Exchange and Movements of Gases and Liquids. 63 



Fig. 38 Awn of Erodium, 



f basal p rti n ' 



small hole should be made in the lid, and the jar set in a 

 cylinder of water. Note the position of the mercury column, 

 or measure the length of the enclosed column of air. As the 

 seeds swell, the bulb will be com- 

 pressed and the mercury driven 

 against the air in the closed arm. 

 The volume of this air will denote 

 the amount of pressure as calculated 

 by Boyle's law. (See "Practical 

 Plant Physiology," p. 176.) 

 55. Hygroscopic move- 



ments. When the particles Of 

 walls are forced apart during imbibi- 

 tion, movements may result of more 



or less importance to the plant. The twisting movements of 

 the awns of certain seeds are of this character. Secure a 

 few awns of Stipa avicennacea or Erodium, which are usually 



curved midway. Warm a 

 cent piece, and put a 

 drop of sealing wax in 

 the middle of one side. 

 Attach the basal end of 

 the seed to the wax, and 

 set the preparation in a 

 small circular glass dish, 

 the edges of which are 

 marked into regular in- 

 tervals. Note the posi- 



t : Qn Q f t u g t j r ^ 



T 



with respect to one of 

 these marks. Pour water 



in the dish, and follow the movements of the awn. After an 

 hour pour out the water, and set the preparation where the awn 

 may dry out. Observe the behavior of the awn. 



Fig. 39. Preparation to show hygroscopic 

 movements of an awn of Stipa. A , small metal 



disk ; B, awn. 



