Taxa treated:
Myriophyllum
alterniflorum
aquaticum
sibiricum
spicatum
verticillatum

References
Key
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Haloragaceae Taxa treated:
Myriophyllum alterniflorum
Myriophyllum sibiricum
Myriophyllum spicatum
Myriophyllum verticillatum
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 © Flora Nordica

by Stefan Ericsson
(6b, 20070531)


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This treatment is partly outdated - the final version will appear in print.

Myriophyllum L.

Linnaeus, Sp. pl.: (1753).
Literature. Aiken 1981a, Aiken & McNeill 1980.
Hydrophytes perennating by roots or by detachable winter buds (turions). Submersed, glabrous, aquatic herbs with irregularly branched shoots, to 3 m. Leaves in whorls of 3–6, exstipulate, submerged or rarely aerial; blade pinnatisect with filiform lobes (segments) which are opposite, subopposite or alternate.
Inflorescence of one or more terminal, bracteolate spikes emerging above the water surface. Spikes more or less erect, with verticillate (or sometimes alternate), sessile, bracteate, unisexual or sometimes bisexual flowers (female in lower whorls, male in upper, often bisexual in the transition zone). Male flowers with 4 caducous petals; stamens 8. Female flowers with or without perianth, gynoecium 4-lobed with tufted, sessile stigmas. Fruit a subglobose schizocarp, of about the same size as the gynoecium, with 4 longitudinal ridges where it splits into 4 one-seeded nutlets with stony, sclerified endocarp..
Chromosome base-number x=7.
Biology. Wind-pollinated and probably also insect-pollinated. The fruits are spread by water. Myriophyllum seeds exhibit prolonged dormancy; germination is erratic and seedlings are rarely seen. Vegetative reproduction is common and efficient; for the taxa which do not have turions (specialized buds functioning as diaspores), fragmentation is the most important mode of reproduction and dispersal.
A Myriophyllum overwinters either as a root crown (M. alterniflorum and M. spicatum) or as turions (M. sibiricum and M. verticillatum; fig.). Since the root crowns are perennial (fig.) and sensitive to freezing, the former species can only survive at depths below the ice, or in running water that does not freeze to the bottom. M. sibiricum and M. verticillatum, on the other hand, can be looked upon as functionally annual; the motherplants die in winter, but the turions can withstand very low temperatures and do survive the winter even when embedded in solid ice. They can therefore grow in shallow lagoons and inlets with harsh winter conditions. Turions start to appear in late summer when the water temperature decreases (late summer to autumn; fig.). In spring they start to grow, elongate and produce roots.
Variation. It seems likely that the species of Myriophyllum show very little genetical variation. The phenotypical variation is however considerable, since the individuals readily adapt their growth to the environment (see Aiken & Picard 1980, Aiken 1981b, Gerber & Les 1994). Plants growing in running water become long and flaccid, those in exposed habitats have shorter leaves, and shoots at great depth have thinner, longer leaves. Plants living at the edge of their niche usually become abnormal, e.g. developing shorter leaves in low-nutrient environments, thicker leaves and fewer leaf-segments when the water is too salty, or succulent leaves and other abnormal growth when the water level is too low or plants continue to grow after drifting ashore. Some of these extremes have been named in the past, but are apparently all modifications..
Even if the environmentally induced variation is wide, it follows a similar pattern in all species; it is likely that a certain character, e.g. leaf length, will show the same tendency in all species present at a locality. Therefore a M. spicatum growing in the Gulf of Bothnia may look very different from an individual in a lake in Denmark, but a M. sibiricum and a M. verticillatum growing side by side in the river Vindelälven in LyL may look confusingly similar. To describe the pattern of these modifications in detail lays beyond the scope of this treatment, but some hints are given in the “similar taxa” sections.
Hybridization. In North America, much attention has been payed to hybridization between M. sibiricum (native) and M. spicatum (invasive alien) (Moody & Les 2002, 2007). Hybridization has not been reported from the Nordic area but could possibly occur.
Identification. Fortunately, there are a few reliable characters that are easy to observe: whether turions are present or not, the shape and position of these turions, the presence and length of pinnatisect bracts, and the position of male flowers. Individuals with inflorescences are usually easy to identify; the exceptions are M. spicatum and M. sibiricum in the middle of the summer when M. sibiricum is usually lacking any trace of turions, and M. alterniflorum before the male part of the spikes develop. Non-flowering individuals are often looked upon as impossible to identify, but individuals found during seasons when the turionic species should have turions or turion leaves from the previous year cause few problems.
Since turions usually rest in a horizontal position, the base of plants emerging from turions tend to be J-shaped. The turion leaves stay attached to the base of the plant until early summer. As the shoot elongates, they become spread along a stretch of 3–10 cm or even more, but finally fall off. On specimens collected later on, detached turion leaves can sometimes be found among the roots or in attached substrate.
The main struggle is the identification of non-flowering individuals lacking turions. Such plants are often abundant and can be confusing to students of water vegetation. To identify them, one usually has to rely on leaf characters. At first this might seem difficult, but there are shortcuts. At any given locality, the width of variation within each species is much narrower than the total span mentioned in the species descriptions, and there are always some differences in foliage between different species. If a representative material of non-flowering plants is collected, its division into morphotypes is usually more or less obvious. Later, one can link these morphotypes to similar plants collected at the same locality at a time when their condition is more favourable for identification, e.g. when the plants are flowering or are preparing for winter.
Leaf characters that should be considered are colour, length, number and density of divisions, outline in water (flat or 3-dimensional, irregular or almost perfectly feather-like), their resilience when emerged (elastic and keeping their natural shape or flaccid and collapsing; to check this hold a wet plant upside-down), and shape of leaf-segments (thick or thin, coarse or flexible, filiform or somewhat tapering in outer part). At first, the leaves are more or less transparent with a greenish or reddish tone. Later they darken, or (in M. verticillatum) remain green.
Hydatodes, i.e. small, usually dark, filiform appendages found e.g. on stem near leaf axils and at floral nodes (between flowers ) are of some taxonomical value but have been little studied.
The species descriptions do not cover terrestrial modifications. Unless stated otherwise, leaf characters in the key and descriptions refer to leaves from the middle part of the stem. Leaves on inflorescence-branches may deviate, and basal leaves may be transitional to turion leaves. Since the fruits are very similar to the gynoecia they are not described separately.
Some floras state that certain species differ in whether the leaf segments are opposite or alternate, but this is not the case. In all species the segments tend to be opposite or subopposite in the basal part of the leaf, and more or less alternate in the apical part. The number of leaves in each whorl is also sometimes used as a character, but this varies too much to be useful.

