Centrifugal Force as Geotropic Stimuli . 751 
occurs almost immediately after stimulation, though the movement can at 
first only be seen by the aid of a microscope. 
Pekelharing (’ 10 ) subsequently remarks that the amount of bending 
which becomes visible to the naked eye is ‘ ein fester Punkt in dem 
Kriimmungsprozess, der als Indikator einer bestimmten Reaktion benutzt 
werden kann \ Using the term ‘ presentation time’ in this sense, we should 
expect marked inconsistencies in the results of different investigators. Also 
the naked eye test is scarcely as refined a test as might be desired, and 
results will involve the personal equation of the researcher as well as the 
individual peculiarities of the plants. 
Some further compromise between the two points of view is possible. 
For it is noticeable that the latter list of authors always employed plant- 
members in which the perceptive region was also capable of response, 
e. g. hypocotyls of Helianthus and coleoptiles of Avena. Now in Avena 
response occurs first near the apex of the coleoptile, and gradually successive 
basal zones become involved in the bending. Thus the reaction time is 
different for different parts of the coleoptile and hypocotyl ; and Trondle 
finds that the reaction time for any particular zone is proportional to its 
distance from the apex of the coleoptile, so that at the apex the reaction 
time will be theoretically zero. Now in roots the responding portion is not 
also sensitive, at any rate to gravity, though Newcombe (’ 09 ) and Jost (T2) 
have shown that it is sensitive to more powerful stimuli, such as prolonged 
centrifugal forces. Thus for radicles reaction time would appear to be 
necessarily a definite period ; and presentation time would be the least 
period of stimulation, the effects of which do not die out before the 
expiration of the reaction time. 
It is true that Czapek, Bach, Pekelharing, and Rutgers also worked 
considerably with members in which the sensitive portion was also capable 
of response, and what value can be attached to their results will no doubt 
be ascertained by further researches of these investigators. Since, however, 
this paper is only concerned with radicles, it is unnecessary to deal generally 
with the subject of presentation time. 
In determining the presentation time for radicles of Helianthus (see 
above, p. 746) it was found that in each experiment some radicles responded 
markedly, bending at right angles or beyond, while others did not bend at 
all. The presentation time was reckoned as that exposure which caused 
a noticeable bend in half the roots experimented on. 
Thus the individual differences between different roots present another 
source of difficulty. It was found that some roots responded to the 
exposure of 3 min., whilst others showed no inclination to bend after an 
exposure of 10 min. One differs from another in delicacy of response as 
much as an expert athlete differs from the slowest ‘hobbledehoy’. Yet 
reaction and presentation times have been worked out for different angles 
