Secondary Xylem Development(II)-Angiospermous Trees ���ݺ���

����It is generally considered that xylem of main stem�� roots and branches of a tree is its woody part. Xylem is an academic term called in life science�� wood is the name for its application as a material. According to the sources of meristematic cells�� xylem is divided into categories of primary and secondary xylem. The primary xylem is located near the pith�� which is in the center of stem. The primary xylem is very thin�� and it is hardly distinguished from the true pith by a magnifying glass. The combinative volume ofthe primary xylem and the true pith is very small�� and remains the same�� but the secondary xylem increases outwards year by year�� and becomes the main part of xylem.����The life period of the major component cells of secondary xylem is only a few months. However�� the life of secondary xylem is long as the tree�� which is reflected in the successive formation ofwood cells. Secondary xylem development is a self-changing process throughout its life controlled by heredity. For a tree�� wood formation takes place in the developing process of secondary xylem. Secondary xylem development and wood formation are two aspects of the same process. Wood formation could be realized from three hierarchies�� including the ultramicro�� the micro and the macro hierarchies. The micro hierarchy is about the formation of major component cells of wood. It involves cell division�� cell volume increase and feature formation of cell wall. The ultramicro hierarchy is about systhesis and precipitation process of organic ingredients of cell wall�� i.e. the final construction forming. Many achievements about wood formation have been got in disciplines of forestry�� fiber chemistry and wood science in the ultramicro and micro hierarchies. The wood differences within an individual tree have been called "variation" in forestry and wood science. The study about this respect is still limited to the structural and performance differences of some appointed local positions within an individual tree�� however�� the formation mechanism of this difference has not been involved. This lack of research has led to that a phenomenonsecondary xylem development�� which is very important for forestry science�� has been covered up for a long time. The fact that wood is formed in the long term life change of secondary xylem has not been disclosed. This subject is the study on the special theme�� and the essence ofits result is an important part of wood formation mechanism.