Lathyrus heterophyllus L. (syn.: L. latifolius subsp. heterophyllus (L.) Asmussen) (Eur.) – A very rare but possibly overlooked, locally naturalized alien. Known since at least the 1970’s from nature reserve “Oude Landen” in Ekeren (Antwerpen) and in 2011 discovered by a railway track near Kortemark (see here for a more detailed account: http://alienplantsbelgium.be/content/lathyrus-heterophyllus-potentially-overlooked-alien-belgium).
Lathyrus heterophyllus is a critical species. It is morphologically intermediate between Lathyrus latifolius and L. sylvestris. It combines the smaller corollas and slightly narrower leaves of the latter with the large stipules of the former (Melderis 1957). The populations from Ekeren and Kortemark are tentatively ascribed to this species (both were previously referred to as either Lathyrus latifolius or L. sylvestris). A broad-leaved form of Lathyrus sylvestris (subsp. platyphyllos (Retz.) Vollm.) seems to be nearly indistinguishable. This taxon is perhaps better accepted as a synonym of Lathyrus heterophyllus or the entire complex reduced to a single, very variable species. Molecular studies might possibly resolve the complex taxonomy of these aggregate. Lathyrus heterophyllus itself also is a variable species. In its typical form (var. heterophyllus) upper leaves have two or three pair of leaflets. However, especially in western and northern Europe, nearly all plants only have a single pair of leaflets and belong with var. unijugus W.D.J. Koch. All plants seen from Belgium also pertain to that variety. Narrow-leaved plants of Lathyrus latifolius that otherwise perfectly correspond with that species in floral characters would key out to L. heterophyllus in Stace (2010).
In its locality in the Oude Landen in Ekeren (formerly a military base) Lathyrus heterophyllus behaves like a very invasive species.
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Selected literature:
Kenicer G. & Norton S. (2008). 632. Lathyrus heterophyllus. Curtis’s Bot. Mag. 25(4): 317-324.
Melderis A. (1957) Lathyrus heterophyllus var. unijugus Koch. Bot. Soc. Brit. Isles Proc. 2: 238-240.
Seferova I.V. (1990) Analysis of taxonomically significant characters of the leaves in species of section Lathyrus of the genus Lathyrus L. Nauchno-Tekhnicheskii? Byulleten 197: 84-87.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.