1. Chapter 8 Bark / 树皮 Although the focus of this textbook is on wood and its utilization, several other components of the tree are both abundant and increasingly valuable. One of these materials is bark, a by-product of wood products manufacture. Ⅰ. Structure / 结构 Ⅱ. Chemical composition / 化学组成 Ⅲ. Moisture content / 含水率

2. Ⅰ. Structure / 结构 The term phloem （韧皮） was used interchangeably with bark in earlier chapter. This usage will now be modified slightly in recognition of the fact that phloem produced by the vascular cambium （维管形成 层） composes only the inner part of the bark layer. The origin of the outer, rough bark is traceable to activity of a second cambium, called phellogen （木栓形成层） that forms subsequent to and outside the true vascular cambium

4. 1. Inner bark Phloem （韧皮部） : produced by the lateral cambium outward Cork cambium （木栓形成层） : produced in phloem in every a few years Phelloderm （栓内层） : produced by cork cambium inward Phellem （木栓层） : produced by cork cambium outward 2. Outer bark Outer bark is all tissue outside the last-formed (functioning) periderm. Included in the outer bark are old periderms and crushed phloem tissue. Periderm 周皮

5. The bark is never as thick as the pith-to-cambium thickness of xylem beneath it. This is because (1) the vascular cambium produces far more xylem than phloem cells (on the order of 3-10 times more xylem than phloem in softwoods and hardwoods respectively), (2) thin walled and unlignified bark cells are crushed during outer bark formation, and (3) outer bark is periodically sloughed from the tree.

6. Ⅱ. Chemical composition / 化学组成 There is significant differences in chemical composition between wood and bark.  Lignin: 40-55%, much higher than that of wood (15-35%)  Cellulose: 20-35%, much lower than that of wood (40-50%)  Extractives: 15-26%, much higher than that of wood (2-9%)  Ash ： 2-5%, much higher than that of wood (<1%)

7. Ⅲ. Moisture content/ 含水率 The moisture content of bark is comparable to that of wood and often exceeds 100% of the oven dry weight. It is calculated by dividing the weight of water present by the moisture-free weight of bark. The difference in moisture content between inner and outer bark is considerable, with an abrupt change between the two layers in some species.

8. Reflection and practice: 1.General structure of bark? 2.Main differences in chemical composition between wood and bark?