For instance, cross-breeding a wild plant with a domesticated one can revitalize crops, building their resistance to new diseases. We saw in the last section that the ample sunlight and extremely wet climate of many tropical areas encourages the growth of towering trees with wide canopies. This thick top layer of the rainforest dictates the lives of all other plants in the forest. When this happens, all of the seedlings on the ground level compete intensely to reach the sunlight . Most other plants survive by taking advantage of the trees that form the canopy layer.
Many plant species reach the top of the forest by climbing the tall trees.
It is much easier to ascend this way, because the plant doesn't have to form its own supporting structure. At the top of the forest, these climbers may spread from tree to tree, making the canopy ceiling even thicker. Some plant species, called epiphytes , grow directly on the surface of the giant trees. These plants, which include a variety of orchids and ferns, make up much of the understory , the layer of the rainforest right below the canopy. Epiphytes are close enough to the top to receive adequate light, and the runoff from the canopy layer provides all the water and nutrients they need, which is important since they don't have access to the nutrients in the ground.