How do the growth rings in a temperate angiosperm tree form and what type of cells are involved?
Answers
Growth rings, also referred to as tree rings or annual rings, can be seen in a horizontal cross section cut through the trunk of a tree. Growth rings are the result of new growth in the vascular cambium, a lateral meristem and are synonymous with secondary growth. Visible rings result from the change in growth speed through the seasons of the year, thus one ring usually marks the passage of one year in the life of the tree.
The rings are more visible in temperate zones, where the seasons differ more markedly. The inner portion of a growth ring is formed early in the growing season, when growth is comparatively rapid and is known as early wood or spring wood or late-spring wood. The outer portion is the late wood and is denser. Early wood is used in preference to spring wood, as the latter term may not correspond to that time of year in climates where early wood is formed in the early summer (e.g., Canada) or in autumn, as in some Mediterranean species.
Many trees in temperate zones make one growth ring each year, with the newest adjacent to the bark. For the entire period of a tree's life, a year-by-year record or ring pattern is formed that reflects the climatic conditions in which the tree grew. Adequate moisture and a long growing season result in a wide ring. A drought year may result in a very narrow one. Alternating poor and favorable conditions, such as mid summer droughts, can result in several rings forming in a given year.
In the tropical areas of the world, trees can grow more continuously and so the rings are layered down less uniformly, because the start of a ring with uninterrupted of growth may be much longer than the temperate zones spring and summer. Since tree growth in tropics is less dependent of seasonal variations the ending the the growth period and subsequent terminus of a ring is very often less pronounced, growth may go on for longer periods of time or stop and commence a number of times with in a short period of time.
So, annual rings are distinct in plants growing in temperate regions and not in arctic regions, grass lands and tropical regions.
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