STRUCTURE OF STEMS 35 



5. Taking the end of the petiole in one hand and the 

 leaf-blade in the other, gently pull the two portions 

 of the petiole a short distance apart. Describe and 

 illustrate by a drawing what you observe. What 

 structures are thus disclosed? 



6. What relation do the fibro- vascular bundles bear 

 to the veins of the leaf? To the root-hairs? 



7. Write a clear statement of how the water passes 

 from the soil into the roots of a plant, and into and 

 through the leaves and out into the air, mentioning, 

 in order, all parts and processes studied. 



B. Internal Structure of an Endogenous (Monocotyledonous) 

 Stem: 



1. Examine with your naked eye a thin cross-section 

 of a corn stem. What general features are to be ob- 

 served? What relation do the dots, seen in this 

 cross-section, bear to the fibro- vascular bundles? 

 What then are these dots? Now study the section 

 under the hand lens and compare the number of 

 bundles near the center and near the periphery. 

 Where are they most numerous ? What mechanical 

 advantage do you see in the observed arrangement 

 of the bundles? The cells intermixed with the bun- 

 dles are pith cells. What kind of tissue? How far 

 from the center do they extend toward the periphery ? 



2. Make a drawing of the cross-section, as* seen under 

 the hand lens, about 10 cm. in diameter, then ex- 

 amine the section under the microscope. Illustrate 

 with a drawing at least 12 cm. wide, the cross-section 

 of a vascular bundle located near the center of the 

 stem. The yellowish ring of cells around the out- 

 side of the bundle is the bundle sheath. What 

 features of the sheath make the color so noticeable? 



