22 PLANT RESPONSE 



which these rapid records are obtained will be described in 

 Chapter XXII. 



From records obtained on a fast-moving drum, with a 

 fairly average specimen of Mimosa, it is found that the 

 leaf does not respond to stimulus immediately, there 

 being a latent period of -f-^ of a second before it begins 

 to move. The maximum fall is attained in the course of 

 2 seconds after the shock. After attaining the position 

 of maximum depression, the leaf remains in its contracted 

 position for a further period of about thirty seconds. It then 

 begins slowly to recover, and perfect recovery takes place in 

 the further course of six minutes. The record given was 

 obtained from the leaf of a plant which was one year old, and 

 in the summer season. It will be remembered that these 

 responsive curves are modified by the physiological condition 

 of the plant ; thus, for example, the time taken by the leaf of 

 a vigorous young plant for recovery may be as short as four 

 minutes, whereas an older specimen in winter may require as 

 long a period as eighteen minutes. We may thus obtain 

 from the record an idea of the physiological condition of the 

 specimen. 



By means of photography the taking of the record is made 

 extremely simple, but there are certain disadvantages insepar- 

 able from this method, which render the devising of other 

 means essential. For example, the motile sensibility of plants 

 like Mimosa and Biophytum is profoundly modified in dark- 

 ness. In the case of the records given, the plants have been 

 kept outside in the light, and brought in immediately before 

 experiment. But even then, after remaining in the dark for 

 half an hour or so, the leaves of Mimosa become abnormally 

 erected, till it can hardly be believed that the plant is sensi- 

 tive, for it often becomes irresponsive to the hardest blow. 

 Biophytum leaflets, again, in the same circumstances undergo 

 closure. For these reasons, long-continued experiments in 

 a dark room are an impossibility. Various sensitive plants, 

 again, flourish only for a short-lived season, and during that 

 period some hundreds of experiments have to be carried out. 



