EXERCISE 7 



STRUCTURE OF STEMS, WITH SPECIAL REFERENCE TO 

 WATER CONDUCTION 



Materials. Cross sections of oak branch or trunk three to six 

 inches in diameter ; microscope slides with sections of woody stems 

 and herbaceous stems. 



The oak sections may be prepared by cutting a seasoned oak 

 branch into sections about one inch long and carefully smoothing 

 one cut surface with fine sandpaper. If red oak is used, the distinc- 

 tion between sapwood and heartwood will be prominent. The sections 

 may be retained as permanent laboratory apparatus if the surfaces 

 are oiled and varnished. 



Directions for work. 1. Examine the smoothed surface for 

 water-conducting tubes appearing in cross section as round pores, 

 just visible to the naked eye but easily seen with a lens. These 

 pores may be filled with dust due to the sandpapering. The 

 dust may be removed by a vacuum cleaner, or a part of the 

 surface may be smoothed with a sharp knife. 



How does the location of the pores correspond with the loca- 

 tion of the water-conducting region discovered in the preced- 

 ing exercise ? Are the pores scattered at random through the 

 branch, or are they arranged in any definite way ? If a new 

 layer of wood grows each year, what relation do the pores 

 appear to have to this annual growth? 



Count the annual layers to determine the age of the tree. 

 About how many years did it take this tree to increase one 

 inch in diameter ? 



Make a diagram of the cross section showing location of pith, 

 wood, bark, annual layers, medullary rays, heartwood, and 

 sapwood. (See textbook for definitions and for assistance in 

 identifying structures.) 



2. With the compound microscope examine thin sections of 

 woody twigs two or three years old. Identify wood, pith, bark, 

 annual layers, medullary rays. Represent the cross section dia- 

 grammatically and label the structures named above. Draw a 

 few cells from the wood, including at least one large pore. 



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