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 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

  • Follow us on Twitter @Plants400

    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 50

    Equisetum sp. (Equisetaceae)




    Equisetum is an ancient genus of plants whose close relatives are fossilized in sediments up to 350 million years old. Today, this once diverse group of plants is restricted to fewer than twenty species. Special characteristics of horsetail shoots made their ancestors evolutionarily and ecologically successful. Today, these resilient shoot systems often give gardeners severe headaches as they try to eradicate horsetails from their gardens.

    Horsetails produce massive systems of underground, branching shoots called rhizomes - the parts of the plant that you see above ground are only the tips of those branches - and it is virtually impossible to remove every piece of rhizome from infected soil.

    While horsetail aerial shoots are generally less than one metre tall today - exceptionally they may be as much as three metres in height - they are dwarfs compared to some of their ancient relatives. During the Carboniferous Period, between 299 and 359 million years ago, close relatives of modern day horsetails formed a considerable proportion of the 'trees' in the world's first forests.

    The giant horsetails, called the Calamitales, formed trees between 10 and 30 metres in height and represent one of the first evolutionary 'experiments' in being tall (or, to put it another way, 'being a tree'). A radical innovation in stem strengthening meant horsetails could to grow upwards without their stems buckling.

    The ability of a plant or an animal to support its own weight is determined by the mechanical strength of its component tissues. Herbs are made of soft tissues and cannot support much weight - try to balance a pound coin on a dandelion head; the head will collapse. Wood on the other hand is mechanically stronger and can support the considerable weight of a large tree.

    In giant horsetails another way evolved; impregnation of the stems with silica (silicon dioxide, SiO2) made them mechanically strong. Silica, chemically identical to the tough quartz mineral that makes granite rock so hard, reinforced the stems and allowed them to grow taller than plants with silica-less stems.

    Structural reinforcement of stems with minerals also allowed another group of plants, bamboos, to grow upwards without collapsing under their own weight. The hard, flinty canes of bamboo canes are enriched in silica.

    The bamboos of today are a reminder of the giant horsetails of the past and demonstrate how plants have co-opted the mechanical qualities of a tough rocky mineral during their evolution to hold them upright.

    Kenrick P, Davis PG 2004. Plant fossils. Natural History Museum.

    Liam Dolan