
THE STEM 
155 
of the cells that are exterior to the cork results in their death, 
and after a time they peel off and disappear. 
The first-formed phellogen does not func- 
Lenticels. Stomata 
allow foranexchange 
of gases through an 
epidermis, and in the 
same way the len- 
ticels allow for an 
exchange of gases 
through the cork. 
Lenticels are formed 
under the stomata 
(Fig. 151) and consist of radial rows of 
cells with intercellular spaces (Fig. 152). 
Like the cork cells they are formed from 
the phellogen. The intercellular spaces 
allow for an exchange of gases. 
Leaf fall. The fall of leaves is brought 
about by meristematic activity of cells 
across the base of the petiole. These pro- 
duce layers of parenchyma cells which 
Separate and cause the leaves to fall 
(Fig. 153). The scars are protected by 

Fig. 153. Abscission 
layer at base of a mul- 
berry leaf (Morus alba): 
(x1) 
tion indefinitely but is replaced by another, 
produced in the underlying tissue. The 
second phellogen is likewise replaced by a 
third, and so on until the phellogen may 
come to be formed in the secondary phloém. 
As the older parts of the bark tend to peel 
off, the bark of old trees may consist only 
of secondary tissues, the cortex, pericycle, 
and primary phloém having been shed. 

Fig. 154. A girdled tree 
trunk, showing greater 
erowth above than below 
the wound 
lignification and suberization of the exposed cells and by the 
formation of a layer of cork continuous with that which covers 
the stem, 
ae i Fico 
