STKUCTL'RK OF STEMS 41 



Stem of Dicotyledonous Plants 



26. Gross structure of an annual dicotyledonous stem.** 



A. Study the external appearance of a piece of sunflower stem 

 several inches long. If it shows distinct nodes, sketch it. 



B. Examine the cross section with the lens and sketch it. 

 After your sketch is finished compare it with Principles, Fig. 

 56, which probably shows more details than your drawing, 

 and label the parts shown as they are labeled in that figure. 



C. Split a short piece of the stem lengthwise through the 

 center and study the split surface with the lens. Take a 

 sharp knife or a scalpel and carefully slice and then scrape 

 away the bark until you come to the outer surface of a 

 bundle. 



D. Examine a vegetable sponge {Luffa), sold by druggists, 

 and notice that it is simply a network of fibro-vascular 

 bundles. It is the skeleton of a tropical seed vessel or fruit, 

 very much like that of the wild cucumber common in the 

 central states, but a great deal larger. 



Structure of bark. The different layers of the bark cannot all be well recog- 

 nized in the examination of a single kind of stem. With lens examine : 



E. The cork which constitutes the outer layers of the bark of cherry or 

 birch branches two or more years old. Sketch the roundish or oval 

 lenticels on the outer surface of the bark. How far in do they extend ? 



y. The green layer of bark as shown in twigs or branches of Forsythia, 



cherry, alder, box elder, wahoo, or willow. 

 G. The white, fibrous inner layer, known as hard bast, of the bark of elm, 



leatherwood, or basswood. 



27. Minute structure of the ordinary dicotyledonous stem. Cut thin cross 

 sections of the stem of one of the perennial species of sunflower (Helianihus), 

 or any large composite. Stain by immersing for a few seconds in a half- 

 saturated aqueous solution of safranin, then wash, and examine in water, 

 first with l.p. and then with m.p. The structural elements of the stem are 

 considerably differentiated by the stain, the outer layers of the cortex ap- 

 pearing yellowish brown, the hard bast magenta, the wood fibers reddish 

 magenta, and the jjith salmon color. 



References. Strasburger-Hillhouse, 6; Strasburger, Noll, Schenck, 

 Karsten, 1. 



