268 PLANT PHYSIOLOGY 



QUESTIONS 



1. What and where are the tallest trees? How high are they? 



2. In what region of the wood (center or outside) does the water rise? 



3. How can a tree be hollow and still live? 



4. In tapping trees for maple sugar, how far into the trunk is the tap placed? 

 Why? 



5. How many atmospheres must the cohesive force of water equal in order 

 to support the water columns in the trees of question (1)? 



6. Define meniscus, cohesion, and adhesion. 



7. In the last chapter it was mentioned that leaves commonly have a higher 

 osmotic pressure than stems or roots. In the light of the past three chapters, 

 name two advantages this may be to the plant. 



8. Why does air rush into the cut ends of plants unless they are cut under 

 water? 



9. Furr reports that in the apple the speed of the flow through the vessels 

 varies with the 4th power of the diameter. Would a dendrograph attached to 

 an apple tree of the same diameter as a pine tree show more or less shrinkage 

 than the pine during active transpiration? Explain. 



10. The highest plants in the evolutionary scale have developed vessels 

 instead of tracheids in their water conducting tissue. Name two advantages 

 that vessels possess over tracheids. 



REFERENCES 



Bose, J. C. — The ascent of sap in plants. Scientia, 36:81, 1924. 



Dixon, H. H. — The Transpiration Stream, London. 1924. 



Furr, J. R. — -The relation between vessel diameter and flow of water in the 



xylem of the apple. Proc. Amer. Soc. Hort. Sci., 25:311, 1928. 

 MacDougal, D. T., and Overton, J. B. — Sap flow and pressure in trees. 



Science, 65:189, 1927. 

 Otis, C. H. — The Askenasy demonstration. Plant Physiol., 5:419, 1930. 

 Smith, F., Dustman, R. B., and Shull, C. A. — Ascent of sap in plants. Bot. 



Gaz., 91:395, 1931. 



