Your students may have memorized the parts of a flower, but do they conceptually understand the role of flowers in relation to plant life cycles, pollinators, and agriculture? One way to gain insight into what students know, to assess what they've learned, and to help them organize and represent concepts and meaningfully evaluate their own growth is to have them create "concept maps."
Before beginning a unit on flowers, for instance, you might ask students to generate a "map" of ideas associated with the concept of flowers. Ask them to consider what mental images or concepts this evokes, and to think about how they could "connect" these images to illustrate their understanding. Some teachers have students simply draw lines to show the connections and hierarchies, while others recommend having students use "linking" words to describe relationships.
If this strategy is new to your students, consider introducing the idea by asking them what image comes to mind when they hear a familiar word, for instance, "dog." Then have them practice using some simple words to link concepts (e.g., "dogs are furry"). Next have them practice further by developing a concept map for a topic that's familiar to them.
By asking students to draw concept maps before beginning an activity or unit, you can gain a sense of what students already know, plan activities and challenges accordingly, and have a benchmark from which to assess student gains. If you also have them do a concept map with the same central concept after they've conducted activities, you and the students will be able to see how their understanding has progressed.