Teaching your preschoolers the three main parts of a flower -- the roots, stem and flower petals -- is best accomplished through simple language. Provide basic information regarding the function of each part. Pair that with hands-on activities at home, giving them an experience to back up the new earth science they've learned.
Introduce your preschoolers to the roots of a flower by telling them the roots are what hold the flower in the ground. If possible, follow this one piece of information up by actually pulling a flower out of the ground. Let your kids see the roots come up out of the ground. If this outdoor option isn't possible, have a flower in the classroom -- preferably potted for similar removal. Encourage your preschoolers to touch the roots. As they examine the roots, tell them the flower's roots are like their mouths: the roots are how the flowers get food from the soil. Preschoolers understand the importance and fun of eating.
Before discussing the role of the stem, re-plant the flower being used in the demonstration. As you finish patting supportive soil around the stem, direct the preschoolers' attention to the stem. Trace your finger or a pointer up and down the stem to clearly indicate to the preschoolers what part of the flower you are now presenting. Inform your children that the stem is what holds the flower up. Compare it to their legs which support their bodies. Tell them that the stem brings the food from the roots to the rest of the flower.
Tell your kids what they may already intuitively know: the flower at the top of the stem with its pretty petals is the first thing you notice when looking at a flower. It's okay to state the obvious with preschoolers -- it reaffirms for them that they are on the right track. Tell them this is the part of the plant that makes more flowers. For now, skip the specifics of asexual reproduction, stamens, pistils, pollen and eggs. There will be plenty of time for those lessons later.
Tissue Paper Craft
Demonstrate the delicate nature of flower petals with tissue paper. Tell the preschoolers that the tissue paper which tears easily is like the flower petals that can be damaged or ruined quickly when not handled gently. Provide a colorful variety of tissue paper for your preschoolers to tear into the shapes of flower petals to paste on to construction paper. When they inadvertently use too much glue, saturating and destroying the tissue paper, tell them this is similar to when nature sends too much rain and flower petals fall apart.