Question

Annual rings and growth rings are formed due to fluctuation in the activity of
A. Xylem
B. Phloem
C. Xylem and Phloem
D. Cambium

Answer 1

Hint: Some plants mainly dicotyledons exhibit an increase in girth. This mechanism is called secondary growth. One of the tissues involved in secondary growth is responsible for annual rings and growth rings.

Complete answer:
Annual rings and growth rings are formed due to fluctuation in the activity of cambium. Cambium is the meristematic layer responsible for cutting off vascular tissues – xylem and phloem. Initially, it is present in patches as a single layer between vascular bundles. Later during the development, it forms a complete ring.
Cambium is very active in the spring season resulting in a large number of xylary elements having vessels with wider cavities. The wood formed during this season is called spring wood or early wood. The cambium is less active in winters and forms fewer xylary elements having narrow vessels, and this wood is called autumn wood or late wood. The spring wood is lighter in color and has a lower density as compared to the autumn wood that is darker and has a higher density. The two kinds of woods that appear as alternate concentric rings, constitute an annual ring.
So, the correct answer is (d) ‘Cambium’.

Note:
- Annual rings give an estimate of the age of trees.
- The Cambial ring cuts off new cells towards both inner and outer sides.
- The activity of cambium is dependent on many physiological and environmental factors.