ASPECTS OF VESSEL DIMENSIONS IN THE AERIAL ROOTS OF EPIPHYTIC ARACEAE

We measured vessel dimensions, most significantly vessel length, in the aerial roots of four epiphytic aroids using a digital camera to photograph sequential sections. Pendulous aerial roots in Araceae can grow from the forest canopy and so reach considerable length (>30 m) before they contact the ground, branch, and become anchored. In the free-hanging state, the length over which tissue maturation occurs can exceed 1 m. We show that the distinctive medullary vessels do not anastomose and each series of vessels, end to end here termed a pipe,'' must differentiate without interruption throughout the length of the root and do not become fully functional until the ground is reached. Measurements show different vessel parameters, including vessel overlap at each vessel end, which is not usually considered in estimates of hydraulic conductivity. Our method of measurement is simple and direct and shows the topographic relation of all vessels in a single organ, suggesting that vessels in long plant organs can be measured precisely, giving results of value in considering the hydraulic properties of xylem elements.