74 



EXPERIMENTS WITH PLANTS 



the plant can compress the spring no further, remove 

 it, invert the bottles, substitute for the inner bottle a 

 larger one of the same diameter, and pour shot into it 



until the spring is compres- 

 sed to the same point as it 

 was by the plant. The weight 

 of this bottle with the con- 

 tained shot, plus the weight 

 of the inner bottle and cotton 

 (since the plant raised these 

 in addition to compressing 

 the spring) , will give the pres- 

 sure exerted by the plant: if 

 we divide this by the area of 

 the cross -section of the stem 

 just back of the crook we 

 shall obtain the number of 

 pounds pressure to the square 

 inch. Thus, in one case the 

 plant exerted a pressure of 

 one pound. The diameter of 

 the stem just back of the crook was one -eighth inch. 

 The equation is 



Modification of tlie apparatus 

 shown in Fig. 59. 



1 pound =1 = 



^R' 3.1416 X {-h) 



2 81.5 pounds per square inch. 



It is interesting to note, in this connection, that 

 the pressure of steam in the boilers of ordinary sta- 



