Crataegus L. – Hawthorn
The genus Crataegus is common in the temperate zones of the northern hemisphere. Species numbers are difficult to estimate (250-1,250 according to Lippert 1995), because, especially in North America, apomixis is widespread in the genus (Muniyamma & Phipps 1979) and depending on the taxonomic concept, either a few variable or very many morphologically uniform taxa are distinguished. In Europe, a few morphologically very variable taxa and their hybrids are recognized. However, it should be considered that in Europe, apomixis may be also common in Crataegus and, similar to North America, many morphologically uniform asexual taxa might have evolved. The taxonomic classification of the Central European Crataegus species does not yet appear to be conclusively clarified, and the classification currently used, for example, by J. Zázvorka in Kubát (2002) or P.A.Schmidt (2011), is rather schematic.
It is urgently needed to clarify the reproductive mechanisms in Crataegus, in particular, whether apomixis plays a role at all in this genus in Europe. Also, the importance of polyploidy for speciation has not yet been sufficiently investigated. A satisfactory taxonomic classification of the genus can only be achieved after such investigations have been completed. Introgressive hybridization, i.e. the occurrence of complete morphological series between individual species, and in addition, hybridizations between several species do not seem to be rare (e.g. Loos 1994), often preventing to species determinations.
Studies on apomixis in Crataegus are rare in Europe thus far (Gosler 1989, 1990; Ptak 1986, 1989), but tri- and tetraploid cytotypes may indicate its presence. The chromosome base number is n = 17; plants with 2n = 34 are diploid, those with 2n = 51 or 2n = 68 are tri- or tetraploid, respectively. There is evidence that triploid plants reproduce via apomictic seed formation (Ptak 1989). The reproductive mechanism of tetraploid plants is unclear. Chromosome numbers have been recorded, notably in Slovakia (see: Karyological database of the ferns and flowering plants of Slovakia).
Linné summarized all species recognized today in Central Europe under Crataegus oxyacantha. However, the type specimen for C. oxyacantha in the Linné-Herbarium belongs to C. praemonticola (= C. rhipidophylla auct.) (Byatt 1974), and this taxon is now called, C. laevigata. The name was rejected by the International Botanical Congress in Sydney in 1981. In the 19th century, a separation of two species according to the number of styles or rather, pyrenes prevailed. From the last decades of the 20th century another taxon has been differentiated, which is characterized by pronouncedly divided leaves and narrow sepals. For this, the name, C. curvisepala, later C. rhipidophylla, was used; we use C. praemonticola. This name was created by Holub (1991) because no name is available for this taxon.
Today's classification is based on Christensen (1992), whereby some of the taxa he interpreted as hybrids are regarded as hybridogenic species (e.g. Hand et al. 2024). The number of species recognized in Central Europe has thus risen to eight. The Crataegus taxonomy is burdened by the multiple descriptions of infraspecific taxa, whose classification is often associated with difficulties. Sometimes a very narrow species concept was also used, as Gandoger (1872) described 18 new species from France (Rhone Valley).
The determination of Crataegus in herbaria and floras is afflicted with great uncertainties. The majority of the plants is determined "incorrectly" according to the classification used here (see Loos 1994). In particular, the apparently very frequent C. calycina was only rarely recognized. Published distribution data should always be viewed very critically. Some species seem to be missing regionally. So far, it has not been possible to obtain evidence for the occurrence of C. praemonticola (= C. rhipidophylla auct.) in Hesse (see: T. Gregor, S. Hodvina & al.: Beiträge zur Pflanzenwelt in Hessen). All specimens identified as C. rhipidophylla turned out to be C. lindmanii. They were most likely intended to be determined as C. rhipidophylla s.l. (including C. lindmanii) without this being labeled.
The European species belong to the section Crataegus and therein to the series Crataegus. Christensen (1992) classified C. rhipidophylla (legitimate name C. praemonticola), C. monogyna and C. ×macrocarpa, C. ×media and C. ×kyrtostyla (legitimate name C. subsphaerica) as belonging to the nominate sub-series, C. laevigata as belonging to sub-series Erianthae (Pojark.) K. I. Chr.
Typical habitats for hawthorns are hedges, forest edges and open forests. The individual species differ according to their need for light, which is highest in C. monogyna and lowest in C. laevigata. It is assumed that the large number of hybrids in our cultural landscape can be explained by the clearing of the forests and the then greater coexistence of these two parental species.
The species occurring in Germany are quite similar. They are sizably, several meters high, abundantly branched shrubs with up to 2.5 cm long thorns. The petiolate and stipulate leaves arise from short shoots and are ovate to rhombic in outline. Depending on the species, they are more or less strongly incised but never to the midvein. The resulting 3-5 leaf lobes have an entire or serrated margin. The shape and division of the leaves can vary on a shrub. The flowers are 0.7-1.5 cm in diameter and have a strikingly unpleasant smell. During fruit ripening, the receptacle grows into a fleshy tissue and thus forms the characteristic apple fruit (pyrene). The fruits are spherical to ellipsoid in shape and 0.7-1.2 cm long. The shape and position of the sepals are important for species identification. The number of pyrenes corresponds to that of the style and is usually 1 or 2, but also higher numbers can be occasionally found. The number of styles (pyrenes) is often variable within flowers of a shrub. The fruits ripen in August till September and often remain on the bush during the winter. The morphological characteristics of the hybrids are particularly variable.
The following characters must be taken into account: Leaf shape, shape and position of sepals in fruiting state and number of styles or pyrenes. A reliable determination can only be made on plants with ripe fruits which show the position of the sepals. Plants with flowers can be only assigned to species groups. Due to the great variability of the leaf shape, sufficient herbarium material should be collected from different branches of a shrub that are exposed as differently as possible.
Classification of plants with one style, narrow, recurved calyx tips, and strongly-incised, dentate leaf lobes is unclear. The names Crataegus curvisepala (illegitimate) and C. rhipidophylla (synonymous with C. subsphaerica) are not applicable, and the name C. rosiformis Janka (e.g. used by Tison & Foucault 2014), which is partly considered to be in accordance with the rules, is not beyond doubt (Holub 1991). This name could also refer to hybrids between C. rhipidophylla auct. and C. lindmanii (Loos 1994, Lippert 1995), which are referred to as C. ×dunensis. We use the relatively new name C. praemonticola Holub, which certainly refers to this taxon. On the other hand it seems to be clear that the name C. laevigata subsp. palmstruchii is synonymous with C. laevigata subsp. laevigata (Christensen 1992, see also species page).
Heinz Kalheber (Runkel) supported us in revising specimens. The Bavarian State Collection provided the specimens.
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