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.

(N/A) $(i)$ Formation of sperms and changes in physical appearance occur.
$(ii)$ Thick hair growth on the face and voice begin to crack (deepen).
$(b)$ $A$ pollen tube grows out of the pollen grain and travels through the style to reach the ovary for fertilization.
$(c)$ $A$ zygote is formed,which is the process of fertilization.
$(d)$ Each piece grows into a complete new organism through the process of regeneration.
$(e)$ Buds may fall on the soil and develop into new independent plants.