Article on the genus Eryngium
Some Botany on the Eryngo Plants and Sea Hollies
(previously published in ‘Botany for Beginners’ part II, Chelsea Physic Garden Florilegium Society)
The genus Eryngium, known also as Eryngo plants, are perennial members of the Carrot family, Umbelliferae. This does not, at first, appear obvious, for they are easily confused with members of the Teasel family, Dipsacaceae, or the Daisy family, Compositae.
What has, in fact, happened over the course of evolution is that the umbel has been telescoped into a solid flower head – known as a CAPITATE UMBEL, which appears to be similar to the capitulum of a composite inflorescence belonging to the family Compositae. Most members of the genus are pollinated by bees, though E.giganteum ‘Miss Willmot’s Ghost’ is polhnated by wasps.
There are about 230 species of Eryngium worldwide, distributed both in the tropical and temperate regions, though absent from tropical and South Africa. Five species are found growing wild in the British Isles, induding the Sea Holly E.maritimum, which likes the sandy and shingle coastline, and the Field Eryngo E.campestre, which grows in grassland and open spaces on calcareous soil near the sea. Other species in Britain are introduced or very rare.
The European species are more likely to have rounded or lobed leaves, while the American species are more likely to have sharply spined, leathery leaves. The leaves are strikingly green or blue-green with contrasting white veins in some species, making it an attractive garden plant. The European Garden Flora lists 24 species commonly found growing in cultivation.
The economic importance of Eryngium is limited. E.pandanifolium is a source of caraguta fibre, which is used in the sack making industry, while E.campestre and E.maritimum have their roots candied and are used as ‘Kissing Comforts’.
The botanical features are worthy of closer study and I have chosen Eryngium bourgatii, which flowers like most other species during the summer months. The plant varies in height from 15cms to 45cms and has spiny-toothed, leathery leaves with conspicuous silver venation. The plant is very decorative, forming up to seven sky blue or silver inflorescences (flower heads). The plant is a native of the Mediterranean region and, like most species, it prefers an open, sunny habitat. There is a cultivar ‘Oxford Blue’ which has a dark silver-blue inflorescence.
Some Practical Hints
In examining the inflorescence you will need to cut the capitate umbel-like inflorescence down the middle longitudinally with a stout knife and then carefully observe prior to pulling the individual florets off with forceps. You will need to use a lens with at least a X10 magnification for observing the L.S. and T.S. of the ovary. Access to a simple dissecting microscope will make life even easier. In the absence of E.bourgatii other species or cultivars will do, but please remember that one or two species are fairly rare, so keep to garden examples wherever possible. Enjoy your dissection work, but be careful as the whole plant is rather prickly!
I would like to thank Dr. James Cullen, Director of the Stanley Smith Horticultural Trust, as well as Clive King, for giving me advice with regards to the interpretation of my dissections.
Dr. J. Cullen et al The European Garden Flora, Vol.V. CUP 1997. D.J. Mabberley The Plant Book, 2nd Ed. CUP 1997. C. Ross-Craig Drawings of British Plants, Vol.4. Bell and