A new hybrid for campus. Image by D. Morris
June saw the publication of a hybrid flora of Britain and Ireland. The book is authored by the luminaries of British botany Clive Stace, Chris Preston and David Pearman, with contributions from many botanists whose expertise extends into the esoteric world of plant hybrids, and is a mine of information. It has coincided with my increasing awareness and interest in plant hybrids, and I thought I’d share a hybrid I recently found on campus.
The image to the left shows the plant in situ. Can you spot what drew may attention to it? Why is it not just ordinary cinquefoil?
Hybrids are understandably perceived as a difficult area for the field botanist, requiring a familiarity with species and their variation, and a capacity for careful observation and discrimination. However, in our well-botanised part of the world, hybridisation is one of the few areas to which amateurs can still make a valuable contribution. Hybridisation forces one to recognise that plants don’t naturally belong in boxes called “species”, but do all sorts of crazy things that upset our tidy human schemes. Rather than find this an inconvenience, I find hybridisation enriches my experience of the natural world! Of course, hybridisation is also an extremely important evolutionary process: it is estimated that around 70% of the world’s flowering plant species have evolved following hybridisation (e.g. Briggs & Walters or Arnold).
The hybrid I found on Whiteknights is an excellent example of the story of hybridisation and the formation of new species. It is also easy to spot if you can count. The hybrid in question is between cinquefoil Potentilla reptans, a very common plant of lawns, grassland, wasteground etc., and trailing tormentil P. anglica, an uncommon plant of dry grassland. The hybrid, which is called P. x mixta is significantly under-recorded and perhaps relatively frequent in occurrence but often mistaken for P. anglica (there are only three Berkshire records of the former from the last hundred years, versus dozens of the latter). It is usually found in the absence of P. anglica, as is often strangely the case with plant hybrids. On campus the hybrid can be seen in the grass around the pond between Childs hall and the lake (approximately SU 7356 7222).
Left: Potentilla reptans with five petals, leaves with five leaflets and with long petioles, and creeping stolons. Right: P. erecta with four petals, sessile leaves with three leaflets and without stolons. Images via Wikimedia Commons
A further species doubtless familiar to many readers is also important to this story, namely tormentil P. erecta, a plant characteristic of all kinds of grassy places on acid soils. It turns out that P. anglica actually arose following hybridisation between P. reptans and P. erecta. In nature this hybrid (P. x italica) is very rare and is sterile, as is usually the case with hybrids. However, rare events such as this are the stuff of evolution, and at some point a hybrid P. x italica underwent chromosomal acrobatics (allopolyploidy) and became the fully fertile object we recognise as the species P. anglica. For what is a species but a phenotypically distinctive and true-breeding bunch of genes?
According to the flora of Whiteknights compiled by David Le Grice and Stephen Jury, all three of these species had been recorded on campus, but not the hybrid P. x mixta. So what does it look like and how does it differ from the species? The descriptions below are taken from the BSBI’s Plant Crib, an invaluable resource for many tricky groups of plants.
Potentilla anglica with a mix of trifoliate leaves and a few simple leaves (arrowed), short petioles and long-trailing stems. The number of petals shown here is within the range of variation of the species, but is more usually four. Diagram taken from Plant Crib.
P. reptans is a distinctive plant that creeps through grassland on long runners (stolons) in the manner of strawberry (to which it is closely related), forming extensive clonal patches. The leaves are compound with five oblong leaflets (hence cinque-foil) attached at one end to a common, usually very long, stalk (petiole), and vivid yellow five-petalled flowers. As in much of the rose family the fruit (not seed) is a little nut-like object called an achene (as in the pips on the outside of a strawberry). P. anglica differs principally in numbers of parts: usually three leaflets per leaf and four petals per flower (as in P. erecta). It also differs in that the stolons take root at longer intervals so that it presents a trailing habit (hence the common name), in the leaves being on short petioles, and in the presence of additional simple leaves (not just stipules) at the base of some of the petioles.
The salient features of the hybrid Potentilla x mixta: very long stolons; a mix of leaves with five and three leaflets and simple leaves; four petals (there was one with five on this plant). Images by D. Morris
The hybrid P. x mixta manifests a mix of these characters. The colony that I found at Whiteknights was almost uniformly four-petalled. Together with its long trailing stolons forming a conspicuous weft in the grass, this made it immediately obvious. By carefully untangling a few of the very long stolons I could see that the leaves were mostly of the cinquefoil type with five leaflets, but there were the odd one or two with three leaflets. Closer inspection showed that there were further simple leaves at the base of some petioles as in P. anglica. As P. anglica never has stolons as long as those I found, this was definitely the hybrid P. x mixta.
A feature that also suggested that the colony I found was of hybrid origin was its apparent sterility, with no fruit being set and the plant putting its energy instead into continuous flowering. P. anglica does not display this consistent sterility because it is self-compatible, i.e. individuals can pollinate themselves, and so one can usually find flowers producing achenes. However, P. reptans is self-incompatible so the lack of fruit seen in my plant does not alone mean that it must be a hybrid. Lack of fruit/seeds is, nevertheless, an important character of many hybrids. Aborted pollen would have confirmed sterility, but I didn’t need to check this with all the other positive features.
These flowers have all finished flowering. None of them show any signs of ripening achenes (‘seeds’). Older flowers on the same plant were simply shrivelled up and brown without any sign of fertility. Image by D. Morris
In conclusion, buy the book, add a few common hybrids to the list of species you know and look for when botanising, and you’ll notice so much more about even common plants. Maybe you’ll decide to become an expert in roses or willows!