Galium verum L. (Euras., N-Afr.) – A variable species, member of a critical species complex (Galium verum group). Two more or less distinct taxa (morphotypes?) have been recorded in Belgium, only subsp. verum being native:
1 Stems usually ascending. Internodes often shorter than the leaves. Leaves up to 1,5 mm wide, with strongly recurved margins. Inflorescence compact === subsp. verum
Stem erect, often robust. Internodes longer than the leaves. Leaves wider, up to 3 mm wide, with less strongly recurved margins. Inflorescence markedly interrupted === subsp. wirtgenii
Subsp. wirtgenii (F.W. Schultz) Oborný [syn.: G. wirtgenii F.W. Schultz, G. verum subsp. praecox (K.H. Lang) Petr., G. praecox (K.H. Lang) Heinr. Braun] (C-Eur.) – A rare but increasing, often persisting alien. Included in wild flower seed mixtures and, as such, found in recently sown roadsides, talus slopes, coal mining spoil heaps, etc. In some places known since many years by now and perhaps more or less naturalized, e.g. in Beringen-Mijn. In recent years also seen, for instance, in the Waaslandhaven (since 2013), Meulebeke, etc.
The exact identity of these plants possibly requires further study. They are clearly distinct from native subsp. verum (the latter usually being a much smaller, cushion forming and more compact taxon). However, native and non-native plants do not seem to differ notably in flower colour (corollas are claimed to be golden-yellow in subsp. verum and citron-yellow in subsp. wirtgenii; see Ehrendorfer 1976, Jäger & Werner 2005). Taxa from the Galium verum group being reputed to hybridize freely, it is not unlikely that plants used in wild flower seed mixtures represent hybrids involving subsp. wirtgenii rather than being referable to the latter itself. Both subspecies usually also differ cytogenetically (subsp. verum being a tetraploid and subsp. wirtgenii a diploid) but southern races of the former are often also diploid and much less easily separated from the latter (Ehrendorfer 1976).
Some authors (e.g. Štěpánková & Kaplan 2000) accept subsp. wirtgenii at species rank.
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Selected literature:
Ehrendorfer F. (coll. Krendl F. & Puff C.) (1976) Galium. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 4. Cambridge University Press, Cambridge: 14-36.
Jäger E.J. & Werner K. (eds.) (2005) Rothmaler Band 4. Exkursionsflora von Deutschland. Gefässpflanzen: Kritischer Band. Springer Verlag, Berlin: 880 p.
Štěpánková J. & Kaplan Z. (2000) Galium L. In: Slavík B. (ed.), Květena České republiky 6: 122-156. Academia, Praha.