Just try to find two trees of the same species within a few minutes in the Amazon rainforest, and the term ‘massive biodiversity’ will take a new meaning: you keep walking across different species. Just the western part of the Amazon River Basin has the highest diversity of trees in the world1.
 

Many species, few specimens

Tropical rainforests set records in biodiversity: anywhere between 40 to 100 species of tree can be found in a 1-hectare plot of land. Take the Cocha Cashu Biological Station in the Amazon floodplain forests of Peru, as an example. There, at least 1,856 species of higher plants have been discovered².

The Amazon is home to as many as 80,000 plant species from which more than 40,000 species play a critical role in regulating the global climate and sustaining the local water cycle.³ But the richness of species is one thing, and abundance another. While there may be many species in tropical rainforests, these often exist in low numbers over large areas.

Amazon plants and trees play critical roles in regulating the global climate and sustaining the local water cycle. The forests they form are home to the huge variety of animals found in the Amazon.

But their greatest riches yet may be the compounds they produce, some of which are used for medicine and agriculture. For Amazon people, both indigenous and recent arrivals, plants are a food source and raw matter for non-timber forest products.

A tight arms race between Amazon plants and invertebrates

Because most rainforest insects feed on plants, these have had to evolve survival abilities to defend themselves. Such techniques involve secreting toxic compounds that will repel the attackers … who in turn evolve abilities to exploit other weak points in the plant.
 

Harsh soil conditions for Amazon plants

Because of the incessant rain that beats down on the Amazon rainforest, soils are generally poor in nutrients. In such harsh environments, plants cannot afford the additional threat of being devoured by voracious insects. To defend themselves, they have evolved an amazing range of strategies and tools.⁴
 

The plant defence systems

It is in the plants' best interests to be tasteless, difficult to eat or just plain poisonous. So some plants have developed tough leaves, resins or latex outer coats that make them tough and able to resist many predators. Other plants produce leaves that are nutritionally poor, so insects have to invest a lot of time and effort in eating, which is not a worthwhile strategy for any species.

In some places, nutrient-rich clays are present and hence, plants are less vulnerable to insect damage. Why is that? Take, for example, the western Amazon Basin, where rich soils cover 50-75 % of the land within about 500 km of the Andes Mountains. Plants use the available nutrients to grow back plant matter when they have been attacked by predators – and are under less pressure to evolve more efficient defence systems.⁵

Epiphytes

Some plants have evolved to the point that they simply do not need to grow on the forest floor – instead, they live on other plants. Such plants are called epiphytes, and in lowland tropical rainforests they may represent up to one-fourth of all plant species.⁶

Epiphytes encompass a wide range of plants: some ferns, orchids, cacti and mosses have the ability to live virtually in mid-air. They trap the little soil they need, which is carried by the wind, and this helps them develop roots and a litter base on tree branches.

Bromeliads

In addition to their terrestrial forms, bromeliads also occur as epiphytes in rainforests, where they accumulate rainfall water and detritus in their cup-like structures.

Tree frogs, snails and other species have evolved to complete part of their development in these structures, which afford them the perfect place at a vulnerable stage in their development. As many as 250 tree frogs, snails, and other species are known to use bromeliads for such purposes.⁷

Other plants that are common both as creepers and epiphytes in the Amazon are the aroids, which include philodendrons. These plants begin life on the ground and grow as a tendril that goes up tree trunks and attaches to them by aerial roots. They eventually lose their ground roots and become climbing epiphytes.

Palms

A majority of palms are easily identified by their distinct shape. Many have commercial value, for example to make brooms, hammocks, necklaces and string bags. 

One palm species - Murumuru (or tucum) - is used for its oil and is said to have three times more Vitamin A than carrots as well as other nutrients⁸. African oil palm is cultivated on an increasingly large scale, posing threats to native rainforests that are cleared in the process.

Vines

Vines are some of the elements that give rainforests their characteristic dense appearance. Apart from their structural role, they are essential food and "highways" for Amazon wildlife.

Vines encompass a variety of plants, including lianas that hang from the treetops, bole climbers, which ascend tree trunks and stranglers, which wrap themselves around trees and sometimes choke them. They are present in both disturbed areas exposed to light and in forest interiors, and regardless of soil type. Humans have long used vines for food, medicine, hallucinogens, poisons and construction materials.⁹
 

¹Morales, K., & Vinicius, T. 2003. Amazon rainforest: biodiversity and biopiracy. STUDENT BMJ, 13, pp 386-7 
²Foster, 1990a in Kricher, 1997
³Morales, K., & Vinicius, T. 2003. Amazon rainforest: biodiversity and biopiracy. STUDENT BMJ, 13, pp 386-7
⁴National Geographic. Plants vs. Insects: An Amazon Epic for the Ages http://news.nationalgeographic.com/news/2005/03/
0307_050307_insects.html. Accessed: 06/10/05
⁵National Geographic. Plants vs. Insects: An Amazon Epic for the Ages http://news.nationalgeographic.com/news/2005/03/
0307_050307_insects.html. Accessed: 06/10/05
⁶Klinge et al, 1975 in Kricher, 1997
⁷Burt-Utley, 1983 in Kricher, 1997
⁸Balick, 1985 in Kricher, 1997
⁹Phillips, 1991 in Kricher, 1997

General source: 
Project Amazonas. Flora of the Amazon.http://www.projectamazonas.com/resources-flora%20&%20fauna.htm. Accessed: 11/10/05.