432 PHYSIOLOGY 
Such curvatures are called in general tropic and the phenomena tropisms. 
To these terms is often prefixed a word indicating the stimulus which 
calls forth the tropism, as geotropism (ge, the earth = gravity), photo- 
tropism (photos, light), etc. (see p. 458). When a curvature evoked by 
either a uniform or a one-sided stimulus is restricted to a single plane by 
the bifacial structure of the organ, the curvatures are called nastic, and 
the phenomena nasties. This term is also applied to like curvatures 
due to unknown (" internal " or " inherent ") causes. Thus we have 
epinasty and hyponasty, photonasty, photepinasty, etc. (see further, 
p. 442). In the organisms capable of locomotion, a one-sided stimulus 
may determine the direction of creeping or swimming. These phenom- 
ena are taxic, collectively taxies, and individually chemotaxy, phototaxy, 
geotaxy, etc., according to the stimulus (see p. 446). 
Energy relations. Not only is the mode of reaction independent of 
the kind of stimulus, but its energy is disproportionate to the amount of 
energy expended in excitation. The stimulus, therefore, cannot be the 
sole cause of the reaction, though the two stand related to each other 
apparently as cause and effect. On the unexpected pricking of the finger, 
little energy is expended; the sudden jerking away of the hand involves 
many times as much. Somewhere this energy must have been released 
and applied; and this is one reaction of the series, whose final one was 
movement. So in the plant, stimulation often involves a mere fraction 
of the energy expended in the final movement; it is released, presumably 
from the protoplasm or some part of it that is particularly unstable, and 
is applied to the work. If this be so, the chemical changes (metabolism) 
ought to be different in a stimulated and unstimulated organ. 
This hypothesis, however, has not yet been verified experimentally. Reinvesti- 
gation of the one case in which such a result was reported has produced a conflict 
of evidence. 
Another hypothesis, that stimulation results in molecular strain only, from 
which there is gradual recovery, sufficiently accounts for fatigue (see next para- 
graph), but does not account for the disparity in energy between stimulus and re- 
action, the existence of which its advocates merely ignore or deny. 
Fatigue, tetanus, and summation. After an organ is stimulated once 
and the response occurs, the original state is presently regained, and the 
organ is ready to respond again as at first (fig. 671). If several stimuli 
follow, each before complete recovery, the responses are of less extent 
than before. This effect is described by the term fatigue, and in manv 
cases the responses gradually become smaller and smaller until they 
