Parque Las Llovizna,
Ciudid Guayana, Venezuela
brief introduction to the Podostemaceae (riverweeds)
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.
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.
The vegetative morphology of plants in this family is often bizarre as
compared to other flowering plants.
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
Flowers are hermaphroditic and not especially different from other
angiosperms. Tepals occur in some
species, while they are lacking in others.
Riverweeds occur in rivers and streams that have distinct high-low water
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.
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