24 STEMS 
twig under observation? ‘They are more clearly seen in 
elm than in horse-chestnut. Cut an elm twig about I cm. 
in diameter, off 3 or 4 cm. above and the same distance 
below an annular scale-leaf-scar ring. The layers 
can be seen better if they are cut obliquely. See that 
the cut ends are smooth. Let them dry a few minutes. 
How many layers of wood do you find above the annular 
ring; how many below ? What does this fact teach? Nu- 
merous small holes may be seen in the layers of wood. 
In which part of each layer are they largest? Can you 
account for this? Have the instructor explain. 
Make an outline drawing of a cross section of a horse- 
chestnut stem (x2). 
Cut cross sections of the following stems, similar to 
those cut of the horse-chestnut; study these sections as 
you did those of the horse-chestnut, and make an outline 
drawing of each: beech, elder, and geranium, or any stem 
that has grown this year, and butternut or walnut. The 
central portion of the butternut or wainut stem will be 
found to consist of dark colored wood. This is called 
heart wood; it is dead. The light colored wood sur- 
rounding it is living and is called sap-wood. 
Note the surface character of the bark (brown bark) 
on all the different trees on the campus. What causes 
it to become much rougher as a stem becomes thicker? 
Does any fall off from year to year?- How is it replaced? 
Peel some of the brown bark from a cherry twig. Note 
the white spots on it, called lenticells. Do they vary in 
form in different parts of the stem? What causes this 
difference in form? 
Exp. 17, Groups of Two: Place three pieces of wil- 
