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How tapetal cells could become bi-nucleate?

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Answer
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Hint: The specialized layer present in the pollen grains that are responsible for the requirement of nutrients. The cells of this layer will undergo cell division that will result in the formation of bi-nucleate or multinucleate cells.

Complete answer:
These tapetal cells undergo endomitosis where the nucleus in the nuclear membrane divides, but cytokinesis does not occur. They become bi-nucleate or multinucleated because of this. The cells generally have several nuclei per cell and are larger in size. The sporogenous cells split the nuclei of the tapetal cells by undergoing mitosis. Sometimes, this mitosis is not normal due to which many cells of mature tapetum become multinucleated.
The anther wall is called the tapetum which is the innermost layer. The cells of this layer have large nuclei and cytoplasm. The tapetal cells are very large cells with many nuclei. Tapetal cells assist with pollen grain growth and development.
The tapetum is found between the anther wall and the sporogenous tissue which consists of a specialized layer of nutritive cells. For the nutrition and the development of the pollen grains, tapetum is required from which a two-layered pollen coat is also originated. Pollen grains consist of a two-layered cell wall. The outer hard layer is exine which is composed of sporopollenin which is one of the most resistant biological materials known. The inner layer is the intine.

Note:
Tapetum cells provide nutrients, but auxin in tapetum cells is not sufficient to support the early stages of pollen development. For the development of the male gametophyte, auxin is required which is produced in the sporophytic microsporocytes. Tapetum is of two types: secretory (glandular), and plasmodial (amoeboid). The glandular tapetal cells are found surrounding the anther lobe while plasmodial tapetal cells dissolve to form protoplasm which is multinucleate.