38 ANATOMY AND PHYSIOLOGY 







you consider the chief structural distinctions 

 between the corn stem and this herbaceous stem? 

 4. Make a diagram of the cross-section as seen under 

 the hand lens, labeling the tissues as thus seen. 

 It is to be noted that endogenous (monocotyledo- 

 nous) herbaceous stems are essentially like the corn 

 stem in cross-section. 



D. Primary Growth in a Woody Exogenous (Dicotyledonous) 

 Stem: 



1. Examine with a hand lens a cross-section of a young 

 Aristolochia, or other woody stem, which has not 

 yet developed tissue from the cambium layer. 



jThe arches of tissue near the periphery (in Aristolo- 

 chia), as seen under the hand lens, form the outer 

 boundary of the vascular bundles. How and where 

 are the latter arranged ? How many are there in the 

 whole cross-section? In what general respects 

 does this stem differ from that of the corn? How is 

 it like the herbaceous stem? Make a diagram of 

 the section. 



2. Examine the section under the microscope. The 

 bundles are seen to be somewhat wedge shaped. 

 The inner part is the xylem, and the outer (toward 

 the periphery) the phloem . The xylem is r ecognized 

 by the large tracheae. The cambium is also present. 

 Where would you look for it? Make a careful 

 drawing of a portion of the cross-section, including 

 one complete bundle and at least a quarter of each 

 bundle on each side; this will be a V-shaped sector 

 with the point at the center of the stem-section and 

 the arc at the periphery. Radiating from the center 

 and between each pair of bundles are the medullary 

 rays. How are these related to the pith at the cen- 

 ter? Into what tissue do they merge toward the 

 periphery? The xylem is composed of the tracheae, 



