XYLEM ANATOMY AND HYDRAULIC CONDUCTANCE OF PSILOTUM NUDUM

Abstract

Psilotum nudum (L.) Beauv. (Psilotopsida) has a simple, vascularized sporophyte with a dichotomously branching aerial axis. The number and lumen diameters of tracheids in the actinostele decrease in each subsequent branch, leading to an approximate halving of the measured hydraulic conductance (Kh) from segment to segment. To understand how the anatomy of P. nudum affects Kh, a biophysical model based on the Hagen‐Poiseuille relation was developed that incorporated lumen diameter, tracheid taper, pit cavities, and pit membranes. Using a technique previously developed for ferns, pit membrane resistance was determined by measuring water flow before and after dissolving the pit membranes with cellulase. Measured Kh was in good agreement with Kh calculated with the model after excluding thick‐walled late metaxylem tracheids that dye studies showed were nonconducting. Model simulations showed that the approximately 40% overlap observed for tracheids of P. nudum was in the range leading to greatest conductance and that Kh decreased to half for 20% overlap. The model also showed that the pit membranes account for an increasing percentage of total resistance to water flow as the lumen diameter increases. Thus, the removal of such primary wall material and the evolutionary origin of vessels would have substantially increased Kh.