CHAPTER IX 



PERIODICITY OF GROWTH 



SECTION 57. General. 



ALL life is rhythmic in character, each life-cycle being a repetition of 

 a preceding one, and during the progress of the grand period 1 of each 

 individual various periodic movements occur in growing and adult organs. 

 Further, all metabolism consists of rhythmically recurrent processes of 

 anabolism and katabolism. In addition to this autogenic rhythm, regularly 

 repeated external factors may induce a secondary or aitiogenic rhythm, 

 and the phenomena observed in nature are the result of the co-operation of 

 these two forms of rhythm. 



Climatic conditions, as well as all external influences due to the 

 organism's own activity 2 , come under the category of external factors. An 

 instance is afforded when a fungus or bacterium, by consuming all the 

 available food or by producing certain metabolic products, ultimately 

 restricts its own development An autogenic reaction, however, lies before 

 us when some internal change in the organism itself causes a constant 

 external factor to act as a stimulus. 



A periodic change in any external factor may not only rhythmically acce- 

 lerate or retard any vital activity, but may also induce formative changes, as 

 in the case of amphibious plants and of those plants which do not flower in 

 darkness. A periodic movement of the leaves of Mimosa pudica is at once 

 produced by stimulation at regular intervals, and in other plants temporary 

 periods of illumination may produce heliotropic curvatures which, during 

 the intervals of darkness, are again neutralized by the action of gravity or 

 by the plant itself. It is also comprehensible that different plants, and 

 different functions on the same plant, should exhibit dissimilar responses to 

 the same agency. Thus darkness accelerates growth, causes photosynthesis 

 to cease, and may, according to circumstances, either leave streaming un- 

 affected or bring about its cessation. Lastly, many algae and fungi are only 

 able to complete their life-cycles when stimulated thereto by changes 

 in the external conditions 3 . 



1 On the prolongation of life cf. Kerner, Pflanzenleben, I. Aufl., Bd. II, p. 448; Hildebrand, 

 Bot. Jahrb. f. System., &c., 1882, Bd. II, pp. 63, 91, 116. 



8 As also when hyphae or shoots grow from one medium to another and so change their external 

 conditions. 



3 Cf. Klebs, Biol. Centralbl., 1899, Bd. xix, p. 209; Jahrb. f. wiss. Bot., 1900, Bd. xxxv, 

 p. 80; Ber. d. Bot. Ges., Generalvers., 1900, p. 201. 



