206 PERIODICITY OF GROWTH 



curopea, and Castanca vcxa will not expand before March or April. In 

 these plants the resting phases form a considerable part of the grand period 

 of growth, and this is also the case when they grow in warm houses, where- 

 as the buds of Prunus aviitm and Cornus Mas begin to expand in December 

 or January under such conditions J . In these latter cases it is, however, 

 possible that growth does not entirely cease under favourable conditions 

 when the plants are freely exposed. 



The result depends in nature as well as in experiments not only upon 

 the changes in the external conditions, but also upon alterations in the 

 power of growth. Hence when prolonged periods of cold delay the 

 renewal of vegetative activity, favourable conditions produce a more rapid 

 response than if they were applied earlier. In this way arises the sudden 

 outburst of foliage when a cold spring is succeeded by warm days at the end 

 of April or in May. Similarly foliation and flowering occur earlier after 

 a warm winter and spring, although the development cannot be indefinitely 

 hastened. The buds of Salix and Cornus Mas may, for example, expand 

 during a warm winter in January, but not those of the oak and beech, 

 which even in South Italy and Madeira do not open until March or April 2 . 

 The resting period of the beech at Madeira averages 149 days, which is 

 15 days less than in Switzerland 3 . As compared with Lesina, on the 

 Adriatic Sea, vegetation begins on the average 43 days later at Paris and 

 ico days later at Pultowa 4 . Plants from southern climates strive to repeat 

 their normal periodicity, and if the external conditions permit it, develop 

 leaves and flowers at unusual times. In the progress of a few years, 

 however, the plant may accommodate itself to the new conditions, and by 

 lengthening its resting period assumes a yearly periodicity corresponding 

 to the changed climate 5 . 



In many cultivated varieties the resting period is considerably shortened or 

 lengthened 6 . The resting period may take place at different stages of development, 



1 Askenasy (1. c., p. 824) kept branches of Pruniis aviuin at 15 to 20 C. with their lower 

 ends in water, and found by weighing and measurement that the buds developed but little between 

 December 4 and 20, but enlarged considerably between December 23 and January 10, and flowered 

 soon after this. Cf. also Geleznoff, Bull. d. 1. Soc. Imp. d. Natur. d. Moscou, 1851, Bd. xxiv, 

 p. 134; Bnsse, Flora, 1893, p. 171 ; Kiister, Beitrage z. wiss. Botanik von Fiinfstiick, 1898, Bd. n, 

 pp. 404-413. On the metabolic changes in buds cf. Fischer, Jahrb. f. wiss. Bot., 1891, Bd. xxii, 

 p. 160. On the time of formation of buds see Mohl, Bot. Ztg., 1844, p. 90; Faist, Bot. Centralbl., 

 iSSS, Bd. xxxvi, p. 43 ; Albert, Beitrage z. Entwickelungsgesch. d. Knospen., Rostock Diss., 1894 ; 

 Jahn, Bot. Centralbl., 1894, Bd. LIX, p. 263. 



2 Cf. Grisebach, Die Vegetation der Erde, 1872, Bd. I, p. 274. 



3 Fleer, Bot. Ztg., 1853, p. 210. 



4 Cf. Kerner, Pflanzenleben, i. Aufl., 1887, Bd. I, p. 528; Drude, Fflanzengeographie, 1890, 

 p. 36; Ludwig, Lehrb. d. Biologic, 1895, p. 146. 



s A. de Candolle, Me'm. presentes p. divers savants, 1806, T. I, p. 349. 



6 Volkens (Gartenflora, 1896, p. 2) states that certain potato- tubers need no period of rest. 

 Cf. also de Vries, Landw. Jahrb., 1878, Bd. vil, p. 244; Schinid, Btr. d. Bot. Gts., 1901, 

 p. 67. 



