The canopy layer of the rainforest forms a roof over the understorey and forest floor layers. It is common for cross pollination to occur between plant species in the canopy layer. While scientists have been unable to research most rainforest plants, some facts are known, such as that the Goliath bird-eating tarantula can be as large as a dinner plate and trees have flowers, and that the understorey layer is warm, wet, and still. Squirrel monkeys are also seen leaping between trees in many rainforest locations.