Teaching about plants is one of my favorite units. When I did my student teaching in a first grade classroom ten or so years ago, I made a big book that shows the parts of a plant, what plants need to survive, etc. It is showing it's age, but I still enjoy using it:
The yellow parts of a plant signs are on Velcro. Each child will have the opportunity to match the vocabulary word with the picture. I also have one with a tree where the students have to identify the roots, trunk, branches, and leaves.
This past summer, I found the neatest thing at the kids section in Bass Pro Shop. I have found LOTS of neat science things there. This is a great way to help students understand the plant parts:
This kit contains three plastic tubes, peat pellets, and a package each of carrots, onions, and radishes. We planted the seeds and borrowed some space on one of my neighbor teacher's window ledge to put our "garden," since, as many of you know, I don't have a window in my classroom.