134 



Science of Plant Life 



The lifting of water in stems. Nothing concerning the 

 physiology of plants has 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 Hke a solid. If a 

 pull is exerted on the upper end 

 of a column of water in the 

 vessels of a tree, the column 



Fig. 77. Expemnent to show the 1 i-i 



lifting power of transpiration and holds together hke a COrd Or 



evaporation. Both tubes were fiUed -^[xe^ and the whole Column is 



with boiled water and placed in a dish mi i ^ J^ 



of mercury. In C the mercury has PuUcd UpWard. As the Water 



been drawn up by transpiration from at the Upper end of the Watcr- 



a branch of arbor- vitse (^); in D, by 1 , • .• • j. j.1. 



=„„„„,of„„ f „ „o rm conductmg tissue moves mto the 



evaporation from a porous cup {Bj. o 



mesophyll cells, additional water 

 is pulled upward into the blades, petioles, and stems. 

 Transpiration is greatest and the largest amounts of water 



