560 LECTURE XXXII. 



intensity of the light decreases in the evening, and the darkness of night follows, the 

 retarding action of the hght- does not disappear instantaneously, but the forces of 

 growth, now set free as it were, obtain more and more the upper hand in the plant, 

 and the growth is accelerated until the morning, attaining a maximum until the 

 moment when the increasing intensity of the light is again in a position to exert its 

 retarding influence. A series of observations made by Prantl on normal green 

 foliage-leaves yielded exactly similar results for these also. 



But however satisfactory this explanation appears to be (and it is certain that 

 the alternations of light and darkness co-operate in the way referred to), further observa- 

 tions show that even quite independent of the daily variation in illumination, a daily 

 increase and decrease of growth nevertheless takes place, e. g. in continuous dark- 

 ness. I had ascribed this fact, which I was the first to observe, hypothetically to the 

 circumstance that my observing-chamber did not admit of complete darkening ; but. 

 the very detailed observations of Baranetzky allow of no doubt with respect to. this 

 very point, that a periodic up-and-down variation of the hourly growth in length 

 takes place entirely independent both of temperature and of hght. Baranetzky, 

 however, is inclined to regard this periodic Arariation, which still exists even in 

 copstant darkness, as an after-effect of the previously existing daily light-period, for' 

 which, moreover, he adduces various apparently plausible reasons ; but, apart from 

 other objections, Baranetzky himself has brought forward the fact that in the case 

 of completely etiolated shoots, which had been developed in the dark from the begin- 

 ning, from tuberous roots of Brassica rapa, the periodicity came out if possible still 

 more sharply than with normal shoots of the same plant. .This single fact suffices 

 completely to set aside the theory of after-effects, since no one, probably, would 

 be inclined to believe that we have here an after-effect which has been for a 

 long time latent, so to speak, in the bud. That would be much as if a pen- 

 dulum commenced to swing spontaneously after it had been standing still for some, 

 time. Moreover, Baranetzky himself adduces the fact that in different plants of the 

 same species the appearance of the maxima in the dark is observed at very different 

 times of the day, although in normal daily illumination this does not occur. 



I, on the contrary, am of opinion that in the plant, or at any rate in its grow.- 

 ing parts, periodic variations occur in some way quite independent of variations of 

 temperature and of light; and these, as I conclude from Baranetzky's observations, 

 may continue for periods of very different lengths. If now the plant is subjected to 

 the regular alternation of day and night, and the variations of temperature are. very 

 small, the above-mentioned influences on growth make their appearance, by which its 

 maximum is transferred to the morning hours, and its minimum to the evening, 

 the above-mentioned periodicity arising from purely internal causes being concerned 

 as the weaker factor in a definite daily period of time. So long ago as 1863 I gave 

 a very similar explanation of the spontaneous periodic movements of foliage-leaves. 

 There also a periodic up-and-down variation takes place in constant darkness, but 

 this, through the alternation of day and night, is modified to a daily period. However, 

 I shall return to this point later on. 



Of special interest is the fact demonstrated by Dr. Vines* in my laboratory, 



1 Sydney H. Vines, ' The Influence of Light upon the Growth of Unicellular Organs ' (Arb. 

 des hot. Inst, in Wzbg. B. II, pp. 133, &c.). 



