380 A LABORATORY MANUAL OF BOTANY 
from extremes of temperature? How do you think hairs 
can protect a leaf against excessively strong rays of light? 
How do they protect against cold and excessive transpi- 
ration? 
Examine all these leaves to see which seem best pro- 
tected against attacks from insects. Is the absence of evi- 
dence of insect attacks a positive proof that the hairs have 
protected the leaf? Upon this point examine the milk- 
weed both from surface view and by cutting the leaf, and 
report your conclusions. 
Examine the catchfly. Note the rings of sticky secre- 
tion about the stem. Will it support the body of such an 
insect as a fly or ant? See whether you can discover how 
this protects the plant. ; 
LESSON XIV 
Plant hairs, spines, and secretions 
A field lesson on points studied in the preceding exer- 
“cise should be added whenever material for the purpose is 
available. Locate as many illustrations as you can of spines, 
hairs, and secretions, and explain the particular use in each. 
case. In addition to further study of the preceding points 
try to locate some hawthorn bushes in various stages of 
growth and note stages in the evolution of the adult bush. 
The young bushes have tender leaves at the top, and these 
are eaten by animals, which procedure, when continued for 
some time, causes the entire clump of plants to become short, 
rough, and spreading. The plants continue to spread and 
to be eaten off until the central bushes are no longer within 
teach of the animals from the outside of the clump. This 
gives an opportunity for the central plants to elongate. 
Side branches that reach out too far are eaten off, while 
the central stalks become tall enough to have their terminal 
branches and leavés out of reach, and finally side branches 
