AND IRRITABILITY OF SENSITIVE PLANTS 187" 



of leaf sensitivity indicate no definite accounts until the eighteenth; 

 century. During this century a number of observations on sensitive- 

 movements were published. Among the early observers were Mairan^ 

 Bonnet, Linnaeus, Acosta, Alpinus, Ray, Hill, Du Hamel and others. 

 Most of the studies were made on the sensitive plant, Mimosa pudica, 

 which had then been introduced into Europe and aroused much interest 

 on account of its leaf movements. 



Miller (35) recognized five species of Mimosa, all more or less sensi- 

 tive. "The first sort (M. pudica) is commonly known by the name of 

 Sensitive Plant, to distinguish it from the others, which are generally 

 called Humble Plants, because upon being touched, the pedicel of their 

 leaves falls downward, whereas the leaves of the other sort are only con- 

 tracted upon the touch. " 



Hill (20) carefully observed the sequence of motion in Mimosa pudica 

 and pointed out that the effect of total darkness on the plant is greater 

 than the rudest touch. He also found that the contact stimulus must 

 be of a sufficient intensity, and that the degree of the subsequent motion 

 depended upon the potency of the stimulus. Hill further observed that 

 the movements of Mimosa are less well marked at a lower temperature 

 than that in which the plants have been reared. His explanation for 

 this more sluggish movement is — "This is probably due to the juices 

 stagnating in the clusters of fibres, and to the contraction of the bark 

 by cold." Hill's explanation of the response to the contact stimulus 

 is interesting because it is an illustration of the view current at the time 

 that such motion was due to the fibres which acted like those of muscle. 

 Hill also made observations on the movements of the leaves of Abrus 

 and Tamarindus, and found that "In these and all others, the degree 

 of elevation or expansion in the lobes, is exactly proportional to the 

 quality of the light, and is solely dependent upon it." Hill's work was 

 severely criticised by his contemporaries. 



Lindsay (24) studied especially the fall of the leaf of Mimosa when 

 stimulated, and discovered that the force which raises the petiole exists 

 in the lower part of the intumescence (pulvinus), and that which 

 depresses it in the upper. He seems to have considered that the tem- 

 porary excess of force in either part is produced by an impulsion of the 

 sap from the vessels of the yielding portion unto those of the opposite 

 portion. 



Dutrochet (16) concluded that the sleep movements were due tO' 

 opposite changes of the energy of expansion in the antagonistic halves 

 of the pulvinus. This writer was the first to show that stimuli are 



