NYCTITROPISM 509 



The periodicity in the after-effects we have referred to are of extreme 

 interest from several points of view ; at the very commencement, however, it 

 must be evident that the studies we have hitherto made on the subject of simple 

 nyctitropic movement are incomplete in one important respect. We are unable 

 in the individual case to distinguish what is the direct consequence of a solitary 

 stimulus and what is an after-effect. Since the sleep position is brought about 

 more rapidly by placing the plant in the dark in the evening than in the morning 

 we may look on that as an instance of after-effect, and if the sleep move- 

 ment is followed after a short time by an opening movement when the plant is 

 placed in the dark in the morning, we must not regard this reverse movement in 

 any sense as due to autotropism, but as an after-effect also. Just because after- 

 effects frequently manifest themselves in nature we ventured to suggest above that 

 nyctitropic movements should be studied in such plants as have been cultivated 

 under constant external conditions ; but this suggestion has not been as yet 

 realized. On the other hand, many authors, and more especially Pfeffer, have 

 shown that in certain plants which exhibit no autonomous movements, after- 

 effects gradually cease when the plants are exposed to continuous light. Thus 

 Pfeffer (1875) noticed that the periodic movements in Acacia lofhantha and in 

 Impatiens, when exposed to continuous light, became gradually weaker, until 

 finally a permanent day position was reached. This result was to be expected, 

 for ScHUBLER (1873) had already pointed out that in the case of plants growing in 

 Northern Norway the periodic movements they exhibited in midsummer disap- 

 peared for a considerable time and reappeared again after the commencement 

 of the arctic night. The cessation of periodic movements as a result of exposure 

 to continuous darkness, presents great difficulties, as we have already pointed out 

 (p. 508). We do not as yet know whether leaves which become normal in 

 complete darkness ( JosT, 1895) assume a fixed night position when the external 

 conditions are quite constant ; the great majority of leaves, however, assume 

 an abnormal rest position in darkness, because either their upper sides or their 

 under sides exhibit increased growth (epinasty, hyponasty) (Vines, 1889). 



The cessation of the periodic movements is especially of interest because it 

 shows that these movements are not hereditarily characteristic of the plant, as 

 one might at first imagine from their continuance in darkness. Other experi- 

 ments lead to similar results, for we may by illuminating plants during the night 

 and darkening them during the day shift the periodicity of movement as much 

 as twelve hours. Periodic movements are, moreover, not developed once and for 

 all ; on the contrary, they apparently arise gradually in the course of the develop- 

 ment of the plant. In order to study the question of the mode of origin of these 

 movements we must at present confine ourselves exclusively to plants which 

 have, when continuously illuminated, lost the power of performing periodic move- 

 ments, since as yet no leaves have been cultivated under conditions which pro- 

 hibited the original development of such movements. After the loss of periodic 

 movement these leaves are in no sense in a state of rigor. In one experiment of 

 Pfeffer's (1875), a plant of Acacia lophantha closed its leaflets as soon as it was 

 darkened, but after a few hours began to open them once more, and in less than 

 12 hours the leaflets were again almost completely extended. When kept in the 

 dark continuously, they exhibited on the two following days, two oscillations, 

 whose turning points were 18 to 24 hours apart. Pfeffer concluded from these 

 and similar experiments that a solitary nyctitropic stimulus induced not one but 

 a whole series of oscillations. If that be so, then the periodic movement must be 

 brought about in such a way that the solitary stimulus is combined with an after- 

 effect. This, however, is only possible if these secondary oscillations occur in the 

 same rhythm as that in which the stimulus affects the plant in nature, that is to 

 say, the periods between opening and closing must be about twelve hours in 

 length. In the experiment with Acacia just described the oscillation was too 



