'54 



PLANT PHYSIOLOGY 



hence may effectively be shown to a class, but the recovery requires an hour 

 or more. A very striking lecture-table experiment to demonstrate the effect 

 of osmotic pressure in giving turgescence, and hence mechanical rigidity, 

 is the arrangement shown by the accompanying figure (Fig. 41). It is made 

 from parchment-paper tubing tied at one end to a stoppered half vial and 

 at the other to a glass stop-cock. It can be filled through the vial with 

 molasses; and then the air may be allowed to escape, or the pressure may 

 at any time be relieved, through the stop-cock, so that it hangs perfectly 

 limp in a tape loop, as shown by the lower part of the figure. If, now, this 

 be suspended in a glass dish of clear, lukewarm water, it will, well within 

 an hour, become strongly turgescent and straighten stiffly out, as shown 



FIG. 41. ARRANGEMENT FOR DEMONSTRATION OF OSMOTIC TURGESCENCE; Xf. 



Details in text. 



in the upper part of the figure. The whole arrangement may then readily 

 be preserved, always ready for use, in a bottle of molasses, as described for 

 the demonstration osmoscope (page 146). 



Both PFEFFER'S and DE VRIES' methods of determining the 

 osmotic pressures in plants being indirect or somewhat com- 

 plicated, even though accurate, one naturally asks whether 

 there is not some simpler and more direct, even though less 

 accurate method. Such a method, albeit but crude, does exist, 

 and may be employed by the student as follows: 



SUGGESTED EXPERIMENT. Select a slender, soft-textured, turgescent 

 :ure, such as a flower-stalk of Tulip or Hyacinth, a petiole of Oxalis 

 ropeolum, and mark off by waterproof India ink a definite length, pref- 

 erably 100 mm., leaving in addition some 10 mm. at each end. Plasmolyze 

 his by some hours' immersion in a normal solution of salt, and then measure 

 distance apart of the marks. Clamp one end firmly between bits of 



