721 
Centrifugal Force as Geotropic Stimuli . 
a microscope actual response to the gravitational stimulus can be observed 
i min. or even \ min. after the commencement of stimulation, and that 
in reality not only presentation time but also reaction time tend to vanish 
altogether. In that case presentation time comes to mean the least length 
of exposure to stimulus which will produce a bend visible to the naked eye , 
and is then of comparatively little value. 
Bach (’ 07 ) and Pekelharing (’ 09 ) have worked out the relationship 
of presentation time to different angles of displacement and different 
centrifugal forces, and have shown that it varies inversely as the intensity 
of the stimulus acting at right angles to the parallelotropic organ. 
Others have tried to determine the relation of presentation time to 
tonic conditions. Thus Bach (' 07 ) and Rutgers (TO) have worked at 
different temperatures ; and the latter author comes to the conclusion that 
presentation time varies with temperature according to Van’t Hoffs law for 
chemical changes, i. e. for each rise of io° C. in the temperature the presenta- 
tion time is reduced of its length, though this of course can only hold for 
the limits of temperature within which the seedlings can grow comfortably 
(i. e. within which other physiological activities are not wholly or partially 
inhibited). Thus Bach’s and Pekelharing’s results point to a physical basis 
for presentation time, whereas Rutgers’s point to a chemical basis. The 
attempt at reconciliation of the two conceptions provides ground for 
speculation. 
Arpad Paal (’ll) showed that presentation time varied with artificial 
changes in the atmospheric pressure. 
Buder (’ 08 ) tried to accommodate the statolith theory to Bach’s 
figures for presentation time in relation to centrifugal force. His object was 
to show that when the seedlings of Vicia Faba and Ricinus were turned 
over, the movable starch-grains in the endodermis of the hypocotyl took 
just such a time to settle as Bach had worked out to be the presentation 
time. The presentation time might thus be nothing more or less than the 
time taken by the starch-grains to settle on the bottom of the statocyte. 
Also this time would vary inversely as the centrifugal force to which 
the seedlings were subjected. This theory is very ingenious and would 
seem to explain the tables obtained by Bach and Pekelharing for the 
relationship of presentation time to centrifugal forces and angles of dis- 
placement. But according to this theory the presentation time must be 
dependent purely on the specific gravity of the starch-grains and the 
viscosity of the cell-sap. Thus the relation between presentation time and 
temperature should be dependent on the relation between viscosity and 
temperature. But this relationship has been found for water to be 
approximately as follows : 
viscosity coeff. = 
0-017941 
(1 +0-023120 t ) 1,5423 
