THE YEARLY PERIODICITY 205 



too early in the year, when a few warm days occur in winter. The details 

 concerning the relationships between climate and periodicity belong more 

 especially to the geography of plants, so that in the following general 

 discussion of automatic periodicity we can confine ourselves to the plants of 

 temperate regions, and may neglect the periodicity induced in warm climates 

 by the alternation of wet and dry seasons. 



In many plants a partial or complete arrest of growth occurs at 

 a certain phase of the grand period, and this is repeated annually at regular 

 intervals in all those plants which prepare for a period of winter rest during 

 their activity in summer. A yearly periodicity is not, however, exhibited 

 by all plants, and naturally not by those whose duration is short, such as 

 bacteria, many fungi, and certain ephemeral flowering plants. Many 

 tropical plants have also no resting period, and even in our hothouses con- 

 tinue to grow during winter, and such plants as Senecio vulgaris and Stellaria 

 media, whose seeds can germinate as soon as they are ripe, may be found 

 during mild winters in all stages of development. All such plants have 

 a resting period forced upon them in severe winters merely owing to the 

 low temperature preventing their growth. 



Both the deciduous and evergreen trees of our climate, however, enter 

 into a winter resting period even when placed in a warm greenhouse and 

 well illuminated. Similarly the oak, beech, and fruit-trees throw off their 

 leaves and rest during the winter period even in such climates as that of 

 Madeira, where the average temperature during January, the coldest 

 month, is i5'4 C., and many of the indigenous plants of this moist island 

 climate grow all the year round 1 . The perennating subterranean parts 

 have in our climate an autonomic winter resting period, so that most 

 rhizomes, tubers, and bulbs will not form shoots in autumn, while potatoes 

 rest during winter in a cellar or clamp, but begin to sprout during spring 

 ! even although the temperature has slightly fallen 2 . 



The duration of the dormant period may be extended by unfavourable 

 I conditions, and also varies in different plants. Thus a rise of temperature 

 i awakens some plants before the end of the year, but others not till spring. 

 , Askenasy 3 was, for example, able to cause Forsythia viridissima and 

 ! Cornus Mas to flower in December, and Prunus avium in January. 

 I Similarly Salix^ Corylus, and Syringa flower unusually early during mild 

 winters, whereas the buds of Fagus sylvatica, Quercus pedunculata, Tilia 



1 Heer, Bot. Ztg., 1852, p. 209; Schacht, Madeira u. Teneriffa, 1859. Cf - also Askenasy, Dot. 

 -, 1877, p. 832. 



2 For other instances see Askenasy, Bot. Ztg., 1877, P- 8l 9 J Grisebach, Die Vegetation d. Erde, 

 1872, Bd. II, p. 399; Ernst, Bot. Ztg., 1876, p. 38 ; Krasan, Beitrage z. Kenntniss d. Wachsthums d. 

 Pflanzen, 1873 (repr. from Sitzungsb. d. Wiener Akad., Bd. LXXVII, I. Abth.). 



3 Askenasy, Bot. Ztg., 1877, p. 793. Cf. also Duchartre (Askenasy, I.e., p. 826) ; Krasan, 

 L c. (Salix nigricans, Euonymus europaeus, Prunus Padus). 



