GENERAL STRUCTURE OF STEMS 185 
forming a lenticel. Through this loose mass of cells air easily 
circulates and reaches the tissues beneath. As the twig increases 
in diameter and the bark is stretched, the lenticels are enlarged 
and, when they remain visible on the older bark, form the char- 
acteristic bands as on the older bark of Cherries and Birches. 
Z 
f i gg \) 
SN \ 
ASGESREY 
Fria. 162. — Section through a lenticel of the Elder (Sambucus nigra). e, 
epidermis; ph, cork cambium or phellogen; J, loosely joined or packing cells 
of the lenticel; pl, cambium of the lenticel. Much magnified. After Stras- 
burger. 
Third, as the twig becomes older, the bark increases in thick- 
ness, cutting off the light from the green tissue beneath, which, 
consequently, loses its green color and no longer functions in the 
manufacture of food. 
On each leaf scar there are dots, which are the severed ends of 
the vascular bundles, known as leaf traces, that branched off from 
the vascular cylinder of the stem to enter the leaf, where by pro- 
fusely branching they form the veins and numerous veinlets of 
the leaf. In turning aside to enter the leaf, the leaf traces leave 
a gap in the vascular cylinder of the stem, and around this gap the 
vascular bundles of the bud in the axil of the leaf connect with 
the vascular cylinder of the stem. (Fig. 163.) Thus through 
the branching and rejoining of bundles at the nodes, a plant’s 
vascular system becomes quite complex, looking like Figure 164. 
Stem tips are not covered by caps, as root tips are. The 
actual tips of stems are the meristematic tissues. During the 
