362 Dr. Lankestcr on the movements 



sule 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 phaenomena, I was induced to 

 submit the seta; 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 elon- 

 gated cellular tissue which is arranged in a spiral manner. 

 (Figs. 1, 2.) 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 direc- 

 tion are quite evident. (Fig. 3.) The immediate cause of the 

 motions appears to be the absorption of moisture by the elon- 

 gated 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 dis- 

 tension of the external tissue, may be a question. The cap- 

 sule 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 capsule 

 when the upper end is wetted is accounted for by the circum- 

 stance of the upper end of the seta being more twisted than 

 the lower end. It does not however appear that the mere spi- 

 ral form of the fibres is the cause of the motion, as this struc- 

 ture exists in the green setae, which are entirely insusceptible 

 of motion from the application of moisture. Nor is merely the 

 dryness of the fibres the cause, as the green setae, though 

 thoroughly dried, do not exhibit any movement. But at the 

 period of ripening the capsule is found bent towards the sur- 

 face of the earth, and although I have not observed it turn- 



