there are seveal good internet sites. i'll have to give them to ya tomorrow when i'm at school. one thing we do is plant a seed and watch it grow. there are some short term plants which will sprout in 10-14 days. then we pull it up and look at the roots, the stem and the flower. i ususally start the unit while we are doing some other science unit because the seed takes a while to germinate.
Plant Lima Bean Seeds in between damp paper towels and put in a ziploc bag. Over several days the seed will sprout. Then take a sprouted seed and pull it apart. You will the seed covering - semi hard shell, the baby plant, and the plant hole where water is taken into the seed.
grow different seeds. Lettuce, pea, green bean. Have them make predictions which will germinate first. Then have them use microscopes and be a scientist and document with illustrations. Don't tell them what each thing is yet. HAve them predict and guess. After you get roots and sprouts turn the cup or baggie they're growing in upside-down. Ask what happens and why.
I always have them start 3 sunflowers or pumpkin seeds to take home. They are thrilled and I've gotten a pumkkin from students returning in the fall!