94 APPLIED BIOLOGY 



rapidly as the water evaporates through the membrane, the 

 oil or mercury rises in the tube. The apparent " suction " is 

 the same as in a pump, and is to be explained by atmospheric 

 pressure on the mercury or oil forcing it up the tube as rapidly 

 as the water evaporates. Possibly the same explanation 

 applies to the evaporation of water from the leaves in the 

 above experiment. 



It has been stated that root-pressure alone does not ex- 

 plain why water ascends stems of high plants, because 

 water sometimes evaporates more rapidly than root-pressure 

 can force it up. The last experiments, showing that evapora- 

 tion of water may exert great lifting force, suggest to us 

 that evaporation from the leaves of plants may suck or 

 lift water up the stems. This is certainly one important 

 factor in causing the ascent of water from the roots to the 

 leaves; but evaporation alone could not lift water up the 

 highest trees. 



Root-pressure and evaporation from the leaves are the 

 two best suggestions concerning the rise of water up the stems 

 of plants; but botanists admit that these together do not 

 explain how a plant can lift water as high as do the highest 

 trees. This is only one of many hundreds of things in science 

 which it has not been possible to explain satisfactorily; 

 and in the attempt to get more knowledge hundreds of scien- 

 tific men are constantly trying experiments in new ways. 



92. In What Part of Stem does Water Ascend? We 

 have already examined ( 69) a stem of bean plant and found 

 rind or bark, wood, and pith. Does the water go up in 

 all these regions of the stem? We can best answer this 

 question by an experiment. 



(D) Cut off a shoot (stem with leaves) of a nasturtium, balsam, 

 corn, bean, or other plant, and place the cut end of the stem in a 

 small bottle with red ink (solution of eosin in water). Watch the 

 rise of the ink along the stem into the veins of the leaves. Now 

 cut across the stem in various places and note that the red color is 



