268 



PLANT RES TON SE 



slower-moving drums, I find that this is attained in different 

 specimens, in a period of 1*5 to 2-5 seconds after the shock. 

 This maximum contraction persists for a further period of 

 about thirty seconds. The leaf now begins to erect itself, 

 and full recovery is attained in the course of a further period 

 of about six minutes. These statements refer to reaction in 

 vigorous Mimosa, at a favourable season of the year, like 

 summer. In an unfavourable season, like winter, however, 

 the reaction becomes very sluggish, and recovery is not then 



complete in less than 

 eighteen minutes, or 

 three times the normal 

 period. 



Effect of cold on 

 latent period. — I shall 

 next refer to the slug- 

 gishness induced by 

 cold, prolonging the 

 latent period. The ex- 

 treme instance of this is 

 seen when iced water is 

 applied to the pulvinus, 

 and too great cooling 

 being thereby effected, 

 the response is abolished. 

 With moderate cooling 

 the latent period is found 

 to be prolonged to 

 several seconds. This 

 effect cannot conveni- 

 ently be shown, however, within the limits of a fast record. 

 In order, therefore, to show the comparative effect of cold on 

 the latent period, in the case of the same specimen whose 

 record is seen in the upper of the two photographs in fig. 1 10, 

 I was careful to cool the pulvinus very slightly. In the lower 

 of these photographs it will be seen that the latent period has 

 become prolonged, from the normal ^ to f 3 ^- of a second. 



Fig. 1 10. Photographic Record of Response 

 of Mimosa, Exhibiting the Latent Period 

 in its Variation 



The upper curve was taken under normal 

 conditions, and the lower when the pul- 

 vinus was slightly cooled. Time-marks 

 = tenths of a second. Original record 

 reduced to half. 



[The duplication, which will be observed in 

 each record, is due to the fact that the 

 heliostatic mirror was silvered behind, 

 thus producing two reflections, one from 

 the surface of the glass and the other from 

 that of the silver.] 



