Autumn/Winter 2011/12 - Harcourt Arboretum - University of Oxford

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6 Botanic Garden News | No. 79 Recent developments at the University of Oxford Botanic Garden and Harcourt Arboretum by Timothy Walker It is universally acknowledged that this has been a very good year for fruits. The possible reasons fall into two groups. Firstly, there was nothing to upset flowering and thus pollination of those flowers. Although December 2010 was bitterly cold, in central Oxford we have escaped serious frost so far in 2011. The winter flowering plants were late but then spring was a bit early and spectacular. Secondly, the conditions through the summer may not have been good if you were camping, but if you were a plant growing and swelling fruits, they were perfect. The many and various fruits of the family Rosaceae are especially abundant this year. The flowers of the members of the Rosaceae are very easily recognised because in them it looks as if the stamens are growing from the base of sepals. In fact the stamens, sepals, and petals all grow around the edge of a cup-like structure called a hypanthium but the petals fall off before the other structures do. The hypanthium is in fact the fused bases of these three whorls of structures. However, the fruits of the members of the Rosaceae are very varied, in part due to the hypanthium and the receptacle that supports all the floral parts. The division of the species in the Rosaceae into genera has been controversial since John Ray (the 17th century English naturalist) was a lad and probably long before then. In particular it has been very difficult to see how the 950 species with pomes can be grouped. A pome is a fleshy, indehiscent (not opening at maturity along lines or pores) fruit, consisting of a modified floral tube surrounding a core, as in apples. 1 The plants with pomes are not just apples but pears, quince, loquat, hawthorn, medlar, cotoneaster, sorbus and pyracantha. To quote a 2007 paper 2 there is “limited Sorbus sargentiana Sorbus domestica forma pyrifera Sorbus aria Sorbus latifolia Sorbus domestica forma pomifera Sorbus torminalis
Botanic Garden News | No. 79 7 Rhododendron bashing at the Arboretum: (l-r) Director Timothy Walker, trainee Katie Benallick and horticulturalists Richard East and Jim Penny resolution of a complex evolutionary history”, or to put it another way, we don’t know what’s related to what. It is a classic example of Darwin’s lament that “we have no written pedigrees [so] we have to make out community of descent by resemblances of any kind.” The situation is complicated by the fact that these species don’t seem to know where the boundaries lie either, and form hybrids with species in quite different genera. 3 So for example at the Botanic Garden we have a hybrid between hawthorn and medlar (x Crataemespilus) and another between rowan and pear (x Sorbopyrus). It is further complicated by the fact that some species indulge in apomixis, where seeds are produced without the need for fertilisation, resulting in very localised variants. It appears that this is a group which evolved very quickly a long time ago, but since then there has been very slow divergence of their DNA. This divergence is very important for building classifications based on DNA. A wise old taxonomist once told me that “where there is confusion at the macro level there will be confusion at the molecular level” and in this case he was right. Confusion can be seen from the fact that in the 1850 guide to the Botanic Garden, written by Professor Charles Daubeny, the applefruited form of the service tree is referred to as Pyrus domestica. In the 1914 guide written by Dr Gunther it is Pyrus sorbus. The current label says Sorbus domestica forma pomifera. Unfortunately this now needs to be changed to Cormus domestica as there is no evidence to support the grouping together of all of the species that were formerly in the genus Sorbus. 4 Interestingly, the genus had already been sub-divided into sub-genera (or sections) along similar lines. 5 If you want to see the differences for yourself, the Botanic Garden is a splendid place to do this. We have Sorbus domestica, S. torminalis, S. sargentiana, and S. latifolia inside the Garden and there are S. aria trees on the High Street outside. Our new orchard includes apple trees and other culinary species which are already starting to bear fruit. We also have Cydonia oblonga, Mespilus germanica, and many other species at both the Garden and the Arboretum and they are all doing very well this year. Many of their fruits also make terrific jelly. Not to be outdone the woody plants at the Arboretum have also fruited well. The squirrels have had a feast on the acorns which have formed carpets under their parent trees. Rhododendron ponticum is an important plant at the Arboretum but it can get carried away. This year, as every year, some of it has had to be coppiced and grubbed out to prevent it from swamping other plants. This also reduces the risk of invasive diseases, such as Phytopthora, spreading. Orchard planting at the Botanic Garden
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Autumn/Winter 2011/12 - Harcourt Arboretum - University of Oxford

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