Myriophyllum verticillatum |
Myriophyllum |
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myriophylle verticillé, verticillate milfoil, verticillate water-milfoil, whorl-leaf watermilfoil, whorled water-milfoil |
myriophylle, water-milfoil |
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Habit | Herbs monoecious, aquatic, sometimes forming dense stands. | Herbs, usually monoecious (dioecious in M. aquaticum and M. ussuriense), aquatic to semiaquatic [semiterrestrial or terrestrial]; with fibrous and adventitious nodal roots. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Rhizomes | usually present, (absent in M. farwellii). |
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Stems | branched or unbranched, to 3 m. Turions present, becoming brown to red-brown at maturity, clavate to obdeltoid, with abrupt transition from foliage leaves to reduced turion leaves, (6–)11–37(–52) × (3–)4–6(–9) mm, apex ± rounded, lateral turions with several whorls of minute, brown prophylls, entire proximallyand toothed to lobed distally, ovate to elliptic or lanceolate in outline; leaves pectinate, strongly appressed to axis throughout, narrowly flabelliform in outline, 4.5–7.5 × 1.2–1.8(–4) mm; segments 8–12(–18), flattened, linear-lanceolate, longest segment 1.5–6 mm, basal segment usually greater than or equal to 2/3 central axis of leaf, apex ± acute, trichomes usually absent. |
erect [prostrate], terete, glabrous, hydropoten (aggregates of transfer cells) sometimes present as scattered yellow, red, or brown splotches. |
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Turions | present or absent, lateral and/or terminal. |
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Leaves | in whorls of (3 or)4, heteromorphic; petiole to 6 mm; submersed leaves pectinate, ovate to elliptic in outline, (7–)12–30(–46) × 9–24(–40) mm, segments (9–)12–22(–34), linear-filiform, longest segment (2–)6–19(–29) mm; emersed leaves pectinate, lanceolate to elliptic to ovate in outline, 2–5(–15) ×0.9–2.6 mm, segments (9–)12–20, greater than 0.5 mm. |
submersed or emersed (not emersed in M. farwellii), usually whorled, sometimes alternate, opposite, subopposite, subverticillate, or irregular [scattered], usually heteromorphic (homomorphic in M. aquaticum, M. farwellii, and M. tenellum), emersed leaves usually becoming reduced floral bracts distally; sessile or petiolate; blade unlobed or lobed, pinnatifid, or pectinate, margins usually entire, sometimes serrate, surfaces glabrous, trichomes, when present, usually in axils of leaves and leaf segments, sometimes scattered. |
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Inflorescences | to 25 cm; flowers proximally pistillate, medially bisexual, distally staminate, in whorls of 4; bracteoles cream, ovate, 0.3–0.6(–1) × 0.1–0.6(–1.3) mm, margins deeply dissected into irregular lobes. |
racemes, simple, 80+-flowered, or flowers borne singly (rarely dichasia in M. humile) in axils of emersed leaves (submersed in M. farwellii); bracteoles paired, alternate, opposite subtending leaf, apex often fringed with glandular, red trichomes, sometimes aristate; flowers unisexual or bisexual, proximally pistillate, distally staminate, often with intermediate transitional zone of bisexual flowers. |
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Flowers | 4-merous, staminate petals persistent or caducous, pistillate petals caducous or absent; stamens usually 4 or 8, sometimes 5–7 in bisexual flowers; ovary 4-locular. |
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Staminate flowers | sepals cream, narrowly triangular to deltate, 0.5–0.7(–0.9) × 0.4–0.6(–0.8) mm; petals persistent, cream, sometimes apically suffused with purple, elliptic to obovate, 1.8–2.2(–2.4) ×0.7–1.5 mm; stamens 8, filaments to 2 mm, anthers 0.8–1.7 × 0.3–0.6 mm. |
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Pistillate flowers | sepals greenish to cream, elliptic to triangular, 0.2–0.7 × 0.2–0.7 mm; petals often caducous, sometimes persistent, cream to purple, elliptic to obovate, 0.4–0.7(–0.9) × 0.3–0.5(–0.8) mm; pistils (1.3–)1.8–2.7 mm, stigmas red to± purple, to 0.8 mm. |
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Fruit(s) | globose, shallowly 4-lobed. |
a schizocarp, light green, tan, olive-brown or -green, brown, red-brown, or purple, cylindric, ± ovoid to oblong, or ± globose, cruciate to ± 4-lobed, splitting into (2–)4 mericarps; mericarps compressed to ± flattened, concave, or ± rounded, adaxially rounded, sometimes with 1–4 abaxial ridges, ridges with or without wings, surfaces smooth to ± papillate to ± tuberculate, sometimes with red punctate glands. |
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Mericarps | olive-green to brown, subglobose to globose, 2–2.7(–3) × (0.9–)1.1–1.3(–1.7) mm, transverselywidely obovate, abaxial surface broadly rounded to moderately flattened, smooth, often with 2 shallow longitudinal ridges, wings and ribs absent. |
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x | = 7. |
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2n | = 28. |
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Myriophyllum verticillatum |
Myriophyllum |
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Phenology | Flowering and fruiting Jul–Oct. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Habitat | Streams, rivers, ponds, lakes, sloughs, tannic waters. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Elevation | 0–2700 m. (0–8900 ft.) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Distribution |
AK; AZ; CA; CO; CT; DE; IA; ID; IL; IN; MA; MD; ME; MI; MN; MT; ND; NE; NH; NJ; NV; NY; OH; OR; PA; SD; TX; UT; VT; WA; WI; WY; AB; BC; MB; NB; NL; NS; NT; NU; ON; PE; QC; SK; YT; SPM; Eurasia; nw Africa (Algeria, Morocco)
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North America; Mexico; Central America; South America; Eurasia; n Africa; nw Africa; Indian Ocean Islands; Pacific Islands; Australia |
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Discussion | Four varieties of Myriophyllum verticillatum have been proposed. Fassett based M. verticillatum var. cheneyi solely on the presence of four stamens. M. L. Fernald (1950) considered var. cheneyi conspecific with M. hippuroides. Some specimens labeled as var. cheneyi examined during this treatment were confirmed to represent M. hippuroides as noted by S. G. Aiken (1981). Fernald recognized vars. intermedium, pectinatum Wallroth, and pinnatifidum Wallroth based on differences in the length of floral bracts. All of these varieties can be found in the flora area though floral bract length is a very plastic character and there is no distinct separation among the forms. The presence and morphology of turions of M. verticillatum can be very helpful in the identification of vegetative material. The clavate to obdeltoid shape and reddish brown color of the turions in this species differ from the cylindrical, typically dark green turions of both M. farwellii and M. sibiricum. Previous floristic studies have reported that the submersed leaves of Myriophyllum verticillatum have 18–34 segments. An examination of specimens unambiguously assignable to this species based on floral and fruit characters found the lower value of the range to be nine. The broad range for segment number and other leaf characters for M. verticillatum and overlap of these values with those for other Myriophyllum species underscores the importance of relying on floral and fruit characters for identifications. (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.) |
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Key |
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Source | FNA vol. 10. | FNA vol. 10. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Parent taxa | Haloragaceae > Myriophyllum | Haloragaceae | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Synonyms | M. verticillatum var. cheneyi, M. verticillatum var. intermedium, M. verticillatum subsp. pectinatum | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Name authority | Linnaeus: Sp. Pl. 2: 992. (1753) | Linnaeus: Sp. Pl. 2: 992. (1753): Gen. Pl. ed. 5, 429. (1754) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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