Question

Annual rings are distinct in plants growing in
A. Tropical regions
B. Arctic regions
C. Grasslands
D. Temperate regions

Answer 1

Hint: Annual rings can be seen when the tree trunk is being cut revealing its cross section. These rings represent the age of the tree as they are formed every year in the roots and stems of the trees and shrubs.

Complete answer:
Annual rings are also known as growth rings present in the cross section of the stem and roots of the plants. They are mostly seen in plants grown in temperate regions as they have a growth period of one year that is why they are named as annual rings. In the case of tropical regions the rings are not discernible or annual. Sometimes growth rings cannot be visible or can be false growth rings even in the temperate regions due to insect defoliation. Annual rings are distinct and clearly visible if the conducting cells are produced early in the growth period and are large as in spring and early wood as compared to those which are produced later like the summer or late wood.
In cold or temperate regions the age of the tree can be calculated by counting the number of annual rings present at the base of the trunk and in case the trunk is hollow then the rings can be found at the base of large roots. Counting these annual rings help in dating the ancient wooden structures. Several fluctuations in the ring width help to know about the changing climatic conditions. Growth rings are visible due to the variations present in the different cell types, and different arrangements of cells. The width of an annual ring, the quality of the soil, rate of cell division, initiation of ring formation, cessation for the radial growth of the year and rate and magnitude at which the cell expands. The radial diameters of the cells present in the axial system are larger during the spring season as the hormone production is high and water stress is low. The thick walled cells mark the end of the growth ring.
As annual rings require sharp climatic variations that are not available in the tropical, arctic or grassland regions so they are now present in the plants over there.

Hence, the correct answer is option (D).

Note: Under adverse conditions the annual rings are not normal as they show incomplete or discontinuous rings, missing rings, eccentric rings that show overproduction towards one side, false rings and fluted rings that show overproduction at various sites around the circumference of the rings.