PCS-Tough to keep track: Helanthium Tenellum (formerly ( proofreader had this as "formally") known as Echinodorus Tenellus before it was reclassified) or is it dwarf?
Here's the scoop on the taxonomy from a profile I wrote some years back.
Pygmy chain sword, Dwarf chain sword
Southern United States through Central America and into South America down to Paraguay.
This plant has long been known under the name Echinodorus tenellus
and will likely still appear as such for some time; the explanation for the recent reclassification is given below. The species epithet tenellus
is changed to tenellum
to agree in gender with the genus name Helanthium
As with all species in this genus, Helanthium tenellum
can be grown permanently submersed in an aquarium or emersed as a bog plant in paludariums and terrariums provided the roots are permanently submerged. When grown emersed, the leaves are oval and on short stems, and flowers will be produced. If planted in the aquarium, the new leaves will be awl-shaped phyllodes (stemless linear and narrow leaves), and the emersed leaves will yellow and die off. Submersed phyllodes (leaves) are variable and there are two generally-recognized variants: the "narrow leaf" has a leaf width of 2mm (1/16 inch) and grows to 3 inches or sometimes a bit taller; under bright light the leaves may turn slightly reddish. The "wide leaf" has a leaf width of 5mm (1/8 inch) and attains 3-4 inches in length but in lower light it may grow to 10 inches.
In both the emersed and submersed forms, it reproduces vegetatively by sending out numerous runners on top of the substrate from which plantlets will emerge every couple of inches and root in the substrate. It spreads rapidly once it is settled into the aquarium, and may be controlled by cutting off the runners as needed; the daughter plants can be re-planted. The lighter green leaves contrast nicely with other plants including the larger swords that are a darker green.
Given the large geographic distribution of this species, it is not surprising that there are several variations, differing in the size of the leaves and the shape of the achenes which are the dried fruits containing one seed (Rataj & Horeman, 1977). Nurseries frequently supply Helanthium bolivianus
as H. tenellus
but the two species are easily distinguishable; H. bolivianus
has three rows of chlorophyll-free pullucid "windows" adjacent to the leaf spine, while H. tenellus
has normal (green) tissue throughout the leaf (Rataj, 2004).
This species was originally described by Buchenau in 1869 and placed in the genus Echinodorus
, the second-largest genus of aquatic plants in the Alismataceae family.
The Alismataceae is a family of aquatic herbs containing 12 genera with about 80 species that are distributed in the tropical and subtropical regions of both hemispheres. Three genera of interest to aquarists occur in the Neotropics [=tropical regions in the Americas]: Sagittaria, Echinodorus
. The species in these genera are quite similar in appearance, making it difficult for aquarists to differentiate between them. To add to the confusion, even within each species the plants can take on quite different leaf lengths depending upon the conditions in the aquarium. With a few exceptions that grow fully submersed, the plants are amphibious bog plants in their habitat, spending roughly half the year emersed when they flower, and half submersed during the flooded period which lasts several months.
The group Helanthium
[the spelling Helianthium with the first "i" is incorrect] was described by Engelmann, Bentham and Hooker in 1883 as a section in the genus Alisma
. In 1905, Engelmann and Britton erected Helanthium
as a distinct genus and they assigned to it the dwarf chain sword species from Echinodorus
. In 1955, Fassett reversed this and considered the species within Helanthium
; however, he divided the genus into two subgenera, Helanthium
held two sections, Nymphaeifolii
(containing one species, Echinodorus nymphaeifolius
) and Tenellii
that contained the several closely-related species with E. tenellus
as the type species. The subgenus Echinodorus
held nine sections containing the remaining species within this genus. In his recent revision of the genus Echinodorus
, Rataj (2004) followed Fassett (1955).
In phylogenetic analysis (Lehtonen 2006; Lehtonen & Myllys 2008) Echinodorus
was found to be polyphyletic [=the last common ancestor is not included in the genus] and in order to obtain a monophyletic [=a clade (here genus) consisting of the last common ancestor and all descendant species] circumscription of the genus, the classification proposed by Pichon (1946) was followed by Lehtonen. E. nymphaeifolius
was transferred into the genus Albidella
, and E. bolivianus, E. tenellus
and E. zombiensis
were transferred into Helanthium
. A number of prominent botanists and institutions including several suppliers of aquarium plants are now accepting this reclassification, including the Royal Botanical Gardens at Kew (a major authority among botanists) and the International Plant Names Index. We are therefore following this classification.
The species now in Helanthium
are those former Echinodorus
species that have traditionally been considered within the generic common designation of dwarf chain sword plants and are smaller than the true Echinodorus
species, though leaf length can vary greatly depending upon conditions in the aquarium. The same species grown in two aquaria can look different, and within the same aquarium two plants of the same species may appear slightly different. The number of species varies according to author, from 5 (Lehtonen) to 9 (Rataj). They are distributed from the temperate USA down to Argentina, and all species are amphibious bog plants that grow emersed and submersed. Regardless of whether they are cultivated emersed or submersed, these species propagate vegetatively via runners up to 50 cm in length from which plantlets arise at intervals of 2-5 cm. They also produce inflorescences when growing emersed which produce flowers but adventitious plants are rare.
Fassett, Norman C. (1955), "Echinodorus in the American Tropics," Rhodora
, Vol. 57, No. 677 (May 1955).
Frank, Neil (2000), The Chain Sword Plants: History and Nomenclatural Perspectives
, Aquatic Gardeners Association [online].
Lehtonen, Samuli (2007), "An integrative approach to species delimitation in Echinodorus
(Alismataceae) and the description of two new species," Kew Bulletin
Vol. 63, No. 4, pp. 525-563.
Lehtonen, Samuli and Leena Myllys (2008), "Cladistic analysis of Echinodorus
(Alismataceae): simultaneous analysis of molecular and morphological data," Cladistics
, Vol. 24, No. 2 (April 2008), pp. 218-239.
Rataj, Karel (2004), "A New Revision of the Swordplant Genus Echinodorus Richard 1848 (Alismataceae)," Aqua--Journal of Ichthyology and Aquatic Biolo
gy, Special Publication No. 1, March 2004.