RHYTHM 361 
minimum turgescence in a radially symmetrical organ may 
lead to a similar nutation. It is not infrequent for the 
rhythmic change in the turgescence to affect only two sides, 
instead of passing regularly round it. The organ, though 
radially symmetrical in structure, will thus behave as a 
bilaterally symmetrical one, its organisation indeed being 
bilaterally symmetrical. Its changes of position will thus 
resemble those of a flattened organ which can only be made 
to oscillate backwards and forwards. 
A similar rhythm can be noticed in the variations of the 
extensibility of the limiting membrane which characterise 
the circumnutation of a ccenocytic hypha. We must sup- 
pose these variations to be due to the protoplasm covering 
the wall, though we cannot explain the mechanism. The 
protoplasm has the power to soften the cell-membrane. 
Rhythmic changes of this kind affect other processes 
than those of circumnutation. We have had occasion to 
notice that the behaviour of a growing organ during its 
grand period shows certain diurnal variations which we 
have called the daily periodicity of growth. Though no 
doubt we have to do here to a certain extent with changes 
in the behaviour of the protoplasm induced by the alter- 
nations of light and darkness, with coincident variations 
in temperature, this daily periodicity of the rhythm does 
not appear to be altogether dependent upon exposure to 
such alternations, for they persist for a longer or shorter 
time during continuous darkness. Their cessation after 
exposure to a period of darkness need not necessarily 
point to their dependence on the intermittent access of 
light and warmth, for, as we shall see later, prolonged 
deprivation of light leads to a peculiar condition of rigidity 
of the protoplasm which eventually causes its death. The 
cessation of the rhythm indeed appears to be a pathological 
phenomenon. The rhythm of the daily periodicity appears, 
however, to bear a certain relationship to the alternation 
of day and night, for plants which have been cultivated 
from seed in continuous darkness do not exhibit it. 
