Wood Formation - 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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