Question

In trees the growth ring represents
A. Primary xylem
B. Secondary xylem
C. Secondary phloem
D. Cambium

Answer 1

Hint: Since all the trees have a growth period of one year so they require new xylem and phloem every year. The xylem and phloem are not formed on their own and they are formed by dividing from the parent cell.

Complete answer:
The growth rings in the trees are also known as the annual rings. These rings are formed as the tree grows over a period of time. Since most of the trees have a growth here of one year then these annual rings are formed. The cells in the inner part of the tree get old and then move towards the outer part of the tree near the bark and as they are further developed these cells will take over the part of the bark. Generally, these rings are formed of the parts of late wood and the heartwood which we have discussed in the previous questions also.
The inside of the tree gets a new vascular cambium every year and the outside of the cambium is responsible for the formation of xylem whose function is to transport the food which is produced in the leaves to the entire parts of the tree like shoots and roots. The inside part of the cambium is developed into phloem having larger cells with then cell walls and their function is to transport water and nutrients from the roots to the leaves of the trees. When the trees grow in cooler temperatures the phloem is of lighter color due to the thin cell walls and in the summer season the growth of trees slows down and the walls of phloem becomes thin in order to prevent water loss. This is responsible for the color variations inside the tree.
The xylem as it ages it from the bark of the tree. Thus, we can conclude that the formation of annual rings is due to the variations in the phloem and xylem of the tree which are formed from the cambium of the tree.

Hence, the correct answer is option (D).

Note: Though the xylem is an outer part and does not show the color variations but it forms the part of the bark and helps in increasing the diameter and girth of the tree which also helps in the formation of outer ring.