Weird and wonderful members of the plant kingdom - air plants

Air plants are amazing because they do not need soil to grow. They are properly called epiphytes - plants which grow above the ground surface, using other plants or objects for support. They are not rooted in the soil nor are they parasitic (ie they do not directly harm the other plant). By growing on other plants, the epiphytes can reach positions where the light is better or where they can avoid competition for light. Many mosses and lichens are epiphytes, as are approximately 10 per cent of all seed plants and ferns. Epiphytes are particularly common in some groups of plants, such as ferns, bromeliads (members of the pineapple family, Bromeliaceae) and orchids: over half of the 20,000 species of orchids are epiphytic.

Most epiphytic seed plants and ferns are found in tropical and subtropical rainforests because they require conditions of high humidity. Since epiphytes have no contact with the soil, the major difficulties they encounter are shortages of water and nutrients. Many have similar adaptations to desert plants, enabling them to collect, absorb and store water and restrict its loss. Some epiphytic mosses and lichens, in contrast, shrivel and stop growing when water is short, but when it becomes available they are able to absorb it rapidly and resume growth.

The major sources of water are dew, moisture in the air and rainwater running down the support tree. In the so-called tank or urn bromeliads of the South American rainforest, such as Aechmea, the leaves are arranged as a tightly rolled funnel, forming a water-tight container. Rain and plant debris are trapped in these funnels or 'tanks', and absorbing tissues at the base of the leaves take up both water and nutrients. The tanks provide habitats for a large number of insects and sometimes even small frogs.

Nutrients in the dust washed off the support tree leaves by the trickling rainwater are absorbed by epiphytes. Other nutrients are obtained from plant debris trapped by the branches of the support tree or amongst the roots of the epiphytes.

Air plant photograph by Ut1ma, used under a creative commons attribution licence