Like most aquatic plants, the pondweeds (Potamogetonaceae) are among the most phenotypically reduced and plastic of all angiosperms. As such, hypotheses of structural homology present difficulties for morphological phylogenetic reconstruction. We used non-coding nuclear and plastid DNA data to address Potamogetonaceae relationships and accompanying issues in character evolution and biogeography. Genera currently assigned to Potamogetonaceae, plus Zannichellia, formed a strongly supported monophyletic group. Potamogeton and Stuckenia (Potamogeton subg. Coleogeton) were both resolved as monophyletic. Within Potamogeton proper, two major clades followed the traditional split between broad-and narrow-leaved species, with the latter condition optimized as basal. Heterophylly (submerged plus floating leaves) has evolved several times, and the ancestral distribution for Potamogeton appears to be Northern Hemispheric. Our phylogenetic results have provided a useful genetic framework from which to interpret morphological, cytological and biogeographical evolution.

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quatic and marsh plants are those species of flowering; whether flowers are submersed, occurring in substrates saturated with water most floating, or emergent; and whether fruits are subor all of the year. These substrates may be in-mersed, floating, or emergent. undated permanently or may have the water ta-Care must be taken to prepare quality specible at the substrate surface. This habitat often mens of aquatic vascular plants. They normally poses a barrier to the collector and special prob-do not need to be pressed immediately following lems in specimen preparation. As a result, aquat-collection. I usually wrap each collection in dry ic and marsh plants are often inadequately col-newspaper and store these wrapped collections lected and, therefore, poorly represented in in plastic bags or styrofoam chests. These bags herbaria.When or chests are kept in the shade to prevent overheating the plants. The moisture from the spec-I examine almost every wet spot, pool, lake, or imens is adequate to moisten the newspaper and stream I encounter and wade to the plants, if to keep the specimens fresh and pliable for sevnecessary. The substrate is often quite muddy eral hours. The plants can be pressed later that knees or deeoer. When water is too deep to wade, I prefer using a small carefully.

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