FUNCTION.] FUN ARIA HYGROMETRICA. 159 



upper portion be moistened in like manner, the capsule will 

 turn round more rapidly in a contrary direction. This 

 phenomenon is exhibited whichever portion of the seta is 

 first wetted. If both ends are moistened at the same time, 

 a tremulous wavering is observed without any motion, but in 

 a few seconds the capsule begins to move in one direction or 

 the other. The direction in this instance is in some measure 

 determined by the quantity of moisture applied, but the 

 upper part seems most easily affected, and the motion arising 

 from moistening it is much more rapid than from the lower 

 portion. If the capsule is held in the fingers the lower end 

 presents the same motions. If both ends are held and the 

 middle left free and moisture is applied, there is an evident 

 effort made to curl the whole stem, but this is not effected. 

 On observing these curious phenomena, I was induced to 

 submit the setae to an examination by the microscope, and 

 their structure explains in some measure the nature of 

 the motions observed. The entire seta is composed of an 

 elongated cellular tissue which is arranged in a spiral manner. 

 The tissue is not, however, continued in the same direction 

 through the whole length of the seta, but at about two thirds 

 of its length it begins to straighten, and at length in the 

 upper part runs spirally in an opposite direction to that of 

 the lower portion, the fibres forming a much more acute 

 angle in the upper, than the lower part of their course. This 

 structure is most apparent in the dried setae. In the young 

 state the fibres are quite straight ; as they increase in age 

 they become more spiral ; and in the green setae, just before 

 the capsule is ripened, the spiral fibres with their double 

 direction are quite evident. The immediate cause of the 

 motions appears to be the absorption of moisture by the 

 elongated spiral tissue. Whether the moisture admitted into 

 the tissue straightens it by the force with which the fluid 

 passes along the bent tubes, or whether it arises from the 

 mere distension of the external tissue, may be a question. 

 The capsule turns round in a direction contrary to that of 

 the spiral of each end, and after the seta has been moistened 

 and has turned round in both directions, its length is greater 

 than it was previously. The more rapid movements of the 



