■48 LABORATORY STUDIES OF THE STEM 



is found at or near the surface. Throughout the looj^e celhilar tissue 

 ((/) the wood is scattered in bundles, or strands (A). Tlie 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 (00") df the monocotylcdoiious stem (x5-](i). 



2. The woody dicotyledonous stem. — Lilac. 



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

 that is, of tlie .stem not n}ore than one year old. 



Xotk: — {„) The pith. 



(/<) Tlie woiid, which seems now to be a solid ring. A high power 

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

 running out to the bark between the wood \\edges. 



(c) Tlie bark, begiiniiug at tlie outer edge of the wood. Careful 

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

 the inner bark region a liug of somewhat glistening bodies, 

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

 bundles of Jiast jihi-rx. "What is a possible use of strong libei's 

 in this positio\i in the twig'.' 



Immediately under the dark outer line of tlie bark are 

 several rows of ct>rk cclh, the examination of which mav 

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

 to the plant, of tliis layer of corkV 



Draw a sector of the cross section (00'^), to show these parts. 



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

 and three years old re.sjiectively. 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 cif Wood (Optioxae) 



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. 



(!') The difference in appearance between spring wood and fall 

 wood. A\Tiat makes the difference (use lens) ? 



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

 rays) . 



The.se features are seen on the cross-sectional face. Look on the 

 ■iither faces for tlie ends of the medullary rays and the sap ducts. 



Show by drawings the points learneil from the sliidv. 



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



