134 



Science of Plant Life 



The lifting of water in 



physiology of plants has 



FIG. 77. Experiment to show the 

 lifting power of transpiration and 

 evaporation. Both tubes were filled 

 with boiled water and placed in a dish 

 of mercury. In -C the mercury has 

 been drawn up by transpiration from 

 a branch of arbor- vitae (A ) ; in D, by 

 evaporation from a porous cup (B). 



is pulled upward into the 

 Transpiration is greatest 



stems. Nothing concerning the 

 interested more people than the 

 transport of water from the soil 

 to the topmost leaves of trees. 

 Yet in spite of much observation 

 and experiment, the process is 

 still only partially explained. 



There can be no doubt that 

 one of the principal factors in 

 the rise of sap is the evapora- 

 tion of water from the leaves. 

 As the water evaporates from 

 the cells of the mesophyll in 

 transpiration, water is drawn 

 from the adjoining water-con- 

 ducting tissue of the veins into 

 these cells to take its place. 

 Water inclosed in tubes has a 

 high cohesive power ; that is, it 

 holds together like a solid. If a 

 pull is exerted on the upper end 

 of a column of water in the 

 vessels of a tree, the column 

 holds together like a cord or 

 wire, and the whole column is 

 pulled upward. As the water 

 at the upper end of the water- 

 conducting tissue moves into the 

 mesophyll cells, additional water 

 blades, petioles, and stems, 

 and the largest amounts of water 



