24 
LABORATORY EXERCISES 
EXERCISE 26 
THE INTERNAL STRUCTURE OF A STEM 
I. Sunflower or Pumpkin Stem. (Dicotyledonous Stems) 
1. Note on a portion of the stem the regular succession of nodes 
and internodes. 
2. Cut thin cross sections through the internodes. Using a lens 
when necessary, examine carefully. If present, study the pith in 
the central region. Note its character. Find the circle of rather 
oval or wedge-shaped structures which are situated about the 
central region. These are the fihrovascular bundles. (The narrow 
regions of tissue between these bundles give rise in perennial plants 
to the primary medullary rays. See Waggoner, Page 124.) Out¬ 
side of the circle of the fihrovascular bundles are the tissues which, 
taken together, make up the cortex. The outer skin-like layer is 
the epidermis. 
3. Fihrovascular Bundle. Examine a fibrovascular bundle under 
the lens. The inner part is the xylem; the outer, the phloem. 
Between the two is the region of the cambium. Remove from a 
longitudinal section one of the fihrovascular bundles. Examine it. 
Test its strength. 
4. Draw a cross section of the stem which was studied. 
II. Indian Corn. (Monocotyledonous Stem) 
1. Examine a thin transverse section. How do the fihrovascular 
bundles differ in position from those of the sun-fiower or pumpkin? 
2. Remove the epidermis and cortex of an Indian corn stem. 
Remove one of the fihrovascular bundles from the pith. Test its 
strength. What is the nature of the pith? 
III. What are the uses of the fihrovascular bundles? Of the 
bark? Why does a girdled tree die? What purposes do stems 
serve? 
