Gymnosperms are a group of seed-bearing plants that do not produce flowers. They include conifers, cycads, Ginkgo biloba, and gnetophytes. In gymnosperms, the process of haploid endosperm formation is different from angiosperms, which have double fertilization. Instead, gymnosperms have a unique mechanism for endosperm development.


The first step in haploid endosperm formation in gymnosperms is the formation of the female gametophyte, called the embryo sac or megagametophyte. The embryo sac develops within the ovule, which is enclosed in the ovary of the gymnosperm.


The female gametophyte in gymnosperms is typically haploid, containing only one set of chromosomes. It is formed through the process of megasporogenesis, in which one of the megaspores produced by meiosis within the megasporangium develops into the embryo sac. The surviving megaspore undergoes several rounds of mitosis to produce the mature embryo sac.


In gymnosperms, pollination occurs when pollen grains are transferred to the female cone or ovule. Once the pollen grain reaches the ovule, it germinates and develops a pollen tube. The pollen tube grows through the micropyle, a small opening in the ovule, and delivers the sperm cells to the embryo sac.


Unlike in angiosperms where double fertilization leads to the formation of both embryo and endosperm, gymnosperms have a different mechanism. After the pollen tube reaches the embryo sac, one of the two sperm cells is discharged and fuses with the egg cell, forming a diploid zygote. This fertilization event initiates the development of the embryo.


The remaining sperm cell in the embryo sac does not fuse with any other cell. Instead, it directly divides by mitosis to produce a haploid endosperm. This process is known as free nuclear division or free nuclear endosperm development. The haploid endosperm nuclei distribute themselves throughout the embryo sac, forming a multinucleate structure.


The haploid endosperm provides nourishment to the developing embryo. It contains stored nutrients such as starch, proteins, and oils, which are utilized by the embryo during its growth and development. The endosperm gradually gets absorbed by the embryo as it matures.


In gymnosperms, the formation of haploid endosperm occurs through a unique process of free nuclear division. Unlike angiosperms, gymnosperms do not undergo double fertilization. The haploid endosperm serves as a source of nutrients for the developing embryo, ensuring its proper growth and development.