48 LABORATORY STUDIES OF THE STEM 



is found at or near the surface. Throughout the loose cellular tissue 

 (a) the wood is scattered in bundles, or strands (6). The bundles 

 are tough and add strength to the stem, and, more important still, fur- 

 nish the means by which water ascends. The sap ducts appear in 

 the cross section as large circular apertures on the periphery of the 

 bundles. 



Draw a sector (60) of the monocotyledonous stem ( x 5-10). 



2. The woody dicotyledonous stem. Lilac. 



(1) The first cross section examined should be of the end twigs ; 

 that is, of the stem not more than one year old. 



NOTE: (a) The pith. 



(6) The wood, which seems now to be a solid ring. A high power 

 of the microscope, however, would show traces of pith tissue 

 running out to the bark between the wood wedges. 



(c) The bark, beginning at the outer edge of the wood. Careful 

 looking, aided by lenses of even moderate power, will show in 

 the inner bark region a ring of somewhat glistening bodies, 

 distantly resembling a string of beads. These are the ends of 

 bundles of bast fibers. What is a possible use of strong fibers 

 in this position in the twig? 



Immediately under the dark outer line of the bark are 

 several rows of cork cells, the examination of which may 

 require the use of a compound microscope. What is the use, 

 to the plant, of this layer of cork ? 



Draw a sector of the cross section (90), to show these parts. 



(2) Make smooth cuts across the twig of Lilac where it is one, two, 

 and three years old respectively. Examine the ends with the lens. 

 In what part of the stem (what part of the cross section) is new wood 

 annually formed? 



Draw the three cross sections in diagram ( x 3). 



EXERCISE XIX. THE STRUCTURE OF WOOD (OPTIONAL) 



First, decide which side of the block furnished for examination was 

 toward the center of the trunk. Then note : 



(1) The annual additions of wood. 



(2) The difference in appearance between spring wood and fall 



wood. What makes the difference (use lens) ? 



(3) The radiating lines, crossing all the annual layers (rnea illary 



rays). 



These features are seen on the cross-sectional face. Look on the 

 other faces for the ends of the medullary rays and the sap ducts. 

 Show by drawings the points learned from the study. 

 Examine also a piece of board containing a knot. Explain the 



