Final answer:
In a tetrasomic individual of the plant species with 2n = 18 chromosomes, there would be 20 chromosomes in a somatic cell.
Step-by-step explanation:
A plant species with 2n = 18 chromosomes typically has 18 chromosomes in its somatic cells. However, a tetrasomic individual of this species has an extra copy of two of its chromosomes. Since the normal diploid number is 18, a tetrasomic individual would have 18 + 2 extra chromosomes, resulting in a total of 20 chromosomes in each somatic cell.