The Analysis of Geotropism. 11 
another is added, the motor reaction does not occur. The time 
that an excitation takes to die out thus comes to have some 
importance, and has received the name Relaxation Time. The 
reaction, i.e. in this case, the geotropic curvature also finally dies 
out. That is to say, if we place a plant horizontal for, say ten 
minutes, and then rotate it on the klinostat, the act of curving 
continues for a certain time and then ceases. 
This is a convenient place to define some other technical terms 
which have been found to be necessary in discussions on tropic 
curvature. If a plant is placed horizontal for a short time, and 
is then removed from gravitational stimulus by placing it on a 
klinostat, we may, or may not, find that geotropic curvature follows. 
By repeating the experiment with increasing times of exposure, we 
can find the minimal period of horizontality sufficient to produce a 
reaction. This is known as the presentation time, 1 and has been 
determined by Czapek for the radicle of the lupine as 20 minutes, 
by Fitting for the epicotyl of the bean as 6 or 7 minutes. It is 
important not to confuse the presentation time with another factor 
in the problem. It is a natural mistake to suppose because the 
plant requires say six minutes’ exposure to produce a curvature, 
that this period is necessary for the act of perception. But this is 
wrong; it is quite conceivable that the plant perceives the stimulus 
within a few seconds of being horizontal, but that such a small 
excitation is produced that no reaction follows. We must in fact 
distinguish between the presentation time, and the excitation time, 
i.e., the period necessary for the perception of the stimulus. If 
every brief period during which a plant is not vertical were to 
produce a reaction it would be stimulated to a geotropic curva¬ 
ture by every breath of wind which bends its stem, and it would be 
constantly bending uselessly as Fitting points out. Fitting has, as 
a matter of fact, shown by experiment that plants are stimulated 
by a few seconds’ horizontality, so that it seems likely that the 
existence of presentation time is an adaptation by which the plant 
avoids useless bendings. The plant may be imagined as so attuned 
to circumstance, that it cannot curve unless the loss of verticality 
has lasted so long that it seems likely to be permanent, and 
therefore gives a situation which must be dealt with. One other 
term which we owe to Czapek must also be referred to, viz., the 
reaction time, i.e., the period of permanent horizontality required to 
produce a curvature. For a Lupine radicle the reaction time is 
1 Czapek. 
