Nature’s Savings and Loan: How Epiphytes Survive Without Soil

Director Lew Feldman

Epiphytes are plants that grow on other plants; their roots are not connected to the soil but are instead often in direct contact with the atmosphere. Consequently, the only opportunity these aerial roots have to obtain moisture is during rainfall, which, in the tropics, occurs as intense, short downpours. This means epiphyte roots have a very limited window of time to absorb water—usually too short a time for the living tissue to absorb sufficient moisture. Thus, epiphyte roots have developed a unique and ingenious mechanism for rainwater capture.

You may have seen this mechanism in operation. For those who grow orchids indoors, you likely recall what an aerial root looks like: green at the tip and grayish behind. This grayish tissue, technically known as the velamen, is the starting point for understanding how these roots function. The velamen consists of dead cells, usually several layers thick, covering the outer surfaces of the root, as shown in the diagrams below (microscopic views).

When a downpour occurs, water rapidly moves into these dead cells via capillary action—the ability of liquids to flow into narrow spaces without any input of energy, similar to what happens when a piece of paper is put into water. Following a brief rain, the velamen cells swell quickly, as water continues to move into them via capillary action. Long after the rain has ended, this stored water is gradually moved out of the velamen and redistributed to the rest of the plant. If you mist your orchids, you will notice the velamen change color; this indicates that water has moved into the cells. In this way, the velamen acts as the plant’s own “soil,” holding onto moisture and slowly feeding the living tissue until the cells eventually return to their original gray hue.