208 PERIODICITY OF GROWTH 



ripe, germinate next spring. Wiesner 1 found that the seeds of the mistletoe, and 

 Kienitz 2 that those of the white fir, pine, beech, hornbeam, and ash, germinate 

 in the following year, and only very exceptionally in the same year. Of sowings 

 of ripe seeds of Cuscuta, Euphorbia Cyparissias, E. exigua, &c., a number germinate 

 very soon, but others not until the following year, as in the case of Euphorbia exigua, 

 some not until after nine years 3 . 



The delay of germination is only in part due to the relative impermeability 

 of the seed-coat 4 , for in those which swell up but do not germinate, some internal 

 cause is evidently operating. During this dormant phase growth may either be 

 entirely or only partially arrested, as in the seeds of Eranthis hiemalis, Ranunculus 

 Ficaria, and other plants in which the seeds possess only a small embryo when shed, 

 and this gradually enlarges at the expense of the endosperm 5 . The previous cultural 

 conditions may apparently modify the length of the dormant period, but the con- 

 ditions which lead to certain seeds resting under the soil for as long as fifty years, 

 and germinating when dug up, have not yet been determined 6 . 



In fungi 7 and algae*, spores, zoospores, and conidia which are developed for 

 rapid multiplication, usually germinate at once under favourable conditions, whereas 

 zygotes, resting-spores, and sclerotia first pass through a resting period. The 

 duration of this period appears to be capable of considerable modification by the 

 prevailing and previous conditions, so that different authors disagree to a certain 

 extent. De Bary gives a resting period of 45 to 145 days for the spores of 

 Saprolegnia, whereas Klebs saw them germinate after 8 to 10 days. Hansen 

 found that the sclerotia of 'Coprinus stercorarius and C. niveus were in some cases 

 capable of immediate development, but in others not until after a resting period of 

 two or three years. 



Periodicity due to drought and light. In the dry seasons of many tropical 

 climates, plants lose their leaves and rest, partly owing to the direct action of the 

 deficiency of water, and partly owing to an autonomic periodicity induced by its 

 regular recurrence 9 . The variable amount of daily illumination is also of impor- 



1 Wiesner, Ber. d. Bot. Ges., 1897, p. 514. 



2 Kienitz, Bot. Centralbl., 1880, p. 52. 



3 Winkler, Ber. d. Bot. Ges., 1883, p. 452 ; Bot. Centralbl., 1889, Bd. xxxvur, p. 830 ; Wiesner, 

 Biologic der Pflanzen, 1890, p. 41. Also Nobbe, Samenkunde, 1876, p. 352 ; Detmer, Physiol. d. 

 Keimungsprocesses, 1880, p. 325 ; de Candolle, Pflanzenphysiologie, 1833, Bd. II, pp. 302, 306. On 

 the germination of unripe seeds cf. Nobbe, 1. c., p. 339; Wiesner, 1. c., p. 40; Arthur, American 

 Naturalist, 1895, p. 804; Kinzel, Versuchsstat., 1900, Bd. Liv, p. 125. 



* Cf. Wiesner, 1. c., p. 41 seq. 



5 Cf. G. Haberlandt, Die Schutzvorrichtungen der Keimpflanze, 1877, p. 50; Goebel, I.e., 



P- 454- 



6 Peter, Nachrichten d. Gottinger Ges. d. Wiss., 1894, p. 373 ; Fr. Muller, Biol. Centralbl., 1886, 

 Bd. VI, p. 513 (rhizome). 



7 De Bary, Vergl. Morph. u. Biol. der Pilze, &c., 1884, pp. 356, 370; Hansen, Bot. Ztg., 1897, 

 p. 121 ; Eriksson, Centralbl. f. Bact., 1898, 2. Abth., Bd. iv, p. 431 ; Klebs, Jahrb. f. wiss. Bot., 1899, 

 Bd. xxxvin, p. 571. 



8 Cohn, Ann. d. sci. nat., 1856, 4 ser., T. v, p. 191 ; Falkenberg in Schenk's Handbuch, 1882, 

 Bd. II, p. 237 ; Gotz, Flora, 1897, p. 99. 



9 Cf. Schimper, Pflanzengeographie, 1898, pp. 286, 370; Huber, Bot. Centralbl., 1898, Bd. 

 LXXVI, p. 259. 



