A MANUAL OF BOTANY 27 



Cross sections of dicotyledons. ISIake clean-cut cross sec- 

 tions of the various stems. Observe in the ordinary dicoty- 

 ledonous stem (as the sumac) a central soft part, inth; an 

 outer part, hark (at first epidermis), and between these a 

 layer of hard material, wood, which again may be subdivided 

 by porous rings into other layers. 



Observe that the larger and therefore the older the stem, 

 the more abundant these latter layers. What is the explan- 

 ation that you could give ? Running from the pith toward 

 the bark are radiating lines (pith rays). Compare the elder, 

 sumac, and oak, and measure the proportion of pith in each. 

 Why is the pith smaller in amount as the stem increases 

 in age and size ? 



Examine (m) the bark and find three layers, white, green, 

 and the outside brown or corky. Study long sections of stems 

 at the junction of branches and find the layers above named. 

 Make careful drawings, recording all the features of the 

 dicotyledonous stems. 



Section of monocotyledons. Examine now a cross section of 

 a monocotyledouous stem and find three features : piY/i, ivood 

 bundles, and outer cortex. Observe the great difference in 

 the arrangements of the parts in the two kinds of stems. 



Where in each stem is the hardest portion ? 



What occupies the position of the true bark in monocoty- 

 ledouous stems ? 



What evidence, if any, to indicate age ? 



Make drawings of cross and long sections of a monocoty- 

 ledouous stem. 



Microscopic structure. Study thin sections of botli kinds 

 of stems (Ij)), to learn about the cell structures. Observe 

 the great difference in size, shape, and cell walls of the ^)?7/i 

 and ivood. 



Make drawings of a quadrant of these thin sections, show- 

 ing the different kinds of cells and cell combinations. Turn 

 to the fern study for the details of vascular bundles (wood). 



