Stipe Anatomy, Water Potentials, and Xylem Conductances in Seven Species of Ferns (Filicopsida)

Abstract

Anatomy and water relations were studied for the desert fern Notholaena parryi, as well as six other ferns representing three different orders which occupied xeric as well as mesic habitats. Tracheid number and diameter, and total xylem cross sectional area increased during leaf development for N. parryi; the whole plant conductance (volume flow of water through a stipe divided by the rhizome-to-leaf water potential drop) increased but tended to level off as the leaves matured. The reported occurrences of very steep water potential gradients (about 25 MPa m–1) in stipes of N. parryi were confirmed. The ferns with the highest whole plant conductances (Alsophila australis, Botrychium dissectum, and Adiantum capillus-veneris) had the largest or greatest number of tracheids. Numerous tracheids in Botrychium dissectum offset a low tracheary conductivity, whereas Marsilea vestita had few tracheids resulting in a low whole plant conductance. Whole plant conductances for the ferns were 2 to 3 orders of magnitude less than those generally observed for angiosperms and 6 orders less than for gymnosperms. However, the relative conductivity (whole plant conductance times stipe xylem length divided by xylem area) was only 5- to 10-fold less than for angiosperms and about the same as for the gymnosperms. Stipe water relations in these ferns are discussed in relation to the evolution of xylem anatomy.