for different plant-members under different 
experimental conditions, and can be used as a test for sensitiveness of the 
object worked upon. The conception was first introduced by Czapek (’ 98 ), 
and was further employed by Haberlandt (’ 03 ), Fitting (’ 05 ), Bach (’ 07 ), 
and many more recent authors. The actual length of this presentation 
time has tended to diminish markedly, with more exact investigation, even 
as determined for the same plant-member under similar tonic conditions. 
Thus, for the hypocotyl of Helianthus annuus : Czapek determined it 
at 20 min., Fitting at 5 to 6 min., Bach at less than 3 min. Still more 
recent authors, Polowzow (’ 09 ), Trondle (T 3 ), &c., have declared that with 
