HYGROSCOPISM. 277 



tended in its dimensions ; and a second that is more 

 hard and close, into which the water can very Httle 

 or not at all penetrate, this therefore retaining always 

 very near the same dimensions, and the other stretch- 

 ing and shrinking, according as there is more or less 

 moisture or water in its pores, by reason of the make 

 and shape of the parts the whole body must neces- 

 sarily unwreath and wreath itself"^ Another grass, 

 which, although not a native, is often cultivated, has 

 very long awns, which are subject to twisting and 

 writhing under increase or decrease of moisture. 

 The whole structure and mode of action in the awns 

 of this species were made the subject of an elaborate 

 investigation by Mr. Francis Darwin." The seed 

 terminates downwards in a sharp, strong, oblique 

 point, armed with a dense plume of barb-like hairs ; 

 upwards it is continued in a strong, woody awn, of 

 which the lower part is strongly twisted on its own 

 axis, and its upper portion untwisted and fringed 

 with a series of beautiful hairs, so as to impart a 

 feathery appearance. It is bent like a knee between 

 the twisted and untwisted portions. When the seed 

 is fixed, and the awn free, moisture applied to it 

 causes the lower portion to untwist, and with it the 



* Hooke, " Micrographia," p. 151. 



- "On the Hygroscopic Mechanism by which Certain Seeds 

 Bury Themselves," " Linnsean Transactions," 2nd series, vol. i., 

 p. 149- 



