Final answer:
A tetraploid individual of a plant species with 2n = 18 chromosomes would have 36 chromosomes in a somatic cell. For Mendel's pea plants with 2n = 14 chromosomes, 128 unique gametes could be produced without crossing over events.
Step-by-step explanation:
If a plant species has 2n = 18 chromosomes, that means its diploid number (the number of chromosomes in a normal somatic cell) is 18. A tetraploid individual has four sets of chromosomes. In other words, instead of the normal two sets (diploid), a tetraploid individual would have twice as many, so you would expect to find 4 x 9 = 36 chromosomes in a somatic cell of a tetraploid individual of this species.
Concerning the number of unique gametes, for the pea plants used in Mendel's studies, which were diploid with 7 chromosome pairs (14 chromosomes total in somatic cells), the formula to calculate the number of unique gametes is 2n, where n is the number of chromosome pairs. Therefore, 27 = 128 unique gametes could be produced by one pea plant, assuming no crossing over events occur.