Usually considered a difficult genus, because aquatic biologists often wish to identify vegetative specimens and there is great phenotypic variation in leaves. The species are distinctive when fertile. Some suggestions for identification of vegetative plants follow the listing of species. Like many aquatics, unusual forms develop, at least in some species, when plants are partly or entirely stranded by a lowering of water levels, or resubmersed upon a later rise in levels. No local species displays quite as dramatic heterophylly as Proserpinaca palustris, except perhaps for the stranded terrestrial form of M. heterophyllum.
Anatomical studies (England & Tolbert, 1964) have shown that the leaves are basically alternate in M. heterophyllum, though they appear (and are said to be) whorled as a result of extremely short internodes. This species often produces a few alternate leaves on its stems, and M. farwellii characteristically has some alternate leaves. In M. alterniflorum and M. tenellum, the flowers are alternate, and in the latter so also are the rudimentary leaves. Some species produce winter buds or turions, specialized shoots with very short internodes and compact leaves. Such structures aid in winter survival and dispersal. The species with emersed spikes are wind-pollinated, the pistillate (lower) flowers beginning to mature just before the staminate (upper) flowers.
1. Leaves all alternate, reduced to minute scales (less than 1 mm long) or bumps on the stem; floral bracts alternate, ca. 2–3 mm long or less, entire.
1. Leaves all or partly whorled, well developed and pectinate; floral bracts whorled, or if alternate then often toothed.
2. Emergent fertile shoots with fully developed pectinate leaves ca. 20–30 mm long, about the same size and form as the submersed leaves; petioles of submersed leaves mostly 2–6 (–8) mm long.
2. Emergent fertile shoots with reduced, entire to pectinate bracts less than 20 mm long (usually much less) and quite different from the submersed leaves, or absent (flowers borne under water in the axils of normal submersed leaves); petioles of submersed leaves mostly (0–) 0.4–2.5 (–4) mm long.
3. Flowers and fruit in the axils of normal submersed leaves; body of fruit ca. 1.5–2 mm long, sharply tuberculate in ridges on the back; leaves partly alternate (or irregularly whorled), extremely limp, the foliage becoming a shapeless mass upon removal from water.
3. Flowers and fruit in the axils of small bracts in an emersed terminal spike; body of fruit various in length (often longer or shorter than in M. farwellii), but smoothly rounded on the back or irregularly roughened; leaves normally all whorled (a few often alternate in M. heterophyllum), usually retaining at least some shape on removal from water.
4. Well-developed leaves 4–12 mm long; distalmost flowers in axils of alternate bracts.
4. Well-developed leaves usually 10–40 mm long; distalmost flowers in axils of whorled bracts.
5. Middle and upper bracts much longer than the flowers and fruits, with distinct and merely toothed blades; leaves crowded, the internodes less than 8 (–10) mm long; stamens 4.
5. Middle and upper bracts about as long as the flowers and fruits or else deeply pectinate; leaves usually less crowded, the internodes often 10–27 (–40) mm long (and only rarely all less than 5 mm); stamens usually 8.
6. Bracts of middle and upper flowers deeply pectinate (more than halfway to the rachis), even the upper equalling or longer than the flowers and fruits; stems greenish or brownish; leaves, or many of them, essentially sessile (the lowest lateral segments arising next to the stem).
6. Bracts of middle and upper flowers (not necessarily ones transitional to leaves) merely toothed or entire, the upper shorter than the flowers and fruits; stems (at least when dry) pale, whitish or light pink (except very near surface of the water); leaves all distinctly petioled.
7. Lateral segments of leaves 5–11 (–13) on a side; turions produced late in the season.
7. Lateral segments of most well-developed leaves (13–) 14–17 (–20) on a side; turions never produced.
Notes on identifying vegetative Myriophyllum
Myriophyllum tenellum poses no problem. The small-leaved M. alterniflorum could be confused only with forms of M. sibiricum with reduced leaves (discussed under that species). Myriophyllum farwellii is extraordinarily limp and grows rarely in northern softwater lakes, nowhere else, and could be confused only with lax vegetative M. heterophyllum. Myriophyllum aquaticum is a rare introduction that usually produces its distinct emergent leaves. The following notes may help with the other four widespread species:
1. If there is a definite tendency to alternate leaves somewhere on the stem, the plant is M. heterophyllum (if not M. farwellii). If all leaves are whorled, it may be any species (except M. farwellii).
2. If many internodes exceed 10 mm, it is not M. heterophyllum.
3. If the foliage is dense (very short internodes, sometimes with 6 leaves) and quite delicate, it is probably M. heterophyllum.
4. If the stems are whitish (with scattered yellow-orange dots), it is not M. verticillatum.
5. Only M. spicatum has as many as 14–17 segments on each side of some or all leaves.
6. Myriophyllum sibiricum and M. verticillatum have fewer than 14 segments on each side of the leaf, but differ in stem color (brownish or greenish in the latter), and the former has the leaves all petioled. Plants with fewer than 8 or 9 segments are M. sibiricum rather than M. verticillatum.
7. If the base of the plant is present, in M. verticillatum and M. sibiricum roots may arise from a strong U-shaped bend in the stem, representing the axis of a turion, which only these species (and M. farwellii) produce.
Note: All four of these species may produce a strongly thickened stem (1.5–2 times the width lower down) shortly below the inflorescence; thus, this is not a reliable character, as sometimes stated, for M. spicatum.