I think vegetables are classified according to which part of the plant is eaten. Some vegetables fit into more than one category when several different parts of the plant are edible, e.g. both the roots and leaves of beetroot can be eaten.
Bulbs Usually grow just below the surface of the ground and produce a fleshy, leafy shoot above ground. Bulbs usually consist of layers, or clustered segments.
Flowers The edible flowers of certain vegetables.
Fruits Vegetable fruit are fleshy and contain seeds.
Fungi When referring to vegetables, fungi are commonly known as mushrooms.
Leaves The edible leaves of plants.
Roots Usually a long or round-shaped taproot.
Seeds (Legumes) apart from sweet corn, seeds grow in pods which are sometimes eaten along with the seed.
Stems The edible stalks of plants when the stalk is the main part of the vegetable.
Tubers Vegetables which grow underground on the root of a plant.
