PROTECTION AGAINST HEAT, COLD, AND ANIMALS 31 
enjoy the same exemption. These then shade the shorter 
plants, which have been eaten off. They soon die, and grad- 
ually disappear. 
Find hawthorn plants showing these stages in the devel- 
opment of the adult bush, and note the character and posi- 
tion of the thorns in each case. 
A careful study of other thorn-bearing shrubs and trees 
will furnish excellent illustrations of the significance of 
thorns among plants. 
LESSONS XV AND XVI 
Evergreen and deciduous trees 
Materials.—Some branches and leaves of pine, spruce, ce- 
dar, arbor vite, and of beech or elm or any deciduous plant. 
Observation and study.—Compare the number, form, 
relative amount of exposure, and general solidity of leaves 
from each specimen. Draw a surface view of each leaf on 
a uniform scale. How many pine leaves are required to 
make as much chlorophyll exposure as has one elm or beech 
leaf? Do you think the number of pine leaves on a branch 
is enough greater to give it as much chlorophyll exposure 
as there is in a similar branch from an elm? 
Make cross-sections of a pine leaf, mount and examine 
under low power of the microscope. Note the following 
points: 
The general form of the cross-section. 
The heavy epidermal layer, and just beneath it other 
heavy walled cells resembling the epidermis. These together 
serve as the protecting and strengthening tissue of the leaf. 
The chlorophyll-bearing cells. Note their arrangement. 
The resin-ducts surrounded by the chlorophyll tissue. 
The wall of each duct is made up of light-colored sheath- 
cells. Is there any regularity in the number and position of 
the resin-ducts ? 
