356 VEGETABLE PHYSIOLOGY 



or bundle which passes from the stele of the stem through 

 the petiole is somewhat reduced, giving greater flexibility to 

 the stalk at that point. The cells upon the upper side of the 

 pulvinus in some cases play only a passive part in the 

 phenomenon, the rhythmic variations only affecting those 

 already described ; in other cases both sides may show the 

 changes of turgidity. 



The same tendency to rhythmic change is shown in what 

 is called the periodicity of the various vital functions. If, 

 for instance, the root-pressure of a plant is examined by the 

 aid of the apparatus already described, in which the water 

 taken up is made to support a column of mercury in a 

 manometer, when the mercury has reached what we may 

 call its mean or average height, it does not remain steady at 

 that point, but begins to oscillate. It rises in the morning 

 till about midday, then sinks somewhat, rises again during 

 the evening, and falls during the night. There is thus 

 a daily variation of the absorptive activity of the roots 

 which is scarcely affected by changes in the environment. 

 It is an instance of an automatic rhythm. 



There is a similar daily variation in the bulk of- a plant, 

 the diameter of its various organs diminishing from night 

 till some time in the afternoon, and increasing thenceforward 

 till dawn. These variations largely depend upon the dis- 

 tribution of the water which the plant contains, which is 

 regulated by the living substance in the way already 

 described. This rhythm is under ordinary circumstances 

 very much affected by variations in transpiration, which 

 we have seen is a process that is very soon modified by 

 variations in illumination and temperature. 



It is difficult to explain the occurrence of these various 

 manifestations of rhythmic change in the protoplasm. Many 

 of them suggest that they are the result of the influence of 

 the alternations of light and darkness, and perhaps of the 

 changes of the seasons, to which plants are exposed. But 

 others are exhibited so regularly under constant conditions 

 of the environment that they cannot be thus explained. 



