.10.2 c i C GENERAL BOTANY 



each season upon the outer face of the wood of the previous year, 

 In the season in which any given wood cylinder was formed it ex- 

 tended from the base of the tree to the apical meristem at its apex, 

 where the new wood cylinder terminated in the meristem of the 

 apical bud of the season. A branch has been covered by the forma- 

 tion of new tissue over it, and lateral buds are shown on the exterior 

 of the tree above. The drawing thus presents a real picture of the 

 main structural features of a tree as they are revealed in a median 

 longitudinal section through the main axis and its branches. 



Growth in diameter and the formation of bark. The gradual growth 

 of the tree in diameter through the agency of the cambium is 

 illustrated in the three sections of the tree cut at different levels 

 of the main trunk (Fig. 55, J5). Fig. 55, B, a, is a section through the 

 young terminal twig as it is developing in the spring from a ter- 

 minal bud, and represents what is often called the primary, or early, 

 structure of the shoot. The epidermis is here intact and forms the 

 only protective layer of the developing shoot. The cortex and pith 

 are wide and are composed of thin-walled living storage parenchyma. 

 The vascular cylinder of phloem and xylem is narrow, and in many 

 instances forms a broken ring of vascular bundles, as seen in a 

 transverse section, with bands of parenchyma tissue joining the 

 pith and cortex. 



At a lower level of the tree trunk very material changes have 

 taken place in the above tissue layers, owing to the formation of an 

 outer corky bark and to the great increase in diameter of the vascu- 

 lar ring of phloem and xylem. The outer layer of corky bark was 

 formed by a cork cambium developed by the cell layer immediately 

 beneath the epidermis. This cell layer begins to divide during the 

 first season in the growth of most young shoots and forms a cylin- 

 drical layer of cambial cells beneath the epidermis, which then be- 

 comes transformed into dead, brown cork cells. It is for this reason 

 that shoots change their color from green to brown during the first 

 season of their growth. As soon as the cork is formed the epidermis 

 dies for lack of water and is finally sloughed off. A shoot in 

 the condition shown in Fig. 55, B, b, would present the external brown 

 bark, the green bark, or cortex, and the inner light bark, or phloem, 

 outside of the wood ring, as shown in the section of the lilac (Fig. 46). 



Structure of the tree at maturity. Fig. 55, Z?, c, shows the structural 

 features of the old stem well down toward the base of the tree. 

 The outer brown bark has here been replaced by a thick bark jacket 



