204 PERIODICITY OF GROWTH 



auxanometer and still further extended our knowledge as regards the periodicity 

 of the growth in length of stems. Reinke * and Friedrich z obtained similar results 

 as regards growth in thickness, while Prantl 3 and Stebler 4 have extended these 

 observations to the leaves of various plants. Strehl 5 found that the root of Lupinus 

 albus, and Macmillan 6 and Golden 7 that the tuber of the potato, also exhibit 

 a daily periodicity of growth, which, when these are kept in equally moist soil at 

 constant temperature and in darkness, must be due to interaction with the aerial 

 organs. Fungi appear also to show a daily periodicity of growth, but the after-effect 

 has not yet been determined. 



The detailed course of the daily period of growth differs according to the plant 

 examined, even when all factors, excepting the daily illumination, are kept constant. 

 The growth of many plants is only slightly increased by the absence of light, 

 whereas that of others is considerably increased. Furthermore, the latent period 

 of stimulation may either be short or of considerable length. In slowly reacting 

 plants the maximum growth may occur at midday, the minimum after midnight, 

 since the retarding effect of light and the accelerating action of darkness take some 

 time to act. The after-effect follows a similar course to the primary one, although, 

 under continuous illumination or darkness, the curves not only diminish in 

 amplitude, but may also be somewhat modified. Darkness, however, produces 

 a greater effect when applied during the period of increasing growth, than when 

 it is decreasing, for in the former case the direct effect co-operates with the induced 

 rhythm, but in the latter case antagonizes it. A similar antagonism takes place 

 when the periods of darkness and illumination are reversed, and ultimately the 

 rhythmic periods are transposed by twelve hours. As regards the confusing effect 

 of other factors, it should be remembered that growth responds very rapidly to 

 changes of temperature. As regards the internal processes involved nothing can 

 be said, for we are entirely ignorant as to the mode of action of light, as to how its 

 after-effect is produced. 



SECTION 59. The Yearly Periodicity. 



In temperate climates plants exhibit a pronounced yearly periodicity 

 due to the alternation of a period of activity in summer with a resting one in 

 winter. This is in the first instance due to the low temperature during winter 

 causing a complete or partial cessation of growth, although many plants so 

 regulate 'themselves that an interval of rest follows each period of activity. 

 The latter is of use in preventing such plants from unfolding their leaves 



1 Reinke, Bot. Ztg., 1876, p. 148. 



2 Friedrich, Bot. Ztg., 1897, p. 369. 



3 Prantl, Arb. d. Bot. Inst. in Wiirzburg, 1873, Bd. I, p. 371. 



* Stebler, Jahrb. f. wiss. Bot., 1878, Bd. xi, p. 47. Cf. Vines, Arb. d. Bot. Inst. in Wiirzburg, 

 1878, Bd. I, p. 128. For observations on the leaf of Victoria regia see Caspary (Sachs, 1. c., p. 187) 

 and Drude, Nova Acta d. Leopoldin. Academ., 1881, T. XLIII, p. 247. 



5 Strehl, Unters. u. d. Langenwachsthum d. Wurzel u. d. hypocotylen Gliedes, 1874, p. 19. 



6 Macmillan, American Naturalist, 1891, p. 462. 



7 Golden, Bot. Centralbl., 1894, Bd. LIX, p. 169. 





