A taxonomic revision of Lamium (Lamiaceae)

The present study deals with the systematics and taxonomy of the genus Lamium (Lamiaceae). The taxonomic revision is mainly based on the study of herbarium collections, and to a smaller degree on field observations and abstracts from literature. The research was done at the Rijksherbarium, Leyden, while many other Herbaria were shortly visited. As the genus Lamium was never before monographed, a short survey is given of the taxonomic history of Lamium, which goes back as far as Linnaeus (1753). In this short survey of the taxonomic history of the genus Lamium the general history of taxonomy and its changing emphasis on different geographical regions can be recognized. It is difficult to give a definition of Lamium in which it stands out from related genera by means of one or preferably two characters. As with so many genera, Lamium can only be defined polythetically: no single character suffices. An important character is the chromosome number: the base number x = 9 is rather uncommon in other genera of the Stachyioideae to which Lamium belongs. As morphological outline of the genus Lamium can be given: annual or perennial herbs, usually not higher than about ½ m, with cordate or reniform, shallow incised leaves; flowers in verticillasters in the axils of the uppermost (= floral) leaves; calyx campanulate with an oblique mouth and with subequal teeth; corolla 2-lipped, the tube straight or sigmoid, the lower lip usually with two rounded lateral lobes, each with one or two teeth; nutlets obovoid with a truncate apex and in cross-section trigonous with one convex and two flat sides. The genus is divided into 3 subgenera; one subgenus ( Lamium) 3 sections, of which one section (Amplexicaule) is divided into is new. A number of 162 names of Lamium species and a more or less equal number of names of infraspecific taxa were known. In this revision 16 Lamium species, 11 subspecies, and 13 varieties are accepted, and also one hybrid. 9 New combinations are made. Type specimens are recorded for all taxa and an identification key to all taxa is given. In all c. 10,000 collections were examined. The most important quantitative characters were measured in 2700 collections, representing specimens from the entire distribution area of individual taxa. To show the variability of these quantitative characters diagrams are added, as diagrams give more information than minimum, average and maximum values. The percentage of ripe fruits appears to vary strongly in herbarium specimens of Lamium: all species with a percentage of ripe fruits higher than 55% are annuals, and all species with a percentage of ripe fruits lower than 20% are perennials. Random studies in other genera led to the same result: the ripening of fruits of annual species takes place in a shorter period than the ripening of fruits of perennial species. Distribution maps of all Lamium taxa are given. The distribution of the genus Lamium may be characterized as temperate Eurasiatic (holarctic), but the centre of diversity is obviously found in the Irano-Turanian and the Mediterranean regions. The habitats of the taxa of the genus Lamium roughly can be listed under two categories: forests on the one hand and rocky mountain slopes/cultivated fields on the other. The differences in habitats and also in distribution areas were the starting point for an intuitive phylogenetic reconstruction of the genus Lamium, in which Lamium galeobdolon is included, in spite of a number of deviating morphological and chemical characters.