AN INTRODUCTION TO PLANT BIOLOGY 



93 



root-pressure alone is not enough to force water to the top 

 of tall trees, and it does not move water upward fast enough 

 to make good the loss by evaporation from the leaves on a 

 hot day. There must be other factors in the elevation of 

 water. 



Another possible explanation is suggested 

 by the following experiment : 



(D) "Suction" Force of Evaporation from 

 Leaves. Cut off * a shoot (stem with leaves) of 

 geranium or other potted plant and, with a short 

 piece of rubber tubing, attach to a glass tube. 

 Fill the tube with water and hold under water 

 while attaching the plant stem. See Fig. 37. 

 The rubber mus-t be firmly wrapped with cord so 

 as to bind it to the shoot and to the glass. It is 

 well to spread a little vaseline around the two 

 ends of the rubber. Support tube with a retort- 

 stand so that the lower end dips into mercury in 

 a small dish or bottle. As water is evaporated 

 from the leaves, the mercury will rise in the tube. 

 If mercury is lifted one inch, it indicates a force 

 able to lift water 13.6 inches. In this way it has 

 been determined that some shoots will lift water 

 several meters. Evidently there is great lifting 

 power or suction force caused by evaporation 

 from the shoot. 



FIG. 37. Mercury 

 (m) lifted in glass 

 tube by evapora- 

 tion of water from 

 leaves. Stem 

 closely fitted to 

 tube by cork at c. 

 (From Detmer.) 



That the lifting power seen in the leafy shoot is due merely 

 to evaporation and not to some force peculiar to living 

 plants may be demonstrated by tying a bladder or other mem- 

 brane over a thistle-tube filled with water and suspended 

 vertically so as to dip into a cup with oil or mercury. As 



* For this and similar experiments it is best 'to bend the stem so as to 

 have the part where the cut is made under water at the moment of cutting. 

 This prevents the entrance into the tubes of the stem of air bubbles which 

 impede the passage of water just as they can be seen to do in a glass tube 

 of small bore. Flowers cut in this way will not wilt as soon as those cut off 

 in the air. (Cut five flowers, or leafy shoots, each way, stand in water and 

 compare as to their wilting after some days.) 



