COMPARED WITH THOSE OP ANIMALS. 79 



The seat of motion in the sensitive plant, is evidently in 

 the little swelling or intumescence at the articulation of 

 the general and partial leaf-stalks. These swellings, when 

 touched directly, communicate motion to the leaves. They 

 appear to have two surfaces, possessing different degrees of 

 irritability. When these swellings are gently touched with 

 a steel point on their upper surface, the leaflets immediately 

 fold together ; but they do not move when the lower sur- 

 face of the swellings is touched. With the swellings at 

 the base of the main petiole, it is just the reverse ; for the 

 irritability resides not in their upper, but in their lower 

 surface. 



Up to the present moment, no satisfactory explanation 

 of these movements has been afforded. The opinion most 

 favorably received amongst scientific men with respect to 

 them is, that they are referable to a power of contractility 

 possessed by the tissues of the plant, analogous to that 

 which exists in certain animal tissues, but especially in 

 the muscular. It is well known to naturalists that certain 

 unicellular and thread-like plants, found amongst the green 

 hair-like vegetation which attaches itself to stones in fresh 

 water rivulets, possess this power of contractility under the 

 influence of external stimuli. If this capillary vegetation 

 be placed beneath the microscope, movements among some 

 of the filaments will be distinctly recognized. These plants 

 have been called oscillatorias, in allusion to these motions, 

 which resemble vibrations or oscillations to and fro, and 

 occasionally writhing movements so well marked, that their 

 vegetable nature has been disputed. This property of con- 

 tractility exhibited by these isolated filaments, is manifested 

 by them when associated together, in the case of the mi- 

 mosa. The movements of the mimosa or sensitive plant, 

 are probably produced by this cause, and are closely allied 



