Flora of North America species comparison

Plants of Myriophyllum ussuriense typically grow in a semi-terrestrial habit in shallow water or on saturated sediments to a height of 20 cm. Shoots often have swollen stem bases that taper dramatically towards the apex. In some populations, extensive production of erect shoots from rhizomes produce dense stands. The floral bracts are distinctive, being opposite or alternate and elongate with usually 2–8 relatively short segments. Dimorphism in size between staminate and pistillate flowers of M. ussuriense is distinctive. Although most populations appear to be unisexual with staminate plants predominating and pistillate plants rare (O. Ceska et al. 1986), the latter are extremely small with a vestigial perianth and are easily overlooked, indicating that monoecy may be more common than thought in this species. S. Ueno and Y. Kadono (2001) reported that seven of 80 populations of M. ussuriense in Japan had some monoecious plants. No fruit was found despite an extensive examination of available material.

Submersed plants have pectinate leaves that are extremely delicate with usually fewer than 12 straight segments. A useful characteristic of some leaves is that the central axis terminates in a right-angled bifurcation.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Species ca. 68 (14 in the flora).

M. L. Moody and D. H. Les (2010) realigned some species of Myriophyllum to conform to the results of their molecular analyses, formally recognizing three subgenera, five sections, and five subsections. Because of strong levels of independent convergence, the subgenera almost completely overlap in vegetative characters. Although we recognize the phylogenetic merits of the work of Moody and Les, use of their classification here would serve only to confuse and not clarify our attempts to present the most straightforward taxonomic arrangement for Myriophyllum.

Although Myriophyllum is easily recognized in the field, positive identification at the species level has been problematic. Much of this difficulty can be attributed to over-reliance on submersed vegetative material for identification. As noted by some authors (for example, C. D. Sculthorpe 1967; G. E. Hutchinson 1975), phenotypic plasticity can greatly alter leaf characters such as size and segment number of aquatic plants in different environments. In some cases, vegetative character states used in taxonomic treatments have been based largely on information repeated from regional floras. As a result, circumscriptions of species have often not encompassed the full extent of variability observed in some taxonomic characters. Reliance on these older treatments for identification, coupled with plasticity in vegetative characters, has led to misidentifications, particularly when flowering and fruiting materials are absent. Most misidentified herbarium specimens of Myriophyllum examined for this flora did not possess reproductive characters. Currently, the most effective method for identifying vegetative specimens of Myriophyllum is by analysis of ITS and cpDNA sequences (see discussion under 8. M. spicatum).

Floral structures and fruits offer good characters to distinguish Myriophyllum species. The inflorescences in Myriophyllum are described here as racemes because most flowers are short-pedicellate. The term spike often has been used in Myriophyllum implying that the flowers are sessile. Admittedly, the distinction is largely semantic, where the pedicels are so short that the flowers appear sessile. It is important to note that mericarp characters used to distinguish these species are not fully expressed morphologically until late in their development. As a result, mericarps that are farthest from the shoot apex typically best display diagnostic characters.

Many authors have mentioned that bisexual flowers often occur on the raceme where there is a transition from pistillate to staminate flowers. Bisexual flowers are more common in Myriophyllum than realized, and we found that many staminate flowers possess pistillodes at greater or lesser stages of development. This condition is most pronounced in M. humile, in which staminate flowers often have complete pistils rather than pistillodes.

Leaf characters in Myriophyllum often have been described using terms for compound leaves, such as pinnate in form, with a central rachis and leaf divisions referred to as pinnae. Because the leaves of Myriophyllum are simple, we use pectinate, central axis, and segments as descriptors.

Many Myriophyllum species will occasionally produce a small emergent form when plants become stranded along shorelines by wave action or when water levels decrease. Only those species that produce an emersed form as part of their normal life history are discussed in some detail.

Small ascidiate (flask-shaped) trichomes are found on the stems and in the axils, or at the bases of leaves and leaf segments. They have often been referred to as hydathodes (A. E. Orchard 1979; S. G. Aiken 1981), but they have been referred to also as enations, myriophyllin glands, pseudostipules, or scales (Orchard). Because the function of these structures is unknown and they do not appear to be secretory, the best approach would seem to be to refer to them as trichomes.

Correct identification of Myriophyllum species in North America is critical in the conservation and management of aquatic habitats because some species are introduced and highly invasive (for example, M. aquaticum and M. spicatum). This issue is further complicated by hybridization of M. spicatum with native M. sibiricum (see 8. M. spicatum discussion). The native M. heterophyllum is considered invasive in the northeastern United States and Pacific Northwest, where there is evidence of hybridization with other Myriophyllum species (see 13. M. heterophyllum discussion).

(Discussion copyrighted by Flora of North America; reprinted with permission.)