From Seed to Seed:
Plant Science for K-8 Educators
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From Seed to Seed: |
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A rainforest is a great place to study plant adaptations for gathering sunlight. With plenty of water and warm temperatures, plant growth is lush and dense, and the availability of sunlight is the limiting factor for growth. Competition for sunlight has resulted in various strategies for capturing light. Light in the rainforest. Plant life occurs in layers in a tropical rainforest. The uppermost layer consists of very tall trees-some more than 150 feet tall-rising out of the dense canopy below. The canopy consists of a dense mass of treetops, vines, and other plants occurring in a layer, usually about 100 feet above the ground. Below that is the understory, which consists of various seedlings, saplings, trees stunted from lack of light, bushes, and shrubs, some growing up to 75 feet tall. Finally, there is a forest floor.Because of the very dense canopy, little sunlight reaches the ground. In a dense forest, the canopy trees can block out more than 95 percent of the sunlight. Although Tarzan movies would have us think otherwise, there is actually relatively little vegetation on the rainforest floor. Aside from some seedlings, young creeping vines, and scattered fallen leaves, the ground is relatively bare. Let's look at some characteristics of rainforest plants, and explore how these characteristics might help plants compete for light. 
Since light levels increase as you rise into the upper layers, rapid vertical
growth would seem to be a good strategy, and indeed it is. One very common
type of rainforest plant is the liana, which is a general term for a woody
vine. In the rainforest, lianas grow to a much larger size than most of
us are used to seeing in our temperate climates. Vines may reach a length
of 200 feet or more, with stems approaching the girth of a medium-sized
tree (6 inches or more in diameter). With a growth rate of up to 3 feet
per day, lianas certainly use the strategy of rapid vertical growth. And
once they reach the upper canopy, the vines can command their share of sunlight.Many rainforest trees also use the strategy of vertical growth. In contrast with a sprawling oak, most rainforest trees grow straight and vertical, with few or no branches on the lower trunk. Many species' saplings are able to remain in a relatively dormant state until a gap in the canopy opens up, perhaps due to a fallen tree. Once sunlight is available, the sapling begins more rapid growth. Interestingly, the leaves of many rainforest trees-and plants-are strikingly similar. Most of their leaves are oblong or oval, with a pronounced "drip tip"-think of the common houseplant philodendron, with its heart-shaped leaves ending in a long point. This shape allows excess water to run off for more efficient photosynthesis. Another interesting group of rainforest plants is called the epiphytes.
These Once you get into the upper canopy, plants face an entirely different challenge from that faced by those plants growing below-intense, relentless sunlight. Leaves in the upper canopy are leathery and have a thick cuticle for protection. And some plants are able to actively alter the orientation of their leaves. By varying the turgor pressure in the leaf joints, plants can raise the leaves perpendicular to the sun for maximum exposure, or lower them to minimize sun stress. |
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plants
grow up in the branches of the canopy-where light is plentiful. Rather
than getting their water and nutrients from the soil, they get them from
the air and the organic matter trapped in the trees' bark 
and
branches. Bromeliad and orchid are common epiphytes in the rainforest.
