how do tapetal cells become binucleate/multinucleate?

Tapetal cells are the layer of cells present inside the pollen sac. The cells are very big in structure and contain two or more nuclei. Tapetal cells help in growth and development of pollen grains.

Tapetal cells undergo endomitosis (type of mitosis) in which nucleus divides within the nuclear membrane but cytokinesis (cell wall formation) does not occur. Due to this reason the number of nucleus increases (binucleate or multinucleate) but the cell is single.