The following key is designed for identification of both flowering and non-flowering plants. Plants with distinctive characters are keyed out first, causing all species to reappear further down. Since the vegetative characters are more or less relative, measurements are seldom mentioned. The key can still be used to compare specimens from localities that obviously hold more than a single species. Terrestrial modifications are not covered, but land plants of M. verticillatum, which will usually flower, can be keyed out anyway. Leaf characters refer to leaves from the middle part of the stem.

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1 Turions present on branch apices or basally on club-shaped shoots (late summer–autumn)
2
- Turions not present
3
 
2 Turions blunt-ended, broadest apically, finally becoming narrowly club-shaped, dense and stiff; inflorescence, if present, with conspicuous pinnatisect bracts longer than the flowers
- Turions more or less pointed, broadest basally or in the middle part, rather loose and flexible; inflorescence, if present, with bracts shorter than the flowers
 
3 Turion leaves from the previous winter present at base of plant (spring–early summer)
4
- No turion leaves at base of plant
5
 
4 Turion leaves more or less rhombic, scale-like, upwards usually decreasing in size, green or almost black, turion leaf segments succulent; leaves coarse, elastic
- Turion leaves more or less spathulate, not scale-like, upwards gradually becoming similar to ordinary leaves, green or brown; turion leaf segments with long, soft, delicate apices; leaves soft, flaccid
 
5 Inflorescence present
6
- Inflorescence not present
9
 
6 All flowers with conspicuous, rather stiff, pinnate or pectinate bracts, usually much longer than the flowers
- Bracts not conspicuous, stiff, and pinnatisect, and usually not longer than the flowers (but the lowermost flowers may have long, flaccid, leaf-like bracts)
7
 
7 Apical (male) flowers alternate
- All flowers in whorls
8
 
8 Stem almost twice as thick below inflorescence; branch apices typically fan-shaped, leaves nearly flat with even outline, almost perfectly featherlike in water, usually delicate but rather elastic, somewhat collapsing when emerged; leaf-segments narrowly spaced, 8–24 pairs (usually more than 12), the proximal segments much shorter than the leaf
- Stem not conspicuously swollen below inflorescence; branch apices often compact and knob-like; leaves often 3-dimensional, irregular in outline, not particularly featherlike in water, usually coarse and elastic and tend to keep their shape when emerged; leaf-segments rather widely spaced, 3–13 pairs, the proximal segments almost as long as the leaf
 
