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. 
