If the cells of root in wheat plant have 42 chromosomes,  then the no. of chromosome in the cell of pollen grain is

Number of chromosome in root cells of wheat plant is
2n = 42
$\mathrm{n}=\frac{42}{2}=21$
The number of chromosome in the cell of pollen grain is 21 because pollen grains are haploid (n) in nature.