466 PHYSIOLOGY 
grains from excitable areas, tumbling them again upon the side corre- 
sponding to the position of equilibrium. 
These mobile grains are called statoliths and the cells containing them statocysts, 
after the analogy of the otocysts of the Crustacea, once thought to be organs of hear- 
ing, but now recognized as organs of equilibrium. The semicircular canals of the 
ear of vertebrates, with their fluid and mineral granules, have a similar function, 
giving the animal a sense of position or equilibrium. 
Extensive anatomical studies have shown a remarkable parallelism between the 
presence of such grains and geotropic sensitiveness. Almost without exception, 
geotropic organs have mobile starch grains, while non-geotropic organs lack them. 
Moreover, when an organ, placed under unfavorable conditions (e.g. low tem- 
perature), has lost its mobile starch, it seems at the same time to have lost its geo- 
tropism, which is regained simultaneously with the rebuilding of the starch grains 
and not until then, although conditions favorable for response (had perception been 
possible) may have existed for much longer than- the usual reaction time. This 
method of experimentation is, indeed, open to some objections ; but the most serious 
one, namely, that the unfavorable temperature which determines the removal of 
the starch at the same time suppresses the geotropic irritability, is largely obviated 
by the fact that perception and the end reaction can be separately interfered with. 
Thus, by a temporary reduction of temperature, perception is not interfered with, 
for upon again raising the temperature with no further stimulation the end reaction 
proceeds as usual. Further, it is executed more promptly after the restoration of 
favorable temperature than it is when the low temperature is first used to eliminate 
the starch, and then at a favorable temperature stimulation is attempted. This in- 
dicates that the failure to obtain the curvature when there is no mobile starch is 
due to an interference with the mechanism of perception rather than with the 
mechanism of transmission or of growth. 
Plagiotropic organs. The erect position of certain organs is not 
necessarily determined by gravity alone, but may be due to the coopera- 
tive action of other stimuli. In like manner the oblique or horizontal 
position may be determined wholly by a response to gravity, or by some 
other single stimulus, or by simultaneously acting stimuli. Experiment 
alone can determine the agents in each case. Among plagiotfopic or- 
gans which owe their position to gravity, some rhizomes that run hori- 
zontally beneath the surface of the ground are noteworthy. When such 
a rootstock is displaced by directing the tip obliquely upward or down- 
ward, curvatures ensue, precisely as in the case of parallelotropic roots, 
though, of course, the growth is much slower. This mode of reaction is 
known as transverse geotropism or diageotropism, corresponding to the 
positive and negative geotropism of parallelotropic organs. Quite 
similar behavior is to be seen in some peduncles, which are pendulous 
while the flower is in bud, but become in bloom horizontal, and in fruit 
