EXERCISE 73 



There are ducts, or vessels, through which fluids may pass in the wood part of a dicot 

 stem and in the bark part of the stem. Some of these ducts carry material up from the 

 roots to the leaves ; others carry material from the leaves to the roots. . 



Problem. Which currents are carried by the vessels of the bark ? 



What to use. Two twigs of willow ; jar of water ; sharp knife. 



What to do. Cut off carefully a girdle of green bark about an inch long, near the 

 lower part of one twig ; place both twigs in water for a few weeks and observe from time 

 to time. Keep replacing evaporated water, so that the water remains at a constant level. 



Record. Note any changes that take place in any part of either stem, or at the upper 

 or the lower edge of the cut surface. From the results observed, tell whether the bark 

 vessels carry materials up the stem or down. 



i 



Questions. 1. Explain reason for any change noted in the above experiment. 



2. Why does a tree die when the bark is girdled ? Why does it not die immediately ? 



3. Why didn't the willow twig die after girdling ? 



4. Is maple sirup obtained from the ascending sap or from the descending sap? How can you tell? 



5. Trace the upward and downward course of sap in a plant 



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