9 Leaves normally 5–18 mm long and extremely flaccid; leaf segments thin (usually 0.04–0.07 mm) and delicate, often wrinkled in herbarium specimens
- Leaves normally 15–50 mm long, elastic or flaccid; leaf segments thicker (usually 0.1–0.3 mm), coarse or delicate, but only rarely wrinkled in herbarium specimens
10
 
10 Not or sparsely branched plant with coarse, usually dense shoots with narrowly spaced leaf whorls obscuring the stem for a considerable length; leaves rather flaccid (since the leaf segments taper in outer half), remaining green during summer
- Extensively or moderately branched plant with coarse or delicate shoots with rather widely spaced leaf whorls (closer at apices); leaves elastic or somewhat flaccid with nearly linear segments, becoming dark green or blackish during summer
11
 
11 Branch apices typically fan-shaped, leaves nearly flat with even outline, almost perfectly featherlike in water, usually delicate but rather elastic, somewhat collapsing when emerged; leaf-segments narrowly spaced, 8–24 pairs (usually more than 12), the proximal segments much shorter than the leaf
-

Branch apices often compact and knob-like; leaves often 3-dimensional, irregular in outline, not particularly featherlike in water, usually coarse and elastic and tend to keep their shape when emerged;   leaf-segments rather widely spaced, 3–13 pairs, the proximal segments almost as long as the leaf

1. Myriophyllum alterniflorum DC.     Map     Ill.: branch       To top

De Candolle, Fl. Franç. ed. 3. 5: 529 (1815). - Described from France, ”dans la rivière d’Erdre près Nantes”.
Hår-Tusindblad. F ruskoärviä. Fa áar-túsundblað. I síkjamari. N tusenblad. S hårslinga.
Perennial hydrophyte, often with branched, woody rootstock with traces of previous years’ growth. The shoots may have branches all along their length; floral shoots have a cluster of branches near the water surface. Stem usually reddish, often becoming straw-coloured in herbarium specimens, (20–)40–200(–250 cm) long, 0.5–1.8 mm thick (but often with a swelling on the main stem at the base of the inflorescence). Whorls of 4 (sometimes 3) leaves, widely spaced on lower part of stems but on growing shoots more crowded, especially near apices, where they often form a brush-like cluster. Leaves reddish (later often darker and greenish or blackish), delicate and soft, completely collapsing when emerged, (3–)5–18(–40) mm, rather irregular in outline, with 3–10 pairs of segments; segments (0.03–)0.04–0.07(–0.1) mm thick, very flexible (often wrinkled on herbarium specimens), almost linear, the proximal ones as long as the leaf or nearly so (much variation).
Spikes 1–8 cm, often much curved, apical part lax and nodding; bracts serrate; bracteoles 0.3–0.5 mm, broadly ovate. Male flowers alternate; their bracts patent, lanceolate to oblanceolate, 0.4–1.8 mm; petals c. 2 mm, yellow with red stripes; anthers 1.2–1.6 mm. Female flowers in whorls; their bracts erect, shorter than fruit, 0.4–1.4 mm, ovate-lanceolate; gynoecium 1.5–2 mm, stigmas cushion-like, red.
[2n=14]
Distribution. Nem–NBor(–LAlp). Alt N Op 1280 m. – D fairly common but declining in western Jylland (VJy, western parts of NJy, ØJy and SJy) and in Brn; rare in Sjæ, previously also in FyL (old finds from Egebjerggård and Gamborg) and LFM Kettinge c. 1895.. N common up to the tree line and north to southern Tr and southern parts of VFi and ØFi; rare in the low-alpine belt and further north. S mostly common on the mainland (up to the tree line), but rare or absent in some regions where oligotrophic waters are scarce (e.g. parts of Sk and Upl); well distributed in Öl; Gtl Hangvar (last seen 1914). F rare or scattered in the coastal areas: three lakes in A, otherwise absent in the SW archipelago and rare in the mainland of western V, coastal U, EP and KP; common in the north and in the southern inland provinces.. Fa common. I common in most parts, but rare in IMi.
Amphiatlantic. NW and W Europe east to Italy, W Ukraine, Baltic states, Russian Karelia and Kola peninsula; NW Africa; Azores; SW Greenland; NE North America from District of Mackenzie east to New Foundland, and south to Michigan and Connecticut.
Habitat. Found in both standing and running water, in both sheltered and exposed situations, at depths safe from freezing, down to at least 6 m; in much of Norden the most common, or only, Myriophyllum species. Typically found in clear, oligotrophic lakes on sand or gravel, but grows in a wide range of habitats, e.g. also in dystrophic tarns, slightly brackish seawater, and lakes on the limestone of Öland. Grows on rock-bottoms as well as gravel, sand, mud or peat. Avoids clayey bottoms, and nutrient-rich or muddy water, and is rare or absent in the agricultural plains of the south. Often abundant in large rivers, even in rapids, but also in minor rivulets, sometimes ditches.
Similar taxa. The most frequent misidentification of Myriophyllum alterniflorum is as M. spicatum (2). Normally developed specimens are easily recognized by short, flaccid leaves in densely set whorls. Misidentified vegetative specimens usually come from extreme habitats, such as exposed sites in the Bothnian Bay, where M. spicatum leaves are thinner than normal while those of M. alterniflorum are thicker, and both species grow unusually short. Misidentified flowering specimens are often individuals with young spikes, where the male part is not fully developed, in which case all flowers might seem to be verticillate.
M. alterniflorum has sometimes been confused with M. sibiricum (3), e.g. in clear lakes in Lapland where M. alterniflorum may be unusually long-leaved, and M. sibiricum unusually thin-leaved and relatively flaccid.
Detached apical shoots of unusually long-leaved M. alterniflorum have sometimes been misidentified as M. verticillatum (4), but the stem is almost always thicker in the latter species.

