How Growth Rings Reveal Plant Age and Environmental Changes
Tissues are the group of cells that are present together to perform a specific function. The tissues are responsible for the growth of the plant. Two types of growth are present in plants. They are primary growth and secondary growth. To study the secondary growth we need to understand the growth ring pant anatomy. The plants that show secondary growth have the presence of growth rings on them. These are derived from the secondary tissues of the plant. Permanent tissues are then formed from these secondary tissues.
Parenchyma
The parenchyma is a simple permanent tissue. It is present in almost all the organs of the plant. It is present in roots, leaves, shoots, buds, and almost each and every organ. The parenchyma is responsible for forming up the main bulk of the plant body. The cells of the parenchyma are known as parenchymatous cells. No protoplasm is present in these cells. They are known as living cells. Isodiametric cells are seen in the parenchyma. This means that these cells have equal diameters. The cells can be in the shape of spherical, oval, or round. These are marked by the presence of thin walls. Cellulose is mainly deposited in the cell wall of parenchymatous cells. There are no intercellular spaces present inside the parenchymatous cells. They help in avoiding the defoliation process. Defoliation is the process by which black spots are developed on the surface of leaves. When defoliation occurs, the leaves start to fall off from the plant. Nectar and oil are secreted by the parenchymatous cells and these are released by the plant body also. The parenchyma plays a major role in the storage of food. They help in storing carbohydrates and proteins. The parenchyma tissues also help in the process of photosynthesis.
Secondary Growth
In the secondary growth of plants, the growth ring plant anatomy plays an important role. They are also known as cambial rings. The vascular bundles start to get themselves arranged in a ring-like manner. They are open in dicot plants. The term “open” means that the cambium is open to growth in these tissues. The cambium grows and then helps in developing the secondary tissues. In the young stages, these rings are present in patches but at the later stages of life, they are present in a continuous form. There are medullary rays that are present between the vascular bundles. These cells are dedifferentiated and by this process, they help in forming the secondary tissues. The growth ring or the vascular cambium is formed by the intrafascicular and interfascicular cambium. This growth ring then becomes active and then they begin to divide and form new cells. These new cells are formed both inside and outside with the help of periclinal divisions. These growth rings are more active on the inner side as compared to the outer side. Towards the periphery, there is an expansion of the cambium rings. The primary xylem is kept intact and there is an amount of increase in the secondary xylem. Due to this increase in the amount of xylem, the amounts of phloem keeps on crushing.
Growth Rings
The activity of these rings is affected by the different temperatures of the year. Mainly in Asia and Central North America continent, we see a difference in temperature so we observe that the growth of the cambium rings is affected. These cambium rings are very active in the month of the spring season. There is an increase in the rate of transpiration and due to this a large amount of water is needed for the growth. These vessels are generally produced with large amounts of lumens. In the winter season, we see the opposite of this happening. In the Winter season, the growth of the cambium ring is slowed down. This is because the cambium is less active in the month of winters. The wood that is formed in the spring season is known as springwood and the wood that is formed in the winter season is known as autumn wood. The springwood is low in density and also light in colour whereas the autumn wood has high density and is dark in colour. The number of rings in the trees helps in determining the age of the tree. The tree that has more number rings means that it is more in age. This study of rings and then determining the age of the plant is known as dendrochronology. These rings are more often seen in trees that grow in temperate regions. The regions that have uniform temperature and rain do not show these cambial rings. Due to constant temperature, we see no activity of the cambium so no rings are formed in this case.
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FAQs on Growth Ring: Definition, Functions & Types
1. What are growth rings in woody plants?
A growth ring is a layer of wood produced by a tree or shrub during a single growth period. In a cross-section of a stem, these rings appear as concentric circles. Each ring is formed by the vascular cambium, a layer of meristematic tissue that produces new xylem cells, causing the stem to grow in diameter.
2. How are annual rings formed in trees from temperate regions?
In temperate regions, the activity of the vascular cambium is influenced by seasonal changes. This results in two distinct types of wood being formed within one year:
- Springwood (or Earlywood): Formed during the spring season when the cambium is highly active. It has larger and wider xylem vessels to transport more water for new growth. It is lighter in colour and less dense.
- Autumnwood (or Latewood): Formed during late summer or autumn when the cambium's activity slows down. It consists of narrower, thick-walled xylem elements, is darker in colour, and is denser.
These two types of wood appear together as one concentric annual ring.
3. What is the difference between springwood and autumnwood?
The primary differences between springwood and autumnwood lie in their structure and function. Springwood is characterised by wider vessels and a lower density, formed for efficient water conduction during peak growth. In contrast, autumnwood has narrower vessels, thicker walls, and higher density, providing more structural support. This anatomical difference creates the visible boundary of an annual ring.
4. What are the major applications of studying growth rings?
Studying growth rings has several important applications, primarily in a field known as dendrochronology. The key uses include:
- Age Determination: Counting the number of annual rings accurately determines the age of a tree.
- Climate Analysis (Dendroclimatology): The width and density of rings can reveal past climatic conditions. Wide rings suggest favourable conditions (good rainfall, temperature), while narrow rings indicate stress (drought, cold).
- Dating Historical Events: Growth rings can be used to date archaeological sites, old buildings, and wooden artifacts by matching ring patterns.
5. Are growth rings and annual rings the same thing?
Not always. A growth ring represents a period of growth, which may or may not be one year. An annual ring is a specific type of growth ring that is formed over a single year. In temperate climates, the terms are often used interchangeably because one growth ring typically corresponds to one year. However, in tropical regions with no distinct seasons, trees may form multiple, less-distinct growth rings in a year, or none at all.
6. Why are growth rings absent or less distinct in tropical trees?
Tropical regions generally have a uniform climate throughout the year with minimal temperature and rainfall fluctuations. Because of this, the vascular cambium remains active at a relatively constant rate. There is no sharp contrast between the wood formed at different times of the year, unlike the distinct springwood and autumnwood cycle in temperate climates. This leads to the absence of clear, well-defined annual rings.
7. How do growth rings relate to the concepts of heartwood and sapwood?
Growth rings, heartwood, and sapwood describe different aspects of a tree's trunk. Growth rings are patterns of seasonal growth. Sapwood is the outer, lighter, and physiologically active part of the wood that conducts water. Heartwood is the central, darker core that is non-living and provides mechanical support. Both sapwood and heartwood are composed of growth rings. As the tree ages, the older, inner sapwood transforms into heartwood.
8. Can monocot plants like bamboo or palms form growth rings?
No, monocot plants cannot form true growth rings. This is because they lack a key tissue called the vascular cambium, which is responsible for secondary growth (increase in girth) in dicots and gymnosperms. The vascular bundles in monocots are scattered throughout the stem and do not form the concentric layers of wood that result in visible growth rings.
