682 PLANT RESPONSE 



a measure of the angular movement at different parts of the 

 day. The dotted line shows the diurnal movement of the 

 younger of the two plants, which was six months old, and the 

 continuous line that of the other, which was one year old. The 

 two, as will be seen, are practically the same. It should be 

 mentioned here that, when this record was taken, 6 A.M. and 

 6 P.M. were the hours of sunrise and sunset, there being no 

 twilight. The records show that the leaves exhibited the 

 erectile or recovery effect with great rapidity during the first 

 part of the night, this movement being almost completed by 

 I A.M. After this there was but slight upward movement, 

 until about y A.M., from which time onwards, during twelve 

 hours, they fell continuously. The maximum depression was 

 reached at almost exactly 6 P.M., and then the leaves again 

 rose, repeating their former movement of recovery. 



Periodic impulses acting on the leaf. — The periodic 

 movements down during the day, and up during the night, 

 are thus seen to be due to periodic impulses of stimulus 

 and recovery acting on the responding leaves. It must be 

 pointed out here that the process of recovery is not alto- 

 gether passive. We saw in the records of Mimosa under 

 light (fig. 256) that, on the cessation of light, the responding 

 leaf continued to move down for a while owing to the 

 positive after-effect. Later, however, owing to the latent 

 energy which it had acquired by the absorption of light, it 

 exhibited the negative after-effect in an erection which 

 carried it beyond the original position. Hence we can see 

 that the increased internal energy due to previous absorption 

 of light plays an important part in that movement of erection 

 which is initiated shortly after nightfall. We may therefore 

 say that the diurnal movements of the leaves are brought 

 about by two alternate periodic impulses, those, namely, 

 of external stimulus and internal energy. The persist- 

 ence of the effect of each of these impulses will depend on 

 the intensity of the two factors respectively, and also on 

 the capacity of the tissue for absorbing stimulus. For 

 example, the positive after-effect by which the leaf continues 



