Hint: It is during the secondary growth of the plant that it increases in diameter and forms annual rings.
- Growth rings are the rings of growth found in trees. The ring also shows the increment of wood added in a single growth period. The tree rings grow under the bark, and the bark is pushed out when the tree is growing. The ring pattern formed in the trees also tells about the climatic conditions in which the tree grew.
- For example, if the tree grew in a dry place, the rings are visibly better than from the tree which grew in a wet place. Switching between good and bad climatic conditions can result in the formation of several rings in a year. It may appear different if the growth is terminated by a layer of thick-walled fibers or by parenchyma. The tree ring growth patterns are affected by several factors like the climate, weather, rain, temperature, soil, pH, plant nutrition, $CO_2$ concentration, and sunspots.
- The increase in the girth of a plant is due to secondary growth. The tissues involved are lateral meristems, vascular cambium, and cork cambium. The vascular cambium passes between the vascular tissues, xylem, and phloem. In a young stem, it is present as patches and later it forms a ring.
- The growth occurs in the cambium which creates new tissue and increases the girth of the tree. Phloem cell, present outside the cambium, carries the food to the branches, trunk, and roots. Some of the phloems die each year and become part of the outer bark. Xylem, which is present in the inside of the cambium, helps in tree growth in diameter. It carries water and nutrients from roots to the leaves.
- At first, the springwood is formed which is light-colored and the xylem cells are large and thin. Later, summerwood is formed which is the dark part of the ring. Here, the xylem cells are thick. Both the woods form the single growth/ annual ring. The activities of the vascular tissues are produced by cambium. Hence, the cambium is responsible for the annual/ growth ring.
Hence, the correct option is A, ‘Cambium’.
Note: The annual rings are formed by the activity of cambium and not because of xylem and phloem. The vascular cambium is the lateral meristem which gives rise to the secondary phloem and secondary xylem.