2. Myriophyllum spicatum L.     Map     Ill.:   perennial habit   branch       To top

Linnaeus, Sp. pl.: 992 (1753). - Type: Burser VII (1) 79 (UPS) lectotype, sel. by Aiken & McNeill, Bot. J. Linn. Soc. 80: 216 (1980).
D Aks-Tusindblad. F tähkä-ärviä. N akstusenblad. S axslinga.
Perennial hydrophyte, often with branched, woody rootstock with traces of previous years’ growth. The shoots may have branches all along their length; floral shoots have a cluster of branches near the water surface. Stem usually reddish, or sometimes greenish, (20–)50–150(–250 cm) long, 1.5–3 mm thick (but inflorescence branches usually conspicuously swollen, 5–20 nodes below the spike being up to twice as thick as the main stem). Whorls of 4 (rarely 5) leaves, widely spaced but more crowded near growing shoot apices, where they often form a fan-shaped cluster. Leaves dark green (at shoot apices often reddish), delicate but rather elastic, (10–)15–40(–50) mm, with (8–)12–24 pairs of rather narrowly spaced segments (gaps 0.5–1.5(–2) mm) in a single plane and of rather even length, making leaves perfectly feather-like; segments 0.1–0.25(–0.35) mm thick, not very flexible, almost linear, the proximal ones much shorter than the leaf (much variation).
Spikes 4–15 cm, usually curved in the basal part, apical part stiff and erect; all flowers whorled, with 4 flowers in each whorl; bracts 1–2.5 mm, equal to or longer than fruit, lower bracts lanceolate, pectinate, upper bracts rhombic to elongate, entire; bracteoles 0.5–0.8 mm, reniform or suborbicular, broader than long, entire; hydatodes between the flowers usually distinct in living plants, but difficult to find in herbarium specimens. Male flowers with c. 2 mm long, pinkish petals, anthers 1–1.8 mm. Female flowers without perianth; gynoecium 1.5–2 mm, stigmas recurved, brushy, pink.
[2n=42]
Distribution. Nem–SBor(–NBor). – D common in most parts, but rare in central VJy. N in the south rare in Øf Fredriksstad, Halden, Ak Asker, Vf Borre, AA Lillesand, VF Farsund and Ro Klepp, Sola and Time; in the north in outer parts of SNo Brønnøy, Dønna, Herøy, Vega, and NNo Bodø and Gildeskål. S in the sea rare in Sk, common from Bl to Upl (including Öl and Gtl), and rare further north in Gst Hille, Hamrånge, Hls Harmånger, Hudiksvall, Mpd Alnö, Tynderö, Ång Nordmaling and Vb Sävar; in fresh water locally common north to BhG, Vg, Nrk and Upl and in Gtl, but rare in SmI Eksjö, Säby and Skärstad, and only a few localities in Hl and Öl; further north rare in Dls Frändefors, Vsm, Dlr Leksand and Gst Ovansjö and Valbo. F in the sea common in A and along the coast of the Gulf of Finland, less common along the Gulf of Bothnia; rare in fresh water in A, coastal V and U. – Erroneously reported from I (the material belongs to M. sibiricum).
Widely distributed in Old World. From Europe east to Japan, and south to NW Africa, Egypt, Middle East, Iran, Himalayas and S China; scattered in E Africa. Introduced to North America, where it has become a troublesome weed, and S South America.
Habitat. Found in standing and slowly running water from depths safe from freezing, down to at least 6 m, in sheltered and exposed situations. Typically in nutrient-rich or base-rich waters, such as eutrophic lakes and man-made water bodies, even heavily polluted, but also found in the Baltic sea, where it is the most salt-tolerant Myriophyllum species. Avoids acidic and dystrophic water. Usually common in agricultural plains of the south, areas where M. alterniflorum is scarce or absent.
Biology. Seed-set is sometimes abundant, but most seeds become dormant and few of them actually germinate. Dispersal and forming of new individuals is mainly by means of fragmentation. Axillary buds which detach easily are formed in winter and early spring, but any detached fragment will easily develop adventitious roots.
Similar taxa. Typically developed Myriophyllum spicatum is very characteristic, with its spaced whorls of neat, feather-like leaves reminding of a series of propellers lined up on a string. It is hardly ever confused with M. verticillatum but often with M. alterniflorum (1) and M. sibiricum (3), see those taxa.

