Why does the vascular system in some climbing palms (Calamus spp.) differ in structure from what has been observed in all other palms? This project, with field measurements to begin soon in Malaysia, seeks to establish a plausible scenario for the evolution of an unusual vasculature in certain rattan taxa.

contact: Alex Cobb

 

In most climbing plants, the xylem structure provides a consistently low-resistance pathway for water relative to that in related non-climbers. Freed of the necessity of thick, lignin-impregnated walls for structural support, climbers tend toward many large-diameter vessels. Interpretation has focused on the need of a vine for a high-conductance path to supply water to a large leaf area through a relatively slender stem. Recent anatomical observations by Professors J. Fisher and P.B. Tomlinson have shown that although Calamus indeed has large vessels, the high-conductance path these vessels might otherwise provide is disrupted by commissures only one-tenth the diameter of the larger vessels.

 

How low-conductance interrupts could have evolved in these very long plants (many grow to over 100 m in length) is intriguing indeed when one considers that, like other palms, they do not produce new vessels through radial growth. The vascular system, presumably subject to great stress over the plant's long life, can never replace itself. Since a low-conductance path for water will increase the pressure gradient along the stem of the plant, lowering conductance leads to greater tensions in the stem for a constant volume flow rate of water. This increases the risk of xylem disfunction.

 

Our current research examines possible functional explanations for this surprising arrangement. Initial results should be available soon.