CH. V] GYPSUM METHOD. 119 



of upright nails fixed in a horizontal board, or secured in 

 some way so that when the other end is pulled the 

 stalk will be stretched. The basal end of the stalk may 

 be simply knotted to a piece of cord, which passes over a 

 pulley let into the board, and supports a scale pan. A 

 millimeter scale having been arranged so that the distan^< 

 between the marks on the stalk can be easily read 96", 

 weights are added to the scale pan until the marks are^ 

 once more 100 mm. apart. The diameter of the stalk 

 must be roughly measured, and the area calculated, so 

 that the force which is equivalent to the hydrostatic 

 pressure in the tissues, may be expressed in grams per 

 square millimeter. It should finally be expressed in 

 terms of atmospheric pressure, — which equals about 

 10 grams per sq. mm. Something between 3 and 6 

 atmospheres may be expected as the result. 



(152) Pfeffer's gypsum method^. 



Pfeffer has devised a method of estimating the 

 pressure exerted by growing plants of which we have no 

 practical experience : the following description is taken 

 from his paper. 



The principle will be understqod from fig. 23. The 

 cotyledons and the basal part of the radicle are contained 

 in the pot n and kept damp by means of sawdust. The 

 extremity of the root is contained in the two blocks of 

 gypsum a and 6, so that as the root grows a and h are 

 separated. Since a is fixed against the pot n, the block h 

 moves, and in doing so compresses the oval spring /. 



' Druck- und Arbeitsleistung i&c. Abhandl. d. Je. Siichs. Ges. Bd. xx. 1893. 



