Parque Las Llovizna, Ciudid Guayana, Venezuela

A brief introduction to the Podostemaceae (riverweeds)  

 Taxonomy

            The riverweeds represent the largest family of strictly aquatic flowering plants.  The number of genera in the family ranges from about 46 to 50, depending on one's taxonomic opinion.  Perhaps as many as 300 species occur in the family.  The actual number remains to be determined as the taxonomy of the family is chaotic in many regions.

Geographic distribution

            Riverweeds are widespread in tropical and subtropical regions.  Species occur in many areas of Asia, north-east Australia, Africa, Madagascar and the Americas.  Few species (e.g., Podostemum ceratophyllum in eastern North America) reach into temperate regions.

            Many authors have reported that species are restricted to very narrow geographic regions, i.e., many species are endemic.  How much of this endemism is real, versus an artifact of superficial taxonomic understanding, is not known.

  Plant form

            The vegetative morphology of plants in this family is often bizarre as compared to other flowering plants.   Distinctions between roots, stems and leaves are often obscured.  Roots can be linear and cylindrical or flat, photosynthesis and lichen-like in general appearance.  They can also occur prostrate, firmly attached to the substratum, or attach at a single location and thus hang in the water current.  Stems can also be variable in form (e.g., upright, prostrate, flattened).  Leaves range from tiny scale-like structures to broad, and thick and somewhat reminiscent of lettuce leaves.

            Flowers are hermaphroditic and not especially different from other angiosperms.  Tepals occur in some species, while they are lacking in others.

  Ecology

            Riverweeds occur in rivers and streams that have distinct high-low water periods.  Most species occur only in open sunny areas.  Plants attach to rocks (or other solid substrata) in the swift currents of river-rapids and waterfalls (haptophytes).   Plants remain vegetative when the water level is high and plants are submersed.  Flowering occurs as plants are exposed when the water level drops.  Both wind and insect pollination occurs in the family.  Few studies on the ecology of riverweeds have been published.

  Conservation

            It is predictable that riverweeds are detrimentally impacted by factors that influence the seasonality of the water level, attachment to solid substrata, and light availability.  For example, dam building changes the high-low seasonal water pattern and no doubt leads to loss of populations in flooded regions of rivers.  Sedimentation from, for instance, run-off from agriculture and construction, covers plants and solid substrata, and is also detrimental.  Nutrient pollution (e.g., from agriculture and domestic sources) can lead to increased algae cover, which decreases light availability.

Although the above statements seem intuitive, there is little empirical data to support them. Such studies are needed.