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BOTANICAL GAZETTE 



plants show a rhythmic alternation of periods of rest and activity 

 of the growth processes. Schimper states: "Internal causes are 

 mainly or solely responsible for the alternation of rest and activity 

 in a nearly uniform climate. Such a rhythmic change, however, is 

 never abandoned, for it arises from the nature of the living organism 

 and not from external conditions; its connection with external con- 

 ditions is a secondary feature, an adaptation." 



From the time of Schimper's observations, tropical vegetation 

 has been favorite material for study in determining the relation of 

 external conditions to rest periods. Klebs (16) has made observa- 

 tions on the tropical vegetation of Java, where climatic conditions 

 show little variation. Exact measurements of twigs were made to 

 determine the amount of growth over a series of months. He also 

 cultivated a number of European and Japanese plants in Java. 

 Native plants of Java were likewise transplanted to Heidelberg and 

 grown in a greenhouse under constant temperature and moisture 

 conditions. These studies, which constitute his most recent 

 research on this problem, led him to deny firmly a periodicity of 

 growth which is independent of external factors, and to formulate 

 the following hypothesis to account for this rest period in tropical 

 trees : When the growth ability is weakened by a deficiency of one 

 or more of the essential external growth factors, carbon assimilation 

 proceeds at first at the normal rate, resulting in a storage of organic 

 materials. These in turn render certain enzymes inactive and thus 

 cause a rest period. All means available for shortening the rest 

 period simply activate these enzymes. Great stress is laid upon 

 salts as the causal agent in the rest periods of tropical plants, since 

 the other three factors are constantly favorable for growth in 

 Java. 



Volkens (28, 29) made more extended observations on the tropi- 

 cal vegetation in its natural habitat. Trees were marked and 

 observed for an entire year. Many new and interesting facts were 

 added to our stock of knowledge regarding rest and activity in 

 tropical plants. Although his observations were made in the same 

 place as those of Klebs and often on the same object, he reached 

 a very different conclusion. He is unable to see any relation 

 between the rest periods in tropical plants and external conditions; 



