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. 



Fia. 162. — Section through a lenticel of the Elder {Samhucus nigra), e, 

 epidermis; ph, cork cambium or phellogen; I, loosely joined or packing cells 

 of the lenticel; pi, cambium of the lenticel. Much magnified. 



Third, as the twig becomes older, the bark increases in thick- 

 ness, cutting off the hght 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 cyUnder of the stem, and around this gap the 

 vascular bimdles of the bud in the axil of the leaf connect with 

 the vascular cyhnder 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 



