On which plants did G. Mendel conduct experiments?

Peas.
Peas are easy to grow, it has many varieties, the offspring of which multiplies well. Of the 34 pea varieties at his disposal, Mendel chose 22 “good” varieties, which clearly differed in any way. To do this, he checked the “purity of the variety” for two years: he gave plants the opportunity to self-pollinate (peas – self-pollinator) and chose varieties where the descendants of all generations were similar to each other and to their parents.
The structural features of the legume flower made it possible to produce artificial pollination and made rare cases of pollination with alien pollen. In a pea flower, a column and anthers containing pollen are surrounded on all sides by a part of a flower called a boat. This prevents alien pollen from falling onto the stigma.