Podostemum ceratophyllum

Podostemum locality on the Mount Hope River in Mansfield, Ct.

Podostemum ceratophyllum, or Hornleaf Riverweed, is the only really good example in Connecticut of a flowering plant that is a rheophyte. Rheophytes are aquatic plants that exclusively live attached to rocks in rapids or waterfalls, in clean, violently fast-moving water. This almost absurdly specialized ecological niche is occupied by species in various unrelated plant families, included a couple of tropical species of Utricularia, the carnivorous bladderworts, but the most diverse family that is exclusively rheophytic is the Podostemaceae.

Podostemum ceratophyllum in the Mount Hope River.

 

 

The Podostemaceae are peculiar things, with a morphology that is difficult to interpret, involving a certain degree of ambiguity between the three classical types of plant organ: roots, stems and leaves. A green, photosynthetic, creeping holdfast-type organ grips the rocks on which the plants grow, and in Podostemum and most other genera this is root tissue (some of the research establishing the homology of Podostemum holdfasts was conducted by UConn's own Tom Philbrick). The roots give rise internally to root born shoots, which bear what are probably leaves (but may be in some cases leaf-like stems) as well as the flowers. The structural oddity of riverweeds, presumably resulting from adaptations to their extreme environment, has also historically made the relationships of the family difficult to determine. Modern molecular evolutionary work seems to have finally placed the Podostemaceae firmly within the order Malpighiales, closely related to the Hypericaceae (Saint John's Wort family).

Podostemum ceratophyllum fruits.

The riverweeds flower and fruit during periods of low water, when the plants are exposed to air. In New England, flowering season is late summer, when there is often something of a drought and the rivers are at their lowest ebb. The flowers are small and non-showy, and probably self-pollinated or wind-pollinated. The fruits are little dry capsules that look quite a bit like moss sporangia (spore capsules). The seeds are sticky when wet, and able to glue themselves to rocks so that they stay in place when their habitat is flooded again.  

Podostemum ceratophyllum only grows in certain special sites, not only needing rock submerged in clean, cool, well-aerated, swiftly flowing water, but also being restricted to exposed, sunny stretches of river. It is apparently a species in decline as a result of development and agricultural runoff, vulnerable especially to sediment and nutrient pollution in its water. One of the  local riverweed sites on the Natchaug River in Chaplin is still extant, but the plants don't seem to be as abundant or healthy as they were 30 years ago, when I was first shown the spot. 

Podostemum locality on the Natchaug River, Chaplin, Ct.

Various people have attempted to cultivate Podostemum and other riverweeds, as far as I know without any long term success. Plants on cobbles moved to aquariums with vigorous aeration and water circulation will languish for a few weeks, then decline and disappear. Seeds have been germinated in artificial conditions, but the seedlings don't seem to persist. It should be possible to grow Podostemum from seed to seed indoors, but it would perhaps require more attention to light, water movement, temperature and water chemistry than anyone has been willing to lavish on a river weed up to this point.