BREAKS OF PLANT LIFE. 275 



CHAPTER XIII 



HYGROSCOPISM. 



HYGROMETRIC and Hygroscopic are two terms which 

 have been applied indiscriminately, or interchange- 

 ably, to indicate certain movements in the parts of 

 plants resulting from a susceptibility to dryness or 

 moisture. These phenomena are often exhibited by 

 dead and dried organs, but sometimes during vitality. 

 It is difficult to mark a distinct line between such 

 phenomena as exhibited by dead and living tissue, 

 nor is this essential, since in all cases the causes are 

 similar, and consist in the different size, form, and 

 density of subjacent series of cells, which expand 

 and contract, at different rates, and to diverse extent, 

 by absorption or loss of moisture, thus producing 

 twisting, curving, or contortion in alternate directions. 

 In other words, it may be accounted for " by supposing 

 that the cells on one side are larger, and have thinner 

 walls than those on the other ; and these will there- 

 fore be most easily distended when placed in water, 

 and will soonest lose their fluid in drying.'' 



One of the oldest and best known illustrations of 

 hygroscopism, is the awn of the wild oat (A vena 

 T 2 



