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University of Oxford Botanic Garden evening tour

Oxford Botanic Garden

After a typical cold, grey English summer week, the sun came out to welcome the SCI Horticulture Group to the University of Oxford Botanic Garden. The garden occupies an enviable position at the end of High Street, opposite Magdalen College and with the gentle waters of the River Cherwell flowing round two sides.

The formal welcome was given by our guide for the day, Dr Alison Foster, Glasshouse Horticulturalist, University of Oxford Botanic Garden. She explained that the garden owed its origin to the dubious exploits of Henry Danvers, later the 1st Earl of Danby. In 1622, after a period of 'exile' he returned to the country and agreed to endow the University of Oxford with five acres of land, opposite Magdalen College, for a physic garden where medical students could gather the plants they required. He also bequeathed the sum of £5,000 (equivalent to £3.5million today). This was used to build the magnificent gateway into the gardens designed by the famous architect Inigo Jones and enclose the area in the thick, high wall that provides the garden with such an excellent protected environment to this day.

Oxford quarters
The money ran out before any plants were installed and it was left to the first curator, Jacob Bobart to lay the foundation of what we see today. He divided the garden into four quarters with the main paths and then each quarter in half to give eight equal planting areas. With true Oxford eccentricity these eight sections remain known as 'quarters' to this day.

In keeping with the gardening style of the times he planted a row of English yew (Taxus baccata) down each side of the main path from gateway in 1645. Sadly only one of these yews remain, but as we paused under its spreading branches, Alison took the opportunity to explain that yew is the source of the drug Taxol that is used in the treatment of breast cancer. Originally extracted from the bark of Pacific yew (Taxus brevifolia), Taxol proved very expensive to manufacture in the laboratory. However, the needles of the English yew were found to contain a similar chemical similar which could be modified less expensively in the laboratory to produce the drug. Hedge clippings from English yews were far less destructive than chopping down Pacific yews for their bark. Taxol initially proved extremely insoluble and thus difficult to administer to patients until it was found that cremophor, derived from the castor oil plant (Ricinus communis), could be used to dissolve the drug.

The original garden was prone to flooding and the soil level was raised using the contents of the college cesspits. Nevertheless the garden's soil smells like any other, a smell Alison pointed out, due to the chemical geosmin released by the decomposition of Streptomyces bacteria living in the soil.

The 'quarters' of the garden contain a series of beds each containing plants from one plant family. The families are also laid in their evolutionary order as dictated by the Angiosperm Phylogeny Group starting with the most primitive Dicotyledons like the buttercup family (Ranunculaceae) to the most advanced daisy family (Compositae) and then the monocotylons like the lilies and grasses. Around the walls of the garden other plants are also laid out in geographical groups. This layout enables the garden to perform its primary function as a teaching aid. In order to satisfy this need along with the wider interest of the general public visiting the garden plants are expected to look good, show the characteristics of the family and be associated with good stories of economic or medicinal use.

Similar compounds in related species
Alison pointed out that this family arrangement also matches their chemistry with similar compounds found in related species. For example hallucinogens are found in the potato family (Solanaceae) and the convolvulus family (Convolvulaceae). The Solanaceae also produces the spicy heat of the chilli pepper (Capsicum) with similar effects coming from ginger (Zingiber) and Euphorbia. In 1912 this led American chemist Wilbur Scoville to create his 'Scoville Organoleptic Test' as a measure of spicy heat. This ranges from the mildest heat of the bell pepper (Capsicum annuum) to the hottest of all, the chemical resinoferatoxin produced by the resin spurge (Euphorbia resinifera). This latter is being investigated as a possible painkiller. The toxic, milky sap of its relative the petty spurge (Euphorbia peplus) is used as a therapeutic agent to remove warts. Recent work also suggests that it may also be effective in treating superficial basal cell carcinomas.

Amazon water lily
Our tour then took us to the glasshouses. The Water Lily House, as with many originally built in the Victorian era, was designed principally to grow the giant Amazon water lily Victoria criziana. The fully formed floating leaves, with their characteristic upturned edges, are reputed to hold the weight of a baby and several children of botanic garden staff have proved this to be the case. The plant is grown each year from seed which must therefore be produced. In the wild, flowers are pollinated by scarab beetles which enter the white flowers one evening and are trapped as the flower closes around them. They are released, covered in pollen, the next day by which time the flowers have turned red. Live scarab beetles are not found in Oxford so the staff have to wade into the pond to pollinate the lily by hand and net the flowers before they fall below the water so they can collect the seed.

The sides of the lily house are adorned with the spectacular climber, the jade vine (Strongylodon macrobotrys) with its unique blue green flower colour. The pigment, as yet unidentified, is water-soluble and the flower turns blue when frozen.

Alison then took us on to the Palm House which is used extensively for teaching, especially young children. It contains plants of coffee (Coffea spp.) which contains caffeine and cocoa plants (Theobroma cacao) with the similar compound theobromine. The palm house is also home to the Madagascar periwinkle Catharanthus roseus. Extracts of this plant have been used to treat numerous diseases including diabetes and malaria. The substances vinblastine and vincristine from the plant are used in the treatment of leukaemia and other cancers. Many young people owe their lives to this plant which is now farmed in Texas.

Flower pigments
Moving on to the herbaceous border, Alison took the opportunity to describe the complexity of flower pigments. The simple terms anthocyanin, carotenoid and betalains hide a vast range of chemicals which are responsible individually and in combination for the array of colours. She also emphasised the importance of chemicals elements in the soil. Thus the blue colour of some hydrangeas comes from aluminium, which is only available to the plant in acid soil. So the secret to making hydrangeas blue on soil which is not naturally acid is to add aluminium sulphate to the soil. It is much easier to change a hydrangea from pink to blue than from blue to pink.

While admiring the neat rows of the vegetable garden Alison pointed to some barren bed which illustrated the problems a plant collection can create. False garlic (Nothoscordum inodorum), originally planted in the family beds, had become a terrible perennial weed which was taking several years to eliminate.

Medicinal garden
We concluded our tour looking at where Alison's own project, a medicinal garden is taking shape. Separate beds have been laid out for plants which produce chemicals that affect different parts of the body such the heart, mind, blood, skin, liver, and cancers.

Our visit ended enjoying a glass of wine in front of the imposing gateway to the garden while the sun gradually set behind the spires of Oxford.

Peter Grimbly

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