During adolescence,several changes occur in the human body. Mark one change associated with sexual maturation in boys.

Explain what happens when:
$(a)$ Testosterone is released in males.
$(b)$ Pollen grain falls on the stigma of the flower.
$(c)$ Egg fuses with sperm cell.
$(d)$ Planaria is cut into many pieces.
$(e)$ Buds are formed on the notches of the Bryophyllum leaf.

In the diagram of the human male reproductive system given below:
$(a)$ Label parts $A$ and $B$.
$(b)$ Name the hormone produced by organ $X$. What is the role of this hormone in the human male?
$(c)$ Mention the name of substances that are transported by tubes:
$(i)$ $C$ and $(ii)$ $D$.

Why are the testes located outside the abdominal cavity? Mention the endocrine and exocrine function of the testes.

What happens if the egg is not fertilized?

In which plant does vegetative propagation occur through leaves?