Northern Prairie Wildlife Research Center
The entire duckweed family has recently been studied on a worldwide basis by Prof. E. Landolt (op. cit.) of the Geobotanical Institute in Zurich. As a result of his investigation, it appears that the surface-floating Lemna species (i.e., excluding L. trisulca, an essentially submersed species) are more diverse than traditionally viewed by authors. Most significantly, populations that have previously passed for L. minor in this region are predominately L. turionifera according to Landolt. Recognition of the latter seems reasonable in light of its unique ability to form small turions which sink to the bottom under unfavorable conditions. This is no doubt an important adaptation for overwintering in this part of the country. L. minor does occur as far north in the Great Plains as s and possibly e SD, but even in the s part of our range, L. turionifera is prevalent. Unfortunately morphological distinction between L. minor and L. turionifera is often difficult and one cannot discount the possibility of mixed populations.
Five other surface-floating species of Lemna occur within or at the s edge of this region, but these are uncommon and local in occurrence. Some occurrences may well represent temporary introductions by waterfowl. These species include L. aequinoctialis, L. gibba, L. minuscula, L. obscura and L. perpusilla. In the interest of completeness, these species verified for the northern Great Plains by Landolt are also included here.
The key that follows is based on Landolt's treatment and necessarily uses some minute characteristics to make distinctions between species. Fruits are valuable for identifying species of Lemna but are rarely present so that most identifications must be based on vegetative traits. For studying internal and some external details of duckweed fronds, Landolt (personal communication) recommends clearing some thalli of pigment by boiling for 2 minutes in a 70% alcohol solution. This enables one to see veins, papillae (on the upper surface of fronds) and air cavities within the fronds more clearly under magnification. Because the production of anthocyanin pigment is of taxonomic importance, uncleared material should also be saved and checked for the presence or absence of red (or purplish when dried) coloration. For purposes of preservation, duckweeds and other Lemnaceae can be pressed and dried like other plants. Their small size suits them well for storage in paper packets or envelopes.
Since duckweeds can colonize calm or slow-moving water almost anywhere (provided salt concentrations are not excessive), it would be meaningless to describe specific habitats for them. Their success is more dependent upon micro- rather than macrohabitat factors . Thus habitat descriptions are omitted from the species treatments that follow. Also, flowering periods are not given as flowering is so rare and sporadic.
|1||Fronds denticulate toward the apex, tapered to a slender stipitate base, the stipe often as long as the main body and commonly attached to the parent frond; colonies star-shaped, usually submersed.||L. trisulca|
|1||Fronds entire on the margin, nearly rounded and not obviously stipitate at the base, solitary or in tight colonies, these not star-shaped, floating on the water surface or stranded on mud.||Lead 2|
|2||Fronds obscurely 1-nerved.||L. minuscula|
|2||Fronds 3- to 5-nerved.||Lead 3|
|3||Root sheath winged at the base; root tip sharply pointed; roots not longer than 3 cm; fronds completely green.||Lead 4|
|3||Root sheath not winged at the base; root tip mostly rounded; roots often longer than 3 cm; fronds often red-tinged beneath or with red spots on either surface.||Lead 5|
|4||Fronds very often with 2-3 papillae in a row on the upper surface above the node (the level at which daughter fronds attach); seeds whitish, with 35-60 faint ribs, not escaping the fruit wall when ripening.||L. perpusilla|
|4||Fronds with only 1 prominent papilla above the node; seeds brownish, with 8-22 prominent ribs, falling out of the fruit wall when ripening.||L. aequinoctialis|
|5||Fronds with several about equal sized, small papillae on the upper surface from the midline to the tip (often obscure), very often red-tinged on the lower surface, forming small, obovate to orbicular, rootless, dark green to brown turions under unfavorable conditions, these sinking to the bottom of the water.||L. turionifera|
|5||Fronds lacking papillae or with one prominent papilla at the apex and another just above the node and with smaller papillae between them, rarely forming turions; if formed, the turionlike fronds have short roots and are slowly forming daughter fronds.||Lead 6|
|6||Fronds very often gibbous, strongly convex and obviously inflated beneath, with air spaces often larger than 0.3 mm in diameter, very often 4- to 5-nerved with the nerves all arising from the same point at the node; ovules 1-6; fruit winged.||L. gibba|
|6||Fronds flat to slightly convex beneath, not inflated, with air spaces less than 0.3 mm in diameter, rarely with more than 3 nerves, but if 4- to 5-nerved, then the outer nerves arising at the base of the inner ones; ovule 1; fruit wingless.||Lead 7|
|7||Papilla at apex of the frond very prominent; fronds often red beneath.||L. obscura|
|7||Papilla at apex of the frond not very prominent; fronds never red beneath.||L. minor|