ORCHIDS
 
In tropical rain forests, it is orchids that have invaded the canopy habitat most successfully. Of the 23,000 or so species of orchids, about half are epiphytes. They receive support from the trees they live on, but do not derive nutrients from it. Malaysia, for example, which has an area about the size of England or Alabama, has 800 orchid species, most of which are epiphytes.

Orchids manage to survive the epiphytic environment by taking up water rapidly when it is available and by restricting its loss during drought. Many epiphytic orchids have thick white roots that spread over the surface of the bark of the trees that lend them support. The outer part of these roots is covered by a layer of dead cells called the velamen. These cells have many small holes in their walls into which water flows with ease. Water-filled velamen cells become transparent and the greenish inner tissues become visible. When the roots are dry, the empty velamen cells scatter the light that falls on them and appear white. By reflecting light in this way, and by forming a sort of mat over the root surface, the velamen must also help to reduce water loss from the orchid's roots.

The water absorbed by the cells of the velamen is transported into the living part of the root and from there to the leaves of the plant. The leaves of many epiphytic orchids are often thick and rigid, and contain tissue that can both absorb and store a great deal of water. The outermost cells of the leaves are usually covered with a thick waxy layer. This restricts water loss to the small pores on the leaf undersides. The pores open and close in response to light and the water content of the plant, and so give added insurance against dehydration.

Huge, spectacular flowers produced by some orchid species may last for month is they are not fertilized. This property makes many epiphytic orchids extremely popular as cut flowers. The industry of orchid-growing and export to all corners of the globe is an important part of the economy in many parts of the tropics, particularly Singapore.