510 MANUAL OF BOTANY 



in support of the existence of rhythm. Though they are ordi- 

 narily exhibited in response to the stimulus of alternate periods 

 of light and darkness, many plants continue to exhibit them 

 when altogether removed from light. That they cease after a 

 day or two of darkness, is probably due to the fact that the 

 plants are then no longer in a phototonio condition. Many 

 flowers continue to open and close their corollas when removed 

 from variable to constant conditions. 



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 be 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 column of mercury has reached what we may call 

 its mean or average height, it does not remain there, but begins 

 to oscillate. It rises in the morning till about midday, then 

 sinks somewhat, rises again towards the evening, and falls 

 during the night. There is thus roughly a daily variation of 

 the absorbent activity of the roots, which is very little affected 

 by changes in the environment ; it is in fact automatic. 



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

 diameter of the various organs diminishing from night till 

 some time in the afternoon, and increasing thenceforward till 

 dawn. These variations largely depend upon the distribution 

 of the water which the plant contains, which is regulated by 

 the living substance. 



Many other instances of the same periodicity or rhythm 

 might be quoted, but it is hardly necessary to multiply examples. 

 Though this rhythmic alteration of the protoplasm in 

 various ways is no doubt inherent in plants, as appears from 

 its occurrence when conditions are kept constant, it is easily 

 affected by external causes. The effect of continuous darkness, 

 we have already seen, is that the movements are made irregular 

 and ultimately stop. In many cases the difference in degree of 

 illumination between day and night affects the readiness with 

 which the nyctitropic movements of the leaves are brought 

 about. After brilliant sunshine they set in more quickly than 

 after a dull light. 



These movements may indeed show an artificially induced 

 rhythm, superposed upon a normal one, in the same way as 

 the movements of heliotropism, geotropism, &c., have been 

 seen to be based upon the ordinary movement of oircumnuta- 

 tion, to be, indeed, an exaggeration of it. In some eases the 



