Endosperm is present in the seeds of most of the angiosperms. It provides nourishment to the growing embryo. Endosperm is replaced by fleshy cotyledons in dicotyledons. In monocotyledons, endosperm persists in the mature seeds too and stores the food. Endosperm tissue is rich in carbohydrates and also contains proteins and lipids.

Salient features of Endosperm

  • Endosperm development takes place from the triploid cell (3n) in the embryo sac. The second male gamete fuses with the diploid secondary nucleus (formed by 2 polar nuclei) to form primary endosperm nucleus (PEN) and a triploid cell. The process is known as triple fusion
  • Endospermic cells are mostly triploid but in some plants, they may be diploid (water lily) or polyploid (up to 15n)
  • In some of the dicots, mature seeds contain food stored in the endosperm, they are called endospermic or albuminous seeds, e.g. castor
  • In beans, peas, gram seeds, the endosperm is completely absorbed during development and food is stored in two cotyledons. They are called exalbuminous seeds
  • Endosperm mostly contains starch as a food reserve, but it may contain fats, e.g. castor
  • Endosperm provide nutrients to seeds during dormancy
  • Endosperm also contains certain hormones like cytokinins and helps in cell differentiation
  • In grains, like wheat, maize, barley, corn, the endosperm is the main source of food
  • Coconut water is the example of a liquid endosperm
  • The white flour, used to prepare bread, only contains endosperm of wheat seeds
  • The outer layer of endosperm is the aleurone layer. It secretes the amylase enzyme, which breaks down the starch contained in the endosperm to sugars for utilisation by seedlings
  • Orchid seeds lack an endosperm

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Types of Endosperm

How the primary endosperm nucleus (PEN) develops into endosperm differs in various plants. Endosperms are divided into three main types based on their development pattern.

1. Nuclear Endosperm: It is the most common type of endosperm found. Here PEN divides repeatedly (mitotic division) without cytokinesis. It results in the formation of a large number of free nuclei in the cell. A large central vacuole is formed and nuclei get arranged at the periphery. There are more nuclei at the chalazal and micropylar end, compared to the sides.

At this stage, cell wall formation takes place from the periphery towards the centre and multicellular endosperm is formed. Examples: maize, rice, wheat, cotton, sunflower

In coconut, the cell wall formation is incomplete resulting in the outer multicellular solid endosperm and inner multinucleated (having free nuclei) liquid endosperm (coconut milk)

2. Cellular Endosperm: It is not very common. In this type of development, division of PEN (karyokinesis) is followed by cytokinesis and two cells are formed due to transverse division, giving rise to the chalazal and micropylar chamber. Further division is similar, which leads to the formation of the cellular endosperm. Examples: Petunia, Balsam, Datura

3. Helobial Endosperm: This type of endosperm development is common in monocotyledons. The first division is similar to cellular endosperm and results in a large micropylar cell and small chalazal cell. The chalazal cell mostly does not divide further and functions as a base cell. The micropylar cell divides further, similar to nuclear endosperm. So it is an intermediate type, a combination of both nuclear and cellular endosperm. Examples: Eremurus

Function of Endosperm

  • Endosperm is important for the growth of an embryo and stores the food reserve
  • They supply nutrients and provide protection to the developing embryo
  • Endospermic tissues have shown to regulate gene expression and seed germination
  • Endosperm induces signals according to environmental conditions and regulates embryonic growth
  • The endosperm contains cytokinin, which regulates cellular differentiation
  • It may induce abortion of seeds from the genetically mismatched cross

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