THE YEARLY PERIODICITY 207 



and does not occur in the same way and at the same time in all organs. Usually 

 the resting period comes after the ripening of the seeds, but in Colchicum autumnale 

 it occurs between the fertilization and fruit-formation \ The seeds and other 

 reproductive organs of many plants require a period of rest, although the vegetative 

 parts show no such inherent periodicity, while in other plants the converse is true. 

 Furthermore, the secondary growth of the xylem in trees begins and ends sooner 

 than that of the phloem, while only a portion of the buds formed are capable of 

 further development. 



Subterranean organs. The different organs of a plant are commonly exposed 

 to different conditions during the progress of the year, and that a localized effect 

 is readily produced is shown by the fact that a branch led into a greenhouse during 

 winter may become covered with foliage, while the branches outside remain quite 

 bare 2 . Since the subterranean organs are subject to less wide ranges of temperature 

 than the aerial ones, and since other conditions may also differ, it is not yet certain 

 whether the dissimilarities in the periodicity of the parts above and below ground are 

 autonomic in origin or are due to the direct influence of the external conditions. 

 It is, however, an obvious advantage that the roots of seeds, rhizomes, and tubers 

 should at first develop more rapidly than the stem, although in adult perennial 

 plants this is not actually necessary. The secondary growth of the root continues 

 much longer than that of the stem, and may be still active in winter or even spring. 

 Since it begins at the same time or only a little later than that of the stem, the 

 resting period of the root is evidently very considerably shortened. This is probably 

 the result of the lesser fall of temperature in the soil, and it is apparently owing to 

 especially favourable circumstances that in some cases no complete cessation of 

 growth could be detected in the root. A similar periodicity is noticeable as regards 

 growth in length of the main root and the formation of lateral roots, which are 

 usually most pronounced in spring, but frequently exhibit a secondary maximum in 

 August according to Resa and Petersen 3 . Since Wieler was unable to detect any 

 secondary maximum it probably, like the opening of buds in August, does not 

 occur in every year. That differences should be shown is not surprising when 

 we consider that hitherto all observations have been made under natural conditions, 

 in which the periodicity is not only influenced by external variations, but also by 

 interaction with the aerial organs. Moreover the different roots of the same tuber, 

 bulb, or rhizome may in some cases exhibit dissimilar periodicities 4 . 



Seeds and spores. Certain seeds are capable of immediate germination, whereas 

 others must first rest for a few weeks, months, or even years, even when they are not 

 dry, but are kept under conditions favourable for germination. Thus the seeds of 

 Trapa natans, and probably of most aquatic plants, which sink to the bottom when 



1 Cf. Krasan, Beitriige z. Kenntniss d. Wachsthums d. Pflanzen, 1873 (repr. from Sitzungsb. d. 

 Wien. Akad., Bd. LXXVII, i. Abth.). 



2 Duhamel, Naturgesch. der Baume, 1765, Bd. II, p. 209; Mustel, Traite d. 1. vegetation, 1781, 

 T. n, p. 326; Goppert, Warmcentvviekelung, 1830, p. 220. 



3 Mohl, Bot. Ztg., 1846, p. 314; 1862, p. 313 ; Resa, Ueber die Periode d. Wurzelbildung, 1877, 

 p. 36; Wieler, Cohn's Beitiage z. Biologie, 1893, Bd. vi, p. 101 ; Fetersen, Bot. Centralbl., 

 1898, Bd. LXXV, p. 272 ; Hammerle, Fiinfstiick's Beitr. z. wiss. Bot, 1901, Bd. IV, p. 149. 



1 Ci. Kimbacii, iJtr. d. Bot. Ges., 1899, p. 30 ; Goebel, Organography, 1900, II, p. 490. 



