68o 



PLANT RESPONSE 



each lasting for twelve hours, and the beginning of each 

 separated from the next by a period of twenty-four hours. 



Diurnal responses of leaf of Biophytum. — From this 

 simple instance of anisotropy we pass on to a more highly 

 differentiated case of dorsi-ventrality, as seen in leaves. We 

 have already seen that the petiole of Biophytum is only 

 provided with a moderately developed pulvinus, and that the 

 petiole also, as a whole, acts as a diffuse pulvinoid. In this 



case also, by the in- 

 ternally and externally 

 diffused action of light 

 during the course of 

 the day, the petiole is 

 progressively depressed, 

 from its position of 

 highest erection in the 

 morning to its lowest 

 depression in the even- 

 ing. Recovery takes 

 place again in the ab- 

 sence of the stimulus of 

 light, and the leaf once 

 more assumes its highest 

 position by morning. If, 

 however, in the course 

 of the afternoon, say, at 

 five o'clock, the plant be 

 taken to a dark room, 

 then, owing to the positive after-effect, the leaf will still con- 

 tinue to fall for about an hour, and then begin to erect itself. 

 Owing to the daily periodicity impressed on the plant, of which 

 we shall presently speak in greater detail, the leaves continue 

 to exhibit the diurnal movement even when kept in continuous 

 darkness. I give here (fig. 273) an interesting photographic 

 record of the diurnal movement of the leaf of BiopJiytnui 

 from 5 P.M., when it was brought into the dark room, till 

 9 A.M. next morning. It will here be seen that, owing to the 



Fig. 273. Photographic Record of Diurnal 

 Movement of Petiole of Biophytum from 

 5 P.M. till 9 A.M. 



First part of record shows maximum depres- 

 sion to be reached at 6 p.m. and maximum 

 erection at 7 a.m. After the latter hour 

 the leaf again begins to exhibit its usual 

 daytime depression. 



