The 25th July 2021 marks 400 years of botanical research and teaching by the University of Oxford.

As a celebration and count-down to this anniversary, the University of Oxford Botanic Garden and Harcourt Arboretum, together with the Oxford University Herbaria and the Department of Plant Sciences, will highlight 400 plants of scientific and cultural significance. One plant will be profiled weekly, and illustrated with images from Oxford University's living and preserved collections.

  • Plant 52: Saccharum officinarum
  • Plant 51: Metasequoia glyptostroboides
  • Plant 50: Equisetum sp.
  • Plant 49: Fraxinus excelsior
  • Plant 48: Rosmarinus officinalis
  • Plant 47: Ptelea trifoliata
  • Plant 46: Acer saccharum
  • Plant 45: Brassica oleracea
  • Plant 44: Helianthus annuus
  • Plant 43: Ricinus communis
  • Plant 42: Simmondsia chinensis
  • Plant 41: Chara sp.
  • Plant 40: Zingiber officinale
  • Plant 39: Aristolochia clematitis
  • Plant 38: Allium cepa
  • Plant 37: Galium tricornutum
  • Plant 36: Artemisia annua
  • Plant 35: Rosa canina
  • Plant 34: Nepenthes rajah
  • Plant 33: Dianthus caryophyllus x Dianthus barbatus
  • Plant 32: Taraxacum sp.
  • Plant 31: Victoria cruziana
  • Plant 30: Lathyrus odoratus
  • Plant 29: Heliconia rostrata
  • Plant 28: Senecio squalidus
  • Plant 27: Paulownia tomentosa
  • Plant 26: Urtica dioica
  • Plant 25: Euphorbia characias
  • Plant 24: Heliamphora nutans
  • Plant 23: Laurus nobilis
  • Plant 22: Tulipa sylvestris
  • Plant 21: Pleurococcus sp.
  • Plant 20: Gleditsia triacanthos
  • Plant 19: Tillandsia usneoides
  • Plant 18: Marchantia polymorpha
  • Plant 17: Daphne mezereum
  • Plant 16: Citrus medica
  • Plant 15: Coffea arabica
  • Plant 14: Gossypium species
  • Plant 13: Stachyurus praecox
  • Plant 12: Encephalartos ferox
  • Plant 11: Aloe vera
  • Plant 10: Araucaria angustifolia
  • Plant 9: Isoetes echinospora
  • Plant 8: Hamamelis virginiana
  • Plant 7: Lithops species
  • Plant 6: Sequoiadendron giganteum
  • Plant 5: Commiphora saxicola
  • Plant 4: Buxus sempervirens
  • Plant 3: Picea abies
  • Plant 2: Cinnamomum verum
  • Plant 1: Taxus baccata

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    The data and images available on this site may only be used for scientific purposes. They may not be sold or used for commercial purposes. All images are copyright of the University of Oxford, unless otherwise indicated.

    The specimens at the Oxford herbaria and the living collections of the Oxford Botanic Garden and Oxford University Herbaria are being digitized using BRAHMS.


    Dr Alison Foster (

    Dr Stephen Harris (

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    Plant 53

    Salvinia molesta D.S.Mitch. (Salviniaceae)




    Salvinia molesta is part of a complex group of closely related floating fern species. Salvinia, apparently native to southeastern Brazil, has become a serious pantropical freshwater weed. Over the course of the twentieth century, the fern has been introduced to many countries, via the international trade in aquatic plants for ponds and aquaria.

    Salvinia leaves are arranged in groups of three along a thin rhizome. In each group, two green, flat, undivided aerial leaves float on the water surface. The third leaf is non-photosynthetic, highly divided and hangs, root-like, underwater. At high light intensities, salvinia aerial leaves are partially folded, raising the plant off the water surface. At low light intensities, such as in the Botanic Garden glasshouse, leaves tend to lie flat on the water.

    Biological, morphological and anatomical features of salvinia have contributed to the fern's success as an invasive species. Salvinia is only known to reproduce vegetatively, from rhizome fragments; even tiny fragments are capable of producing new plants. Spore-producing structures, for sexual reproduction, form underwater but the spores are infertile; this has led to the suggestion that salvinia may be of hybrid origin. Furthermore, under appropriate nutrient conditions, salvinia grows rapidly; some studies have shown that salvinia plants may double their size in just over a week. The effects of heavy salvinia infestations are profound. Thick carpets of plants clog lakes and block sunlight to other plants that oxygenate the water. Eventually, as the plants decompose lakes stagnate and overall biological diversity drops. Infestations of salvinia also interfere with water management, for example, through flooding and irrigation controls, and provides conditions for mosquito breeding.

    The aerial leaves show clear adaptations for buoyancy and water repellence. The two leaf surfaces are separated by rows of cells that divide the leaf into a series of air-filled chambers, which creates a buoyant platform. The surface of the leaf in contact with the air is densely packed with long-stalked, wax-covered hairs that look like egg whisks. The main part of the hair is hydrophobic but the extreme tip is hydrophilic. The effect is to produce an extremely water-repellent surface from which water droplets are readily lost and when submerged the plant is covered by an air film. Curiously, from a developmental perspective, salvinia leaf surfaces are reversed: the surface in contact with the air is the lower surface, whilst the surface in contact with the water is the upper surface.

    Barrett SCH 1989. Waterweed invasions. Scientific American 261: 90-97.

    Kaul RB 1976. Anatomical observations on floating leaves. Aquatic Botany 2: 215-234.

    Stephen Harris