Xylem  - Xylem is not wood.  Xylem is a transport tissue in vascular plants.  It transports free water and substances dissolved in it, from absorbing non-woody roots to leaves.  When xylem is lignified it is then correctly called wood.  Lignified means that high amounts of the natural "cement" called lignin is deposited within the cellulose strands in the cell walls.  This makes the cell walls very tough.  Having tough, lignified cell walls is a unique feature of trees.  Xylem is born from the cambium zone.  Xylem rays connect radially to form phloem rays. The phloem rays connect circumferentially to form the phellogen. 
    In some tree species xylem and phloem cells do not begin to form until the leaves are halfway through their development.  When the leaves are fully formed the xylem begins to become lignified.  The first xylem cells have no lignin and the cells are like jelly.  This is why bark can be removed easily in spring when leaves are forming.  After the formation of the leaves more xylem and more lignification of xylem to form wood takes place.  In most trees about 90 % of the growth increment will be formed in about 6 to 8 weeks after leaf formation.  There will always be exceptions.  The current growth increment (cg) does not store starch until near the end of the growth period.

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