184 
STEMS 
from the leaf bearing portion of a branch to the regions behind 
where food manufacture is being abandoned, the following struc- 
Vic. 160. Twig of 
the White Walnut, 
showing leaf scars (a). 
tural features are plainly seen. 
First, there are the leaf scars, left where 
the leaves fell away, and interesting because 
of the way they are formed. (Fig. 160.) As 
the time approaches for leaves to fall, a cork- 
like layer, known as the absciss layer, forms 
across the base of the leaf, severing the direct 
connections of the leaf with the twig and re- 
maining as a covering over the scar after the 
leaf falls. The absciss layer closes the open- 
ings which would otherwise be left by the 
falling of the leaf, and thereby prevents the 
entrance of destructive organisms into the 
twig. It is in connection with leaves which 
still remain on the tree after the absciss layer 
is formed that the various autumn colors oc- 
cur due to changes in the dying leaf tissues. 
Second, there are the lens-shaped dots, 
known as lenticels, which, although common 
on the branches of all woody plants, are espe- 
cially conspicuous on 
the branches of the 
Cherry and Birch. 
(Fig. 161.) The for- 
mation of lenticels accompanies the forma- 
tion of bark. In the young twig, where 
the protective covering is an epidermis, 
air is supplied to the tissues beneath 
through the slit-like openings of the sto- 
mata; but, as the twig becomes older and 
bark is formed, the stomata are replaced 
by lenticels. Lenticels are stomata dis- 
torted and transformed in structure by the 
development of bark. Just beneath each 
stoma, instead of cork, there is formed a 
loose mass of cells, and this loose mass of 
Fic. 161.— Twig of Birch, 
showing lenticels. 
cells is pushed up into the opening of the stoma, as shown in 
Figure 162, rupturing the stoma and surrounding cells and thus 
