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| RAINFOREST LIBRARY |
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| Plant types of the Rainforest |
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| Trees form the superstructure of the rainforest. The crowns of which these trees form aerial walkways for animals
and roof gardens for epiphytes, their foliage that plunges the under storey in to the cool, humid shade which nurtures seedlings and other delicate plants,
which these plants roots, collect scarce nutrients, and their mighty trunks that support the great weight of the canopy. The diversity of tropical trees
species is immense. There are commonly fifty to two-hundred different species per 2.5 acres, compared with ten species in a similar area of temperate
woodland. In spite of the diversity, many tropical trees look superficially alike and for this reason are difficult even for botanists to identify.
The great majority are about thirty meters(100 feet) tall, although some reach 60 meter(200 feet) or more. They have slender, unbranched trunks, smooth bark, and a simply
branching system. Few exceed one meter (3.3 feet) in girth, although giants 17 meters(56 feet) across have been found. the wood can be extremely hard and resistant to termites
and other wood-boring than insects. Many tropical trees are evergreen, shedding a few leaves
throughout the year. the leaves are generally oval, undivided, thick, and leathery. An evergreen rain forest tree
grows all year round and therefore has no annual growth rings. It may live in anything from
150 to 1400 years and usually takes 30 to 60 years to reach maturity and begin flowering. Even when mature, many do not flower
and fruit annually, but only once in 3 to 10 years. There may be an element of time-sharing between species using the same pollinators and dispensers.
Semi-evergreen and seasonal forest are common at the outer limits of the equatorial rain forest belt, in rain shadow areas and regions adjoining savanna.
During the dry season, the deciduous trees not only shed their leaves, but most of then also flower, so that the seed are ripe and ready
to germinate when the rains come. |
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| Emergents |
In almost any temperate forest some mosses and lichens are ordinarily visible on the bark of the trees. These epiphytes-plants that live on other plants-often form a thin layer over the surface of their host. In addition to the usual mosses, lichens and ferns, there are orchards, cacti, bromeliads
(members of the pineapple family, Bromeliaceae), aroids (plants of the arum family, Araceae), and representatives from numerous other plant families.
So numerous are the epiphytes on many trees that their leaf area may exceed that of the tree on which they are growing.
A quarter of all plant species in low land rain forest are epiphytes. Tropical America alone has an estimated 15,500 epiphytic species.
African rain forests have fewer epiphytes than other rain forests, perhaps because many of them became
extinct during the successive dry periods that were a feature of the last Ice Age, whose impact was more severe than
elsewhere. Even so, 47 different species of orchard have been recorded on a single West African tree. Like other rain forest plants, epiphytic species tend to occur in strata. Canopy epiphytes are exposed to sun, wind, and occasional dry period. As a result, they show many of the same adaptions of desert species. In contrast,
epiphytic species in the under storey have to contend with low light and permanently damp conditions. Consequently, they often have thinner leaves, "drip tips", at the ends of the leaves to get rid of excess water and corrugated or velvety textures which increase the surface area, and thus the light-gathering capacity of the leaf.
The epiphytic way of life is precarious. The host tree may shed its bark periodically to rid itself epiphytes, or it may lose limbs under the weight of wet vegetation(which may total several tonnes per tree). |
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| Climbing Plants |
| Reaching for the sunlight, about 8 percent of the plants in lowland rain forest are climbers, the highest percentage occurring in secondary forest. They show several different approaches into the problem of getting to the light.
The simplest is to have a barbed stem and to use brute force, thrusting upwards and gaining purchase of thorns which hook onto the surrounding vegetation. Rattans (climbing palms), which can reach 200 metres (650 feet), in length, use this technique.
They also have barbed, whip like extensions to their new leaves which do not unfurl until the "whip" has lashed itself to the next support. An alternative approach is used by the climbing aroid group (genera of the arum family Araceae, which include Philodendron, Mostera, Rhaphidophora and Scindapsus).
They produce two kinds of roots, short ones put out at right angles to the stem, which develope adhesive hairs when they make contact with the climbing surface, and long feeding roots which may dangle in mid-air initially, but branch profusely as soon as they encounter the soil.
The clasping roots often attach the plant so securely that it takes considerable force to tear them away. The aerial roots are extremely flexible and strong. When anchored, they act like guyropes, tethering the host tree and playing an important role in the structure of the forest.
Local people use the roots of many different species for cordage, from fine weaving to heavy-duty ropes. Some climbers of this kind may become almost epiphytic. Although they germinate and begin growth on the forest floor, they may eventually flourish entirely in the tree tops,
losing connection with the ground as the original roots die. Another approach is to put out tendrils from leaves or stems. An outstretched tendril moves away from the light and makes sweeping movements as it searches for a suport. Brushing against another plant stimulates it to curl.
As most tendril climbers are attached only at their extremities, the heavy water-conducting stems tend to sag and loop through the vegetation. Some species have stems filled with water, which can be tapped by forest peoples. |
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| Flowering and fruiting plants |
Two thirds of the worlds flowering plants are found only in the tropics. One reason for this tropical diversity is the large number of relationships they have forged with animals in the rain forest, this web of relationships they have forged with animals and
flowering plants is so complex that the loss of one species can lead to the extinction of many others. Another reason for this diversity lies in the fact that the climate conditions in a rain forest vary little throughout the year, or even over centuries. This has allowed the evolution
of a stable community of long-lived perennial plants, in which long periods elapse between generations, and the opportunities for new individuals to become established are few and far between. Consequently, competition between juveniles is intense. Smaller trees as well as climbers and epiphytes
all face the same uphill struggle. There are three main strategies that a plant in this situation can adopt to gain an edge over its rivals. First, it can multiply its chances by reproducing sexually so that its offspring are as genetically diverse as possible. The majority of the flowering plants in the rain forest
cannot fertilize themselves. To reproduce they must be cross-pollinated. The second strategy that a plant can adopt is to produce a mass of seeds to ensure that its genes have as many opportunities as possible for expression. Flowering and fruiting is therefore a major investment in terms of energy,
and takes place only when conditions are right. Because many trees take 30 to 40 years to mature and seldom flower annually. An Estimated 2.5 acres of lowland rain forest produces as much fruit and seed, by weight, as 30 acres of temperate oak woodland. The third strategy is to ensure widespread dispersal, so that if the conditions
are unfavorable in one location a few of the seeds have a chance of succeeding elsewhere. Most plants use highly mobile animals, such as birds, bats, and monkeys for this. |
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