RHYTHM 353 
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 coenocytic 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 alternations 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 inter- 
mittent 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 dark- 
ness do not exhibit it. 
23 
