236 



PART III. VEGETABLE PHYSIOLOGY. 



the mercury in the tube at the beginning of the experiment. After a time, the 

 water forced out of the stem will begin to drive the mercury up the long arm of 

 the tube against the pressure of the atmosphere, 

 and, since the volume of the air will be inversely as 

 the pressure, the extent of the root-pressure may 

 readily be determined. 



5. Cut off the stem of a thriftily growing 

 woody plant which is in full leaf, and immediately 

 plunge the cut end into an aqueous solution of 

 aniline blue. After two or three hours remove the 

 stem from the staining fluid, and immediately cut 

 transverse sections of it at various heights and 

 observe how high the staining material has ascended 

 and along what tissues. The juice of Poke-berries 

 or a decoction of Brazil-wood may be used instead 

 of aniline-blue with good results. 



6. Wipe the inside of a bell jar perfectly clean ; 

 place underneath it a growing plant and expose it 

 to the light. Drops of water will soon gather on 

 the inner surface of the jar and after a time will run 

 down its sides. 



Vary the experiment as follows : Cut off a leafy 

 branch of some thriftily growing plant and immedi- 

 ately immerse the cut end in water contained in a 

 rather tall and narrow glass jar ; stop the mouth of 

 the tube so as to prevent evaporation from the sur- 

 face of the liquid. Mark on the outside of the jar the height at which the water 

 stands on the interior at the beginning of the experiment, and expose the leaves 

 to the sunlight. After a few hours, it will be found that the level of the liquid 

 is very perceptibly lowered. By measuring the quantity of the water at the 

 beginning of the experiment, and then again at its close, and noting accurately 

 the time, the rate at which evaporation has taken place may readily be deter- 

 mined. 



7. Cut off two twigs of some woody plant that has large leaves, and is 

 growing vigorously. Let one of them be cut rather high up where the tissues 

 are young and little lignified, and the other lower down where they are con- 

 siderably lignified. Immerse the cut ends of both in water immediately. The 

 former will wither sooner than the latter. Why ? 



8. Select two twigs, both from the same plant, which are of about equal 

 size and possess about the same number of healthy leaves. Cut one of them 

 off in the air, immediately immerse the cut end in water and let it remain in 

 sufficient water until it has withered away ; but arrange to cut the other off 

 under water, and take care not to expose the cut surface to the air, but keep it 

 immersed in water ; every day cut off a thin piece from the immersed end with- 

 out removing it from the water. Continue this until the leaves begin to wither. 

 It will be found that the twig thus treated will survive much longer than the 

 other. What is the reason ? 



Fig. 463. 



