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Species of Podostemaceae grow in rapidly flowing tropical rivers, streams and water falls, strongly attached to rock substrates via expanded, adhesive-secreting holdfasts (haptera). Vanroyenella is a monotypic genus endemic to Mexico, occurring in the northern extent of the tropical distribution of Podostemaceae in the New World. Embryo sac development and nucellar plasmodium formation of Vanroyenella plumosa Novelo and Philbrick was studied. The numerous ovules are anatropous, bitegmic, exostomous, and tenuinucellate, each with a four-celled monosporic embryo sac of the Apinagia type. Disintegration of the nucellus into a plasmodium takes place prior to fertilization. Embryo sac development of Vanroyenella is similar to other members of the Podostemoideae subfamily.

The purpose of this study is to present and analyze empirical data on seed production and seed characteristics of Mexican and North American species of Podostemaceae. Species of Marathrum, Oserya, Podostemum, Tristicha and Vanroyenella were studied. Podostemaceae flower annually in abundance. Eight of the nine species examined produced seeds in abundance. Podostemum ricciiforme (Liebmann) Royen seldom produces seed. Significant differences occurred between annual and perennial species in mean ovule and seed number. In Podostemaceae, the outer integument of the seed plays a central role in the establishment of seeds on solid substrate. Significant differences between species were observed when comparing overall seed width and length for dry versus wetted seed (cf. percent contribution of the hydrated outer integument). In all species there was a significant positive correlation between integument size and seed size. It remains to be shown whether the observed differences in seed characteristics are ecologically significant, e.g. equate with differences in microhabitat attachment, or reflect differential selection for seed establishment. Clear patterns of relationships between seed production and geographic distribution of species studied herein are lacking. Questions that remain to be addressed concerning seed ecology are discussed.

The Podostemaceae are the largest family of strictly aquatic angiosperms; however, relatively little is known about the palynology of the family. Pollen micromorphology and ultrastructure of five representative New World taxa are described, including Marathrum rubrum Novelo & Philbrick, Oserya coulteriana Tul., Podostemum ceratophyllum Michx., Tristicha trifaria (Bory ex Willd.) Sprengel, and Vanroyenella plumosa Novelo & Philbrick. Pollen grains from all five species are relatively small, spherical, microechinate, have a tectate-granular sexine and a thick nexine in non-apertural regions, and a semitectate sexine and a thin nexine in apertural regions. Characters that vary among the taxa include dispersal unit (monads or dyads), sculptural element morphology, infratectal granule size, and aperture morphology and ultrastructure. This is the first study to describe the pollen morphology of these five taxa in detail, and it is the first to illustrate the ultrastructure of pollen wall characters for any member of the family.

A taxonomic treatment of Podostemaceae in Mexico is presented, based on herbarium study and extensive field collections. Five genera and eight species of Podostemaceae (including Tristichaceae) occur in Mexico: Marathrum Humb. & Bonpl. ( M. rubrum Novelo & Philbrick, M. schiedeanum (Cham.) Tul., M. tenue Liebm.), Oserya Tul. & Wedd. ( O. coulteriana Tul., O. longifolia Novelo & Philbrick), Podostemum Michx. ( P. ricciiforme (Liebm.) P. Royen), Tristicha Du Petit-Thouars ( T. trifaria (Bory ex Willd.) Spreng.) and Vanroyenella Novelo and Philbrick ( V. plumosa Novelo & Philbrick). More species occur on the Pacific slope (especially the states of Jalisco, Michoacan and Guerrero) than the Atlantic slope. In Mexico, Podostemaceae occur predominantly in three main climate types (A, hot and humid; B, dry; C, temperate and humid), and eight Mexican physiographic provinces. Podostemaceae are found primarily in river rapids that occur in the following vegetation types: oak-pine forest, moist mountain forest, tropical rain forest, tropical subdeciduous forest and tropical deciduous forest.

The systematic position of the river-weed family Podostemaceae remains enigmatic due to taxonomic difficulties imposed by the radically altered morphology of these alga-like angiosperms. Although previous workers have placed this group phylogenetically among a wide variety of monocotyledons and dicotyledons, most contemporary authors have proposed that river-weeds are closely related to members of the dicotyledonous order Rosales. A diversity of opinion also exists as to whether the Hydrostachyaceae are related to Podostemaceae. We have investigated the phylogeny of river-weeds by comparing DNA sequences of the chloroplast encoded rbc L gene for eight river-weed genera together with 84 other angiosperm and 11 non-flowering seed plant taxa. The high level of sequence divergence in rbc L that exists between river-weeds, Hydrostachyaceae and other angiosperms presents systematic problems that parallel those associated with the highly divergent morphology of these groups. Rooting rbc L sequences with distant non-flowering plant outgroups results in a topology where Podostemaceae comprise a basal angiosperm clade, but in which other renditions of angiosperm family relationships are depicted unreasonably. Restricting the comparison of river-weed sequences entirely with angiosperms places the group as a sister clade to the Hydrostachyaceae as some authors had anticipated, but this result is only weakly supported. The high level of both morphological and molecular divergence in the river-weed clade confounds efforts to correctly ascertain their phylogenetic relationships. A tentative hypothesis from rbc L data is that the Hydrostachyaceae and Podostemaceae are sister taxa whose closest relatives are the rosid families Crassulaceae and Haloragaceae.

Laboratory studies of seed germination were conducted for five species of Podostemaceae from Mexico: Marathrum haenkeanum Engler, Marathrum rubrum Novelo & Philbrick, Oserya coulteriana Tul., Tristicha trifaria Bory ex Willd. Sprengl, Vanroyenella plumosa Novelo & Philbrick. This represents the first comparative study of seed germination for North American Podostemaceae. Germination occurred within 4 days when seeds were placed in water and under lights. Germination was ⩾95% after 9 days for all but one of the 11 collections examined. Seed placed in water but in the dark did not germinate. Marked interspecific differences in seed germination were not observed. The biology of Podostemaceae is poorly known, especially the more cryptic stages as seeds and seedlings.