There are a number of activities that can be done around identifying parts of a plant. You can start the conversation by showing a potted plant and ask if the child can identify the stem, flowers, leaves etc.The main parts of a plant are Roots, Stem, Leaves and Flower. Each part of the plant has a specific function.
Roots absorb water and nutrients from the soil and hold the plant to the ground.
The stem carries water and minerals to the leaves.
Leaves collect energy from the Sun and make food for the plant.
Flowers produce seeds from which a new plant is formed.
There are many activities which can be done to explain the parts of a plant for preschoolers. We have listed three of them here:

Puzzle: You can take a printout of this puzzle or even draw it on your own. Cut the pieces of the puzzle and ask the child to put the parts into the correct place. Preschool kids will not be able to read the names of the parts so the parents will have to help with that. However the parents can mention the starting sound of each name to help them start with reading.

Grow your own Plants: Plant some beans in a small transparent jar. Fill the Jar with paper towels and keep some seeds near the outer wall of jar. Spray the paper towels with some water and put enough water everyday to just keep it moisturized. In a few days the roots and shoots will appear. The kids will be thrilled to watch the small plant grow out.

Art Project : Kids learn better when more than one sensory activation is involved in doing a project. You can make this parts of plant project using muffin cup, straw, wool, seeds. The kids will have fun pasting different parts of plant and also able to feel the different textures and shapes involved.