150 ft in the air, form the basic structure of the rainforest. Their top branches spread

　　wide in order to capture maximum sunlight. This creates a thick canopy (樹冠) level

　　at the top of the forest, with thinner greenery levels underneath. Some large trees

　　grow so tall that they even tower over the canopy layer。

　　As you go lower, down into the rainforest, you find less and less greenery. The

　　forest floor is made up of moss, fungi, and decaying plant matter that has fallen from

　　the upper layers. The reason for this decrease in greenery is very simple the

　　overabundance of plants gathering sunlight at the top of the forest blocks most

　　sunlight from reaching the bottom of the forest, making it difficult for robust plants to　thrive。

　　The, Forest for the Trees

　　The ample sunlight and extremely wet climate of many tropical areas encourage

　　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. New tree seedlings rarely survive to

　　make it to the top unless some older trees die, creating a "hole" in the canopy. When

　　this happens, all of the seedlings on the ground level compete intensely to reach the　sunlight。

　　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。

　　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(養(yǎng)分)they need, which is important since they

　　don't have access to the nutrients in the ground。

　　Stranglers and Buttresses

　　Some epiphytes eventually develop into stranglers. They grow long, thick roots

　　that extend down the tree trunk into the ground. As they continue to grow, the roots

　　form a sort of web structure all around the tree. At the same time, the strangler plant's

　　branches extend upward, spreading out into the canopy. Eventually, the strangler may

　　block so much light from above, and absorb such a high percentage of nutrients from

　　the ground below, that the host tree dies。

　　Competition over nutrients is almost as intense as competition for light. The

　　excessive rainfall rapidly dissolves nutrients in the soil making it relatively infertile

　　except at the top layers. For this reason, rainforest tree roots grow outward to cover a

　　wider area, rather than downward to lower levels. This makes rainforest trees

　　somewhat unstable, since they don't have very strong anchors in the ground. Some

　　trees compensate for this by growing natural buttresses. These buttresses are basically

　　tree trunks that extend out from the side of the tree and clown to the ground, giving

　　the tree additional support。

　　Rainforest trees are dependent on bacteria that are continually producing

　　nutrients in the ground. Rainforest bacteria and trees have a very close, symbiotic (共

　　生的) relationship. The trees provide the bacteria with food, in the form of fallen