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TEXTBOOK OF BOTANY 



exemplified by the carrot-root demonstration without any artificial man- 

 ipulation of the structures involved. In other words, guttation may be 

 the result of the processes by which pressure is built up in the xylem 

 vessels of the root system by the osmotic absorption of water across the 

 complex membrane of tissues external to the xylem. This pressure may 

 become great enough to push water up the vessels and out of the ends 

 of the veins of some of the leaves of a plant. 



Fig. 134. Guttation in leaves of tomato in a moist atmosphere. Photo from J. H. 



Gourley. 



During early autumn, when the soil is still warm and moist and the 

 atmospheric humidity is near saturation, drops of water exude from the 

 points on the leaf margin of oaks, maples, cottonwoods, and other trees. 

 This is another example of guttation, often mistaken for dew. 



Absorption of water by pull of transpiration and osmosis. The phe- 

 nomena of "bleeding" from cut stems and guttation have sometimes led 

 to the conclusion that the pressure developed in roots is the cause of the 

 upward movement of water in tall stems under all conditions. 



However, if a hole is bored through the bark of a tree and an inch or 



