DISCUSSION QUESTIONS 243 



ground organs. In general, however, such an effect is to be expected only 

 when an internal water deficit exists in a plant. 



The cohesion theory of the movement of water will account equally well 

 for the conduction of water in either the upward or the downward direction 

 through the plant. If conditions are ever such that the diffusion pressure 

 deficit of the cells of roots or some other basal organ of a plant exceeds that 

 of the apical parts of the same plant a reversal of the direction of movement 

 of water in the xylem will occur. Although downward movement of water 

 can be readily induced in plants under certain experimental conditions, such 

 movements of water are probably of infrequent occurrence in intact plants. 



Discussion Questions 



1. If the water columns in the vessels are under high tensions at times of 



rapid transpiration why does not air enter the vessels through the pits in 

 their walls from the intercellular air spaces in the stem? 



2. When colored dyes are applied to the cut ends of the topmost branches of 



a tree they often move downward for considerable distances and out into 

 lateral branches. Is this behavior consistent with the idea that tensions 

 exist in the vessels? Explain. 



3. If water is pulled up through the vessels and tracheids of trees by forces 



produced in the leaves why does not water rise as freely in dead trees as 

 in living trees? 



4. Why do the leaves on a dead branch dry up while adjoining branches with 



living leaves continue to obtain water? 



5. What explanations can you suggest to account for the fact that many woody 



stems will "bleed" in the early spring but show no sign of this phenomenon 

 after the leaves appear. 



6. Suggest an experimental set-up by which the direction of the movement of 



the "transpiration stream" in a plant could be reversed. 



7. If the cohesive force of water is so great why is it not held rigidly in place 



when occupying tubes of small diameter? 



8. The "imbibition theory," formerly advocated by some botanists, held that 



movement of water occurred through the walls of xylem elements as a 

 result of imbibitional forces. How would you undertake to check upon 

 the validity of this theory? 



9. Some botanists have postulated that upward movement of water occurs 



through the stems of trees largely or entirely in the form of water-vapor. 

 Evaluate this hypothesis. 



ID. Is it logical to assume that since the redwoods are the tallest and largest 

 of trees they must have the most effective water-conducting system? 



II. Suppose the primary xylem of the young roots of a twenty year old tree to 

 be injected with a dye. Assuming that no lateral movement of the dye 

 from the xylem to adjacent tissues occurs, trace the exact course of the 

 dye as it rises through the tissues of the root, stem, and leaf. 



