HYGROSCOPISM. , 285 



African tree {Pentaclethra macrophylld) possess this 

 property in an exaggerated degree. When fastened 

 together by strong wires they break themselves in 

 pieces in their efforts to become free. 1 The length of 

 these pods is from twenty-two to twenty-five inches, 

 with a breadth of about three and a-half inches. 

 From the observations of Professor Oliver, it has 

 been ascertained that the increase and decrease of 

 length between dryness and moisture is sixteen per 

 cent., so that the contraction in one pod would not 

 be less than three inches. Contractility of a similar 

 character but to a less extent has been observed in a 

 plant of the cucumber family (Echinocystis lobatd)? 



An illustration of a different kind to those hitherto 

 adduced is furnished by Dr. Darwin, in one of his 

 works. It refers to a plant which bears a name in 

 allusion to its hygrometric predilections (Porlieria 

 hygrometricd). "In the Botanic Gardens at Wurzburg," 

 he says, " there was a plant in a pot, out of doors, 

 which was daily watered, and another in the open 

 ground which was never watered. After some hot 

 and dry weather there was a great difference in the 

 state of the leaflets on these two plants ; those on 

 the unwatered plant, in the open ground, remaining 

 half, or even quite, closed during the day. But twigs 



1 Oliver, in " LinnEean Transactions,'' xxvi., p. 415. 



2 Wyman, " Proceedings American Academy," iii., p. 167. 



