Question

Explain the development of different types of endosperms in angiosperms.

Answer 1

Double fertilization in angiosperms form a zygote and a triploid nucleus. The zygote develops into the embryo while the triploid nucleus undergoes repeated nuclear divisions to a triploid nutritive tissue. It is rich in lipids, sugars, and carbohydrates and nourishes the developing embryo.

Complete answer:
Angiosperms are the flowering plants and exhibit two separate nuclear fusions during fertilization. Out of the two male gametes carried by the pollen tube, one fuses with the egg cell to form a zygote. The other male gamete fuses with the two haploid polar nuclei of the central cell to form triploid primary endosperm nucleus (PEN). The primary endosperm nucleus undergoes repeated divisions to form nuclear, cellular, or helobial endosperm.

The nuclear endosperm is formed when the primary endosperm nucleus undergoes a series of free nuclear divisions. It results in the formation of a large cell with multiple nuclei. The nuclei are arranged in a peripheral layer while the central part is occupied by a large central vacuole. The nuclear endosperm is mostly converted into the cellular endosperm due to the formation of the cell wall in the later stages of development. Example: coconut.

Cellular endosperm develops when the repeated nuclear divisions of the primary endosperm nucleus are accompanied by the cell wall formation. Example: datura

Helobial endosperm is an intermediate form of the nuclear and cellular endosperm. The primary endosperm nucleus is present at the chalazal end of the female gametophyte and divides unequally to form a large micropylar cell and a smaller chalazal cell. The nucleus of the chalazal cell may or may not divide further while the micropylar cell undergoes repeated free nuclear divisions. Example: water lily.

Note: Since the central cell consists of two haploid polar nuclei, its fusion with the haploid male gamete forms the triploid PEN. Irrespective of the type of endosperm, PEN always divides mitotically. Therefore, the endosperm is always triploid.