20 PLANT RESPONSE 



degrees of mechanical sensibility — from those in which it 

 is shown in an extreme degree, to others again in which it is 

 apparently almost non-existent — will be demonstrated in this 

 and succeeding chapters, 



In order to study the responsive movements of plants, 

 we may take either the leaflets or the main petiole of 

 Mimosa pudica. The leaflets, however, in this case are so 

 excessively sensitive that even the contact for experimental 

 adjustment is sufficient to produce a closure from which they 

 do not recover for a considerable time. The pulvinus of the 

 main petiole, on the other hand, is considerably less sensitive. 

 Of intermediate sensibility are the leaflets of Biophytum 

 sensitivum, which on the whole furnish the most suitable 

 specimens for the general purposes of these experiments. 

 This plant, which is known to be sensitive, grows in a wild 

 state near Calcutta, and is so common as to be considered 

 a weed. It is a low-growing herb, with simply pinnate 

 leaves, each bearing from ten to sixteen pairs of leaflets. 

 A better specimen could hardly perhaps be found for the 

 exhibition of some of the most important characteristics of 

 mechanical response. It is not, under ordinary conditions, 

 excessively sensitive. A gentle touch does not, as a rule, 

 produce the closing effect, but under specially favourable 

 circumstances its sensitiveness may equal, if not surpass, that 

 of the Mimosa leaflets. The closing of the leaflets takes place 

 not upwards as in Mimosa^ but in the downward direction. 

 I shall presently give details of the response obtained with 

 Biophytum. But as this plant is not universally obtainable, 

 and as it flourishes only for a short season, during and after 

 our tropical rains, it may be best first to give an account of 

 experiments made on the more generally accessible leaf of 

 Mimosa. 



Response of Mimosa. — As the responsive movement of 

 the leaf of Mimosa is of considerable extent, no magnifica- 

 tion is necessary for the record. Indeed, on the contrary, 

 for the illustrations in the present work, the records had fre- 

 quently to be taken on a reduced scale. This was accomplished 



