214 PERIODICITY OF GROWTH 



be directly due to the external conditions \ There can, however, be no 

 doubt that it is due to the co-operation of an hereditary tendency to rhythm 

 with the after-effect of the periodic repetition of aitiogenic reactions induced 

 by the external conditions. Buds appear to have an inherent hereditary 

 rhythm, which may, however, be shortened or lengthened by appropriate 

 external conditions. 



Owing to the persistent after-effect it takes some years before a 

 deciduous tree becomes evergreen at Tjibodas in Java 2 , and if the tree 

 were brought back to Europe it would presumably gradually revert to 

 its original periodicity. Similarly a tree transplanted from the Southern 

 hemisphere keeps at first its original periodicity, and often after a time 

 accommodates itself to the altered seasons. 



The origin of the rhythm which persists under constant conditions is 

 quite unknown, and several facts contradict the assumption that it is due 

 to the hereditary fixation of a retardation of growth produced by external 

 conditions recurring regularly for countless generations. Many plants of 

 temperate regions have, for example, no autogenic resting period, and 

 instances have been given of the gradual disappearance under constant 

 conditions of a periodicity induced by an annual climatic rhythm repeated 

 for thousands or millions of years. Many of the plants of Java and of other 

 regions throw off their leaves at stated intervals and then become again 

 clothed with foliage 3 . This rhythm is independent of the external con- 

 ditions, and is carried out at different times by plants growing close together. 

 In this case each tree keeps its own rhythm, whereas in oaks and pears 

 which have become evergreen, each bud has its own individual rhythm and 

 the tree as a whole has none. 



Internal factors. With regard to the internal mode of origin of resting 

 periods, it can only be said that the same end may be attained in a variety 

 of ways, that chemical influences due to the production or removal of 

 metabolic products may be of great importance, and that in general 

 metabolism may in a variety of ways directly or indirectly accelerate or 

 retard growth. 



Owing to internal stimuli, growth may temporarily or permanently cease or 

 be retarded although optimal nutritive conditions are ensured. It is in this way, 

 and not by a deficiency of food, that an autogenic resting period is produced and 

 regulated. That the resting cells have a sufficiency of food is shown not only by 

 their appearance, but also by their power of responding to injuries by callus-formation 

 and increased respiration. The effects produced by low and high temperatures show 

 that buds and rhizomes have the power of forming sugar from starch and starch 

 from sugar during the entire resting period. Miiller-Thurgau and Sachs are hence 



1 Grisebach, Die Vegetation d. Erde, 1872, Bd. I, pp. 273, 279; Askenasy, Bot. Ztg., 1877, 

 p. 840. Cf. Pfeffer, Periodische Bewegungen, 1875, P- 4 2 > footnote. 



2 Schimper, Pflanzengeographie, 1898, p. 266. 3 Id., I.e., p. 264. 



