354 VEGETABLE PHYSIOLOGY 



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 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. 



