After the megaspore of Pinus germinates, it goes through a series of developmental stages before forming the endosperm with archegonia.



The megaspore of Pinus germinates to form a multicellular female gametophyte called the prothallus.



The prothallus grows and develops into a mature female gametophyte. It consists of a small number of cells and is located within the archegonial chamber, which is surrounded by the nucellus.



Within the mature female gametophyte, one or more archegonia are formed. Archegonia are specialized structures that contain the egg cells or oospheres.



Once the female gametophyte is mature and the archegonia are formed, pollination takes place. Pollen grains released from the male cone are transferred to the female cone, where they land on the ovule.



Upon landing on the ovule, the pollen grain germinates and produces a pollen tube. The pollen tube grows towards the archegonial chamber, guided by chemical signals.



The pollen tube enters the archegonial chamber and delivers two motile male gametes. These male gametes are released into the archegonium, where they fertilize the egg cell.



After fertilization, the zygote develops into an embryo, and the primary endosperm nucleus within the archegonium undergoes multiple divisions to form a triploid endosperm. This endosperm provides nourishment to the developing embryo.



The endosperm and embryo continue to develop, and the seed matures within the female cone. Eventually, the cone opens, and the mature seed is released.

Therefore, in Pinus, the megaspore after germination develops into endosperm with archegonia, as the archegonial chamber contains the archegonia with the egg cells, and the endosperm provides nourishment to the developing embryo.