RHYTHMS IN PLANT LIFE 



EVERY one recognises that, in a general way, Summer 

 is a season of great activity among plants, and 

 Winter a season of rest. This is one of the broad 

 expressions of the fact that all life is rhythmic. Professor 

 Pfeffer, one of the highest authorities on the physiology 

 of plants, has summed up the situation in one of 

 his terse paragraphs : l " All life is rhythmic in character, 

 each life-cycle being a repetition of a preceding one, and 

 during the progress of the grand period of each in- 

 dividual, various periodic movements occur in growing 

 and adult organs. Further, all metabolism consists of 

 rhythmically recurrent processes of anabolism and kata- 

 bolism. In addition to this autogenic rhythm, regularly 

 repeated external factors may induce a secondary or aitio- 

 genic rhythm, and the phenomena observed in nature are the 

 result of the co-operation of these two forms of rhythm." 



Some of the clearest examples of periodicity are found in 

 the alternations of vegetative and reproductive periods, and 

 it has been shown in a variety of cases, especially among 

 Algae and Fungi, that reproduction is induced by appro- 

 priate changes in the external conditions. It can be hurried 

 on, or kept back, or suppressed for prolonged periods. The 

 beautiful experiments of Klebs on Algae, such as Vaucheria, 

 Spirogyra, and Hydrodictyon, show that the external con- 

 ditions punctuate the life-processes, determining whether 



1 The Physiology of Plants, vol. ii. Trans, by Ewart, Oxford, 1903, 

 p. 197- 



