CH. V] GYPSUM METHOD. 131 



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 milli- 

 meter. It should finally be expressed in terms of atmo- 

 spheric pressure, which equals about 10 grams per sq. 

 mm. Something between 3 and 6 atmospheres may be 

 expected as the result. 



(152) Pfeffers gypsum method 1 . 



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 understood from fig. 25. 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 b, so that as the root grows a and b are 

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

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

 The degree of compression, and therefore the force 

 exerted, is estimated by reading, with a horizontal 

 microscope, the distance between the needle-points fitted 

 to the inside of the spring. 



The following is the method of fitting the plant into 

 the apparatus. 



The seedling bean is placed in the flower-pot n filled 

 with damp sawdust so that 15 30 mm. of the root 



1 Druck- und Arbeitsleistung, &c. Abhandl. d.k. Sachs. Ges. Bd. xx. 1893. 



92 



