Our large aroids are characterized by their inflorescences with a spadix bearing small flowers subtended or even surrounded by an enveloping, often colored bract, the spathe. The tiny duckweeds (Lemna, Spirodela, and Wolffia) are highly modified and specialized aquatic Araceae, and were treated as their own family, Lemnaceae, in Michigan Flora. Mixed collections consisting of members of any or all genera of duckweeds are common, for the plants often grow thoroughly mixed.
1. Plants relatively large (> 10 cm in diameter or > 15 cm tall) with clearly differentiated normal leaves and with rhizomes or tubers.
2. Leaves compound.
2. Leaves simple.
3. Plants forming free floating rosettes, rooting in water; leaves densely hairy, sessile.
3. Plants not free floating, leaves not arranged in a rosette, rooted in soil near shores or terrestrially; leaves glabrous, petiolate.
4. Leaf blades ± sagittate, with a prominent vein extending from base of midrib into each basal lobe; spathe and spadix elongate, the flowering inflorescence usually at least 10 times as long as its diameter at the middle.
4. Leaf blades rounded or cordate at base, with veins of basal lobes (if any) no more prominent than others; spadix short, ellipsoid or nearly spherical, the inflorescence less than 5 times as long as wide.
5. Spathe white, on peduncle 7 cm or more long; plant without strong odor.
5. Spathe green and/or reddish brown, the inflorescence almost sessile, barely emerging from the ground; plant with strong skunk-like odor.
1. Plant bodies tiny, floating or submerged aquatics less than ca. 4 mm wide and 15 mm long, without differentiation into leaf, stem, and rhizome or tuber.
6. Plants with roots.
7. Internodes each with a single root (very rarely forked), sometimes flushed with purplish but seldom solid dark-colored beneath; upper surface with at most 1–3 very obscure nerves, these never radiating from a main dark spot; internodes usually averaging smaller than 3 mm in breadth or else not rounded in general outline.
7. Internodes (individual joints or “fronds,” each with several rootlets), solid dark reddish purple beneath; upper surface above point of attachment of the roots often with a purple dot, from which radiate 5–7 (or more) obscure nerves; larger internodes averaging 2.5–3.5 mm in breadth, rounded to obovate.
6. Plants without roots.
8. Plant bodies circular or ellipsoid (as viewed from the top), not more than about twice as long as wide; globose, ovoid, or flattened above and rounded below,
8. Plant bodies ±sword shaped, curved and much longer than wide.