which part of the plant is used often as food?

Answer 1

It depends on the plant.

Tomatoes, peppers, squash are all the berries of a plant.
Carrots, turnips, potatoes are all the roots of a plant.
Lettuce, spinach, kale are leaves.
Celery & rhubarb are the stems.

Answer 2

I would say fruits and seeds (including grains / kernels / nuts) because this is the most convenient place for plants to store food to insure reproduction of the plant.

Secondly, would be bulbs, roots, tubers - these like onion, potatoes, taro, etc. are all specialized for plant food storage.

Thirdly, would be stems...most of the stems we like to eat just taste good...salad greens, celery...and do not have much in the way of stored food. There are a few like sugar cane where the stem is specialized for food storage.

Answer 3

Depends on the plant.
Some plants, just to name a few, you can eat the foliage:
Lettuce
Kale
Spinach
Basil
Thyme
Sage

Some plants you eat the fruits and seed pods:
Tomatoes
Grapes (foliage too)
Peppers
Peas
Beans
Cucumbers
Rose hips

Some plants you eat the flowers:
Broccoli
Califlower
Asparagus
Roses
Brussels sprouts

Some plants you eat the roots:
Potatoes
Onions
Carrots
Radish
Ginger
Turnip
Beets

Answer 4

Which plant?