3. Myriophyllum sibiricum Komarov      Map     Ill.:  branch  turions     To top

Komarov, Feddes Repert. 13: 168. (1914). – Type: Russia, Kamchatka River basin, Komarov 4855 (LE) lectotype, sel. by Aiken, Taxon 37: 958 (1988).
M. spicatum subsp. squamosum Laest. ex Hartm. (1864).
M. spicatum var. capillaceum Lange (1880).
M. exalbescens Fernald (1919).
D Grønlandsk Tusindblad. F kalvasärviä. I vatnamari. N kamtusenblad. S knoppslinga.
Literature. Aiken et al. 1979, Aiken & McNeill 1980, Aiken & Walz 1979, Fægri 1982.
Turionic hydrophyte with an annual root system. The shoots may have branches all along their length; often several main stems, especially in shallow water. Stem at first reddish, but soon straw-coloured or almost white, (15–)40–80(–150 cm) long, 1.5–3 mm thick (may be thicker below the inflorescence, but not conspicuously swollen). Whorls of 4 (sometimes 3 or 5) leaves, widely spaced except near growing shoot apices where they often form a knob-like bud. Leaves dark green (finally almost blackish), coarse and elastic, 10–)15–30(–45) mm, with 3–13 pairs of rather widely spaced segments (gaps (1–)1.5–4(–6) mm) which often curve in an erratic way and are of more or less uneven length, usually making leaves 3-dimensional and irregular in outline; segments (0.1–)0.15–0.3(–0.4) mm thick, almost linear, somewhat tapering but remain thick close to apex, the proximal ones almost as long as the leaf. Turions 15–80 mm, at shoot apices, pointed, broadest basally or in the middle part, loose and flexible; turion leaves dark, stiff, smaller and thicker than ordinary leaves, more or less rhombic, with densely packed, succulent segments.
Spikes 2–11 cm, stiff and erect, but may be curved in the basal part; all flowers whorled, with 4 flowers in each whorl; bracts 1–2 mm, shorter than or rarely equalling fruit, lower bracts ovate, serrate, upper bracts spathulate, ovate or oblong, entire; bracteoles 0.7–1 mm, ovate to ± orbicular, entire; hydatodes between the flowers insignificant or absent. Male flowers with c. 2 mm long, pinkish petals; anthers 1–1.4 mm. Female flowers without perianth; gynoecium 1.5–2 mm, stigmas recurved, brushy, pink.
[2n=42]
Distribution. (Nem–)BNem–NBor. – N found in two separate areas: in the south very local in Gudbrandsdalen in Op Ringebu (Trøstakermoen), Sel (Laurgård) and Vågå (Selsvatn), and in Dovrefjell He Folldal (Ryhaugen), Op Dovre (Hjerkinn), and ST Oppdal (Gåvåli), also an isolated, undated record from Op Jevnaker; in the north scattered and locally common from NNo Ballangen, Evenes and Fauske to VFi Alta (Alta), Kautokeino (many stations), Porsanger (Veines) and ØFi Nesseby (Mortensnes) and Sør-Varanger (Pasvikselva); a somewhat dubious record from Vf Borre (Borrevatnet 1923, a single individual in a mixed collection). S in the sea rare in Bl Bräkne-Hoby, Karlskrona, Ramdala and Ronneby and in Klm, more frequent from Srm northwards, being common in Upl and från Ång to Nb; in fresh water rare in the south, known from Srm Botkyrka, Dunker (Nälen), Upl Lovö (Drottningholm), Värmdö (Ösby träsk), Österåker (Valsjön), Hls Norrala (Morvikstjärn), and more isolated in Dlr Leksand (Limsjön); in the north more common but local, found in base-rich parts of Jmt, along the Scandes from western Jmt to TL, and in lower parts of TL south to northern Nb; an unsubstantiated record from Hrj Storsjö (Öjön). F usually common in the sea along the entire coast; rare in fresh water in a few localities