124 



PLANT PHYSIOLOGY. 



scribed for large tubes on page 38) the lower end of the vial, and 

 fit into both ends tight narrow corks. Through the diameters of 

 each cork force two fine needles at right angles to each other cut to 



\ ' 



34. ARRANGEMENTS FOR DETERMINATION OF GEOTROPIC PRESSURE OF 



ROOTS. One-half the true size. 



such a length from the points that they are slightly shorter than the 

 diameter of the test-tube. (Or the bottom may be left in the vial, 

 and the needles attached to it by sealing-wax.) Such a vial will 

 float upon the top of mercury in the test-tube, with the needle- 

 points against the glass holding it in an upright position, and allow- 

 ing it to slip up and down in the mercury practically without fric- 

 tion. In the top of the upper cork, left considerably longer than 



