Final answer:
Mendel's original pure strains of plants are designated as P1 and the offspring of the P generation are called the F1 generation.
Step-by-step explanation:
The designation for Mendel's original pure strains of plants is P1. These plants were the parental generation, also known as the P generation. Mendel collected the seeds produced by the P plants resulting from each cross and grew them the following season. These offspring were called the F1 generation, or the first filial generation.