in A, V and U, in EH Lammi (Ormajärvi), ES Lappeenranta (ditch), Ruokolahti (Kuokkalampi), Parikkala (Siikalahti), PS Joroinen (Kotkatjärvi, Uskalinlampi and Valvatusjärvi), PK Liperi (Särkijärvi), Outokumpu (Sysmäjärvi), and Kn Paltamo (several stations) and Suomussalmi (Kiannankylä); further north more frequent, locally rather common.. I INo more than 20 localities in Austur-Húnavatnssýsla (Hnausar), Mývatn and the vicinity of Öxarfjörður; IAu [Hornefjörður] 1934, ISu Þingvellir (1870), IVe Snæfellsnes (Syðri Garðar) and Reykjavík (1868).
Circumpolar. From N Europe east through Russia to Kamtchatka, and south to Latvia, Mongolia and N Japan; northern and western N America from Alaska to W Greenland and New Foundland, and south to California, Indiana and New York.
Habitat. Found in standing and, more rarely, slowly running water down to at least 3 m, usually in sheltered situations. Typically found in clear, base-rich water on soft bottoms. Avoids dystrophic and oligotrophic waters, but often common in brackish seawater. The turionic hibernation allows this species to grow in waters that freeze solid in winter, such as rock pools, delta lagoons, shallow sheltered bays and inlets with deep, soft sediments, but it is also found in lakes in base-rich areas.
Biology. From the end of summer, nutrients are allocated upwards to the turions. During winter, the turions become less buoyant and sink to the bottom, and become embedded as the mother plant starts to wither and dies. They survive even when embedded into solid ice and will start to elongate in spring.
Variation. Varies much in its vegetative parts. Especially deviating are plants from inland localities in southeastern Finland (and the Ladoga region in Russia), which have several traits in common with M. spicatum (more flaccid leaves with denser segmentation, a tendency of swollen stem close to the spikes etc), but clearly do possess turions. The material from this region is, however, rather sparse and seldom appropriate for studies.
Similar taxa. Typically developed Myriophyllum sibiricum and M. spicatum (2) are easily distinguished by leaf characters alone, the leaves of M. sibiricum being more irregular, and not propeller-like. In certain waters, however, this difference is less evident. The biggest problem seems to be in the middle part of the Baltic (SW Finland and EC Sweden), where both species commonly exhibit unusual growth form, and are easily confused. The best way to determine such populations is probably to return at a more suitable season and check hibernation characters.
M. sibiricum and M. verticillatum (4) hardly ever coexist, but when they do, and the locality is relatively nutrient-poor, they can look surprisingly similar; e.g., they were confused by Erixon (1981) in some lagoons along the river Vindelälven in LyL, where both are rather long-leaved and M. verticillatum has more widely spaced leaf whorls than usual. In such localities, turion shape is the best diagnostic character.
For M. alterniflorum (1), see under that species.
Taxonomy. Although observed already by L.L. Læstadius (Hartman 1858) and later described as a subspecies (Hartman 1864), M. sibiricum was long neglected in Europe. It was again observed by Swedish botanists from the 1960’s, but only in 1980 it was definitely identified and formally recognized as the North American M. exalbescens (Aiken & McNeill 1980), later found to be a synonym of M. sibiricum (Ceska & Ceska 1986).

4. Myriophyllum verticillatum L.    Map    Ill.:  branch  turions  autumn aspect   To top

Linnaeus, Sp. pl.: 992 (1753). - Type: LINN 1123.3 lectotype, sel. by Aiken & McNeill, Bot. J. Linn. Soc. 80: 218 (1980).
D Krans-Tusindblad. F kiehkuraärviä. I tjarnamari. N kranstusenblad. S kransslinga.
Turionic hydrophyte with an annual root system. Often unbranched or with a few long branches, usually not branching in the upper part. Stem more or less greenish, (30–)50–150(–300) cm long, 1.5–4 mm thick (not swollen below the inflorescence), sometimes with prominent hydatodes at leaf axils. Whorls of 4 or 5 leaves, usually densely spaced and obscuring the stem for a considerable length in apical parts, making growing shoots look brush-like. Leaves persistently green, coarse but flaccid, collapsing when emerged, (15–)20–45(–60) mm, rather irregular in outline, with 9–17 pairs of segments; segments (0.07–)0.1–1.5(–0.2) mm thick, tapering for most of their length, the proximal ones shorter than the leaf (much variation). Turions 15–35 mm, on specialized, short branches at base of plant, at shoot apices, or at nodes, blunt-ended, broadest apically, finally becoming club-shaped, dense and stiff; turion leaves structurally similar to ordinary leaves but smaller, more or less spathulate, with long segments with soft, delicate apices.
Spikes 5–15 cm, stiff and erect; all flowers whorled, with 5 flowers in each whorl; bracts 2–25 mm, up to 10 times longer than fruit, pinnate or pectinate, never entire; bracteoles up to 0.2 mm, pectinate, or absent. Male flowers with c. 2 mm long, greenish-yellow petals; anthers 1.2–1.7 mm. Female flowers with indistinct perianth of 4 lanceolate-ovate, up to 1 mm long lobes; gynoecium 1.5–2.5 mm, stigmas recurved, brushy, pink or red.
[2n=28]
Distribution. Nem–NBor. – D fairly rare in most of the country; Brn latest Hammeren c. 1955. N rare and local in the southeast from Øf to Te Porsgrunn and inland to He Hamar and Bu Ringerike, being most common in Øf and lower parts of Bu; isolated localities in He Alvdal (Savalen), Ro Hå (Nærland) and ØFi Sør-Varanger (Svanvik and Noatun). S well distributed north to BhG, Vg, Nrk, southeastern Vsm, southern Dlr and Gst; within this area common in nutrient-rich regions like Sk, BhG, Vg, Ög and Upl, but rarer in poor regions like SmI, and in Öl only known from Vickleby (1868); further north rather common in Nb, but otherwise very local; a peculiarity is its rarity in some agricultural districts, e.g. southern Vrm, contra its presence in remote areas in Lapland, such as mountain lakes in ÅsL Vilhelmina (e.g. Krutsjön and Vardofjäll) and along the river Vindelälven up to LyL Sorsele (Bräskafors); provinces with few records are Dls Gunnarsnäs, Hol, Laxarby, Skållerud, Vrm Sunne (Kolnäs), Visnum (Björneborg), Jmt Håsjö, Kall (Spjuttjärnen), Lit, and LL Gällivare (Överselet in Råneälven). F north to KiL Kittilä and SoL Sodankylä; rather common in agricultural and populated areas in the south, in parts of PS and along the Bothnian Bay, but elsewhere local, being almost absent in nutrient-poor regions; increased due to eutrophication. – Records from Iceland (see Grøntved 1942) are erroneous.
Circumpolar. From N Europe east to Anadyr and Kamtchatka and south to NW Africa, Caucasus, Kashmir, C China and Japan; North America from Alaska east to New Foundland, and south to Utah, Minnesota and Florida. Introduced in S South America.
Habitat. Found in standing and slowly running water down to at least 3 m, usually in sheltered situations. Typically found in nutrient-rich water on soft bottoms. The most hemerophilous of our Myriophyllum species, found in many man-made habitats, but also in lakes and some rivers. The turionic hibernation allows this species to grow in waters that freeze solid in winter, such as shallow ponds, ditches, seepage in fens and seashore meadows.
Biology. From the end of summer, nutrients are allocated downwards to the turions. In late autumn they become actively released from an abscission layer. The mother plant then dies. The turions will survive even when embedded into solid ice and are capable of drifting over considerable distances.
Variation. Since the turions are free-floating and easily drift ashore, terrestrial plants are much more common than in other species. They are erect, usually 10–15 cm or even taller, and remind of stout, leafy flowering spikes. The leaves are fleshy, bract-like and bright green, and the stem is often red-toned. Flowers are usually spread over most of the length, and bisexual flowers are commonly found. Basal leaves which grow in seeping water or on damp soil are transitional to (or at least similar to) ordinary leaves.
Another modification occurs in rivulets and seeping water in fens and shore meadows. In such places an individual might have a horizontal growth of considerable length, with long, leafless stems connecting clumps of assimilating leaves, and abundant formation of turions in late season.
Similar taxa. Myriophyllum verticillatum is usually easily recognized by its long, flaccid, green leaves. It is hardly ever confused with M. spicatum (2), but sometimes with M. alterniflorum (1) or M. sibiricum (3); also the weedy aquarium escape M. aquaticum (rare casual) has been mistaken for M. verticillatum.

