Dr. Jeffrey M. Osborn

Publication Abstract

Philbrick, C. T. and J. M. Osborn. 1994. Exine reduction in underwater flowering Callitriche (Callitrichaceae): Implications for the evolution of hypohydrophily. Rhodora 96: 370-381.

Both aerial and underwater pollination systems (hypohydrophily) occur in Callitriche (Calitrichaceae), and, as such, the genus can serve as a model system in which to probe questions concerning the evolution of hypohydrophily from aerial pollination systems. Evidence from scanning and transmission electron microscopy reveals strikingly different exine thickness in the pollen of four aerially flowering species (C. heterophylla var. bolanderi, C. marginata, C. peploides, and C. stagnalis), which have a distinct sexine layer (0.55-1.1 µm thick) in comparison with that of the hypohydrophilous C. hermaphroditica. The exine of the latter species is rudimentary (< 0.1 µm thick) in pollen of the annual form of the species, and virtually absent in the perennial form. The degree of exine reduction in pollen of C. hermaphroditica relative to that of the aerially flowering species contrasts with the otherwise general morphological similarity observed among the flowers of the respective species.

Aquatic plants, Callitriche, Callitrichaceae, Underwater pollination, Pollen, Ultrastructure