Happy birthday, Charles Darwin!

 

Today, 200 years ago, Charles Darwin was born – his vivid interest in varieties of animals and plants has led him to the insights of evolution: Species are not created once and forever but are rather the result of a process, which is partly still continuing. Especially the relatively young family of orchids is still in the midst of its development, and nature is trying to go new ways. One of them are colour variations like white orchids.

In his book about “The various contrivances by which orchids are fertilised by insects” (1862, 2. Auflage 1877), Darwin studied the structure of orchid flowers in relation to their pollinators. About Platanthera chlorantha (which he called Habenaria chlorantha) he wrote, that they are pollinated by moths – due to their long spur filled with nectar and an intense scent at night: “The remarkable length of the nectary, containing much nectar, the white colour of the conspicuous flower, and the strong sweet odour emitted at night, all show that this plant depends for it fertilisation on the larger nocturnal Lepidoptera.”(p.85). In a famous forecast Darwin estimated that there must be a pollinator in Madagascar matching the orchid Angraecum sesquipedale which has a spur with a length of 25 cm: “In Madagascar there must be moths with probosces capable of extension to a length of between ten and eleven inches!” (p. 198). In 1903, 41 years later, the appropriate butterfly was found, Xanthopan morgani.

Dactylorhiza traunsteineri

The magazine of the Arbeitskreise Heimische Orchideen (AHO) has an article of Norbert Griebl in its latest edition giving an overview about the Dactylorhiza species in Austria. His contribution presents two photos of white varieties – a Dactylorhiza traunsteineri, the photo taken at lake Kochel in Bavaria,and a bright flowering Dactylorhiza incarnata, which is defined als Dactylorhiza incarnata f. ochrantha- with a yellowish accent in the lower part of the inflorescence but not as yellow as Dactylorhiza incarnata ssp. ochroleuca – Griebl views this taxon not as a subspecies but as a species of its own, since he argues that there are almost none hybrids between incarnata and ochroleuca.

In an article about maintenance of biotops in Rhineland-Palatinate Juergen Passin mentions a habitat near Vallendar on the Rhine “Orchis militaris, also var. albiflora”.

Overview on all the orchid genera

The shipment took more than a month but now the book has arrived from Cornell University Press in Ithaca, New York. It’s really intersting, the Illustrated Dictionary of Orchid Genera. The authors Peggy Alrich and Wesley Higgins present an overview on all the genera of the family of orchids – from  Aa to Zygostates.

Aa is quite a nice story – this valid taxon was defined by Heinrich Gustav Reichenbach in 1854. One explanation for the unusual name is that Reiuchenbach took the first and the last letter of the related genus Altensteinia in order to be always at the first place of  the compendia – the new reference book demonstrates his success. Another explanation says that Aa with its 27 species in Central and Southern America commemorates the Dutch printer Pieter van der Aa.

The preface states that there are about 850 accepted genera in the world of orchids – but the number described in the book is more than three times greater. The two authors have introduced a colour scheme to present them: A vivid green stands for the validly published and “currently accepted” names of genera. Light green is the colour of the validly published but “not currently accepted” taxons – among them are e.g. Listera (now: Neottia) or Aceras (now Orchis). The deprecation is explained with molecular-genetic studies: “Current DNA testing of this genus shows that Aceras is clearly nested within Orchis.” With the same reason Nigritella is put to Gymnadenia – but Barlia is printed in vivid green as an accepted taxon, though Barlia robertiana was classified as belonging to Himantoglossum  by molecular biologists.

The compendium explains classification and etymology for each genus listed, followed by a synopsis of the genus with the number of species, geographic distribution, favoured habitat and a short description of morphological features. For each genus there is an illustration of a flower. In the global context the European genera of orchids appear in a a rather limited scope. The authors state that they expect the discovery of new species but these may find their place in the now existing structure of genera.

White flowers may be colourful in the view of insects

Yoshikazu Tanaka of the Institute of Plant Science in Osaka sent me an article about the biosynthesis of plant pigments und and pointed out in an e-mail exchange that white petals often contain pigments from the group of flavones and flavonols. “Flavonols and flavones are very pale-yellow and are mostly invisible to the human eye”, Tanaka and his co-authors Nobuhiro Sasaki and Akemi Ohmiya explain. “As they absorb UV, which insects recognize, they give color and patterns to flowers to attract insects.” 

Just as anthocyanins flavons and flavonols belong to the flavonoids. Under the impact of the enzyme dihydroflavonol 4-reductase (DFR) certain flavonols are transformed to a pigment of the anthocyanin group. With certain species, the authors explain with regard to orchids of the tropical genus Cymbidium, DFR does not unfold this effect due to a strict substrate specifity. “This is the reason that these species lack pelargonidin-based anthocyanins and thus lack flowers of an orange/brick red color.”