THE STEM 101 
rings of bud-scale scars if growth in length is seasonal and the 
buds are protected by scales. 
When stems continue to grow and bear leaves throughout 
the year, there are of course no rings of bud-scale scars, and it 
is not possible to tell the age of a branch 
by inspection. 
Lenticels. ‘The epidermis of the very 
young parts of a stem contains stomata. 
As the stem grows older these are re- 
placed by lenticels, which are groups of 
loosely arranged cells that allow an ex- 
change of gases between the interior of 
the stem and the external atmosphere. 
On the younger internodes these can 
usually be seen 
as brownish spots 
with raised bor- 
ders (Fig. 93). 
Arrangement of 
leaves. ‘The leaves 
occur on the stem 
in anumber of dif- 
ferent arrangements. The system of leaf 
arrangement is known as phyllotary. 
When two leaves are attached opposite 
_each other at the node, they are called 
opposite leaves (Fig. 96). On vertical 
stems each pair of opposite leaves is borne 
at right angles to the point of attachment 
of the pairs above and below it, so that 
the attachment of the leaves occurs on 
the stem in four vertical rows, or ranks 

Fia. 97. Diagram show- 
ing the arrangement of 
two-ranked leaves 

Fic. 98. Diagram show- 
ing the arrangement of 
three-ranked leaves 
(Fig. 96). When three or more leaves are attached to the same 
node, they are designated as whorled. Whorled leaves are at- 
tached above and below points between the points of attach- 
ment of the leaves of neighboring whorls, except when there 