Rare casuals

Myriophyllum aquaticum (Vell.) Verdc. 1973 (M. brasiliense Cambess. 1830, M. proserpinacoides Gillies ex Hook. & Arn. 1833). S storslinga. – A robust, up to 2 m long creeping aquatic plant, rooting at nodes, with soon decaying submerged leaves. Emergent leaves persistent, glaucous, erectopatent or at shoot apices nearly erect; they remind of bird's feathers, beeing petiolate, blunt-ended, and having 18–36 pairs of uniform, narrow segments. Flowers are borne singly in axils of upper emergent leaves in an indeterminate, erect spike somewhat similar to the terrestrial shoots of M. verticillatum. Dioecious; only female plants, with 4 minute white sepals, are known outside South America.
D LFM Sydfalster (Birkemose near Gedser, found 2000 in pond on seashore). – South America; an aquarium plant now widely established on all continents, in some regions as a troublesome weed; in Europe known to be established north to Britain, the Netherlands and Germany. - Map (not in the book).

References To top

Aiken, S.G. 1981a: A conspectus of Myriophyllum (Haloragaceae) in North America. Brittonia 33: 57–69.
Aiken, S.G. 1981b: An experiment relating vegetative morphology of Myriophyllum alterniflorum DC. (Haloragaceae) to growth substrate. Aquatic Botany 10: 383–388.
Aiken, S.G. & McNeill, J. 1980: The discovery of Myriophyllum exalbescens Fernald (Haloragaceae) in Europe and the typification of M. spicatum L. and M. verticillatum L. Bot. J. Linn. Soc. 80: 213–222.
Aiken, S.G., Newroth, P.R. & Wile, I. 1979: The biology of Canadian weeds. 34. Myriophyllum spicatum L. Can. J. Plant Sci. 59: 201–205.
Aiken, S.G. & Picard, R.R. 1980: The influence of substrate on growth and morphology of Myriophyllum exalbescens and Myriophyllum spicatum. Can. J. Bot. 5: 1111–1118.
Aiken, S.G. & Waltz, K.F. 1979: Turions of Myriophyllum exalbescens. Aquatic Botany 6: 357–363.
Ceska, A. & Ceska, O. 1986: Notes on Myriophyllum (Haloragaceae) in the far east: The identity of Myriophyllum sibiricum Komarov. Taxon 35: 95–100.
Erixon, G. 1981: Några karaktäristiska växtbiotoper vid Vindelälven. Svensk Bot. Tidskr. 75: 173–176.
Fægri, K. 1982: Et bortglemt fennoscandisk tusenblad (Myriophyllum).taxon. Blyttia 40: 149–153.
Gerber, D.T. & Les, D.H. 1994: Comparision of leaf morphology among submersed species of Myriophyllum (Haloragaceae) from different habitats and geographical distributions. American Journal of Botany 81: 973–979.
Hartman, C. (ed.) 1858: C. J. Hartman. Handbok i Skandinaviens flora. Ed. 7. Stockholm.
Hartman, C. (ed.) 1864: C. J. Hartman. Handbok i Skandinaviens flora. Ed. 9. Stockholm.
Moody, M.L. & Les, D.H. 2002: Evidence of hybridity in invasive Watermilfoil (Myriophyllum) populations. Proceedings of the National Academy of Sciences of the United States of America 99: 14867–14871.
Moody, M.L. & Les, D.H. 2007: Geographic distribution and genotypic composition of invasive hybrid watermilfoil (Myriophyllum spicatum x M. sibiricum) populations in North America. Biol. Invasions.
Patten, B.C. Jr. 1956: Notes on the biology of Myriophyllum spicatum L. in a New Jersey lake. Bulletin of the Torrey Botanical Club 83: 5–18.

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