Question

How tapetal cells could become bi-nucleate.

Answer 1

Tapetal cells are the nutritive cells of an anther which helps in the development of pollen. Tapetal cells consist of a large number of nuclei and divisions take place by mitosis, but in its mitotic division cytoplasm does not divide due to which the cell becomes enlarged.

Complete Answer:
The anther is divided into four layers like an epidermis, endothecium, middle layer, and tapetum. Tapetum is the innermost specialized nutritive layer located between sporogenous tissue and anther wall. Tapetal cells consist of a large number of the nucleus and it is generally polyploidy in nature. Division of the tapetal cell takes place by mitosis and after mitosis cytokinesis does not take place and this type of mitosis is known as endomitosis and it results in the binucleate or multinucleate tapetum. A large number of nuclei results in the large size of the tapetum which provides high nutrition or regulatory substance to pollen grain.

Additional information:
Other than nourishment tapetum also perform other functions like it provides enzyme callose for binding microspores, it secretes hormone-like indole-3-acetic acid that helps in the early growth of pollen, it secretes Ubisch granules that help to provide outer covering of pollen grain like sporopollenin and it also provides compatible and incompatible proteins to pollen grains.
Tapetum is of two types: amoeboid and secretory or glandular. Amoeboid tapetal cells grow and pass between the spore mother cells which help in providing nourishment and other material whereas secretory tapetum remains in situ which secrete nourishment passes into sporogenous cells.

Note:
In some species, endomitotic division in tapetal cells shows untypical properties of the anther. DNA methylation level of cells helps to understand the role of nucleolus for spindle formation during mitosis. Endomitosis is also known as inhibited mitosis because of the absence of cytokines.