At what age do boys typically reach puberty?

How can the chromosomes be identified?

$(a)$ Give the functions of the following :
$(i)$ Pollen Tube
$(ii)$ Ovary
$(b)$ List any two changes observed in the body of a female during puberty.

What is the number of chromosomes in a human sperm cell?

Why is the temperature of the scrotal sac $2\,^{\circ}C$ less than the body temperature?

$(a)$ Name the human male reproductive organ that produces sperms and also secretes a hormone. Write the functions of the secreted hormone.
$(b)$ Name the parts of the human female reproductive system where:
$(i)$ fertilisation takes place.
$(ii)$ implantation of the fertilised egg occurs. Explain how the embryo gets nourishment inside the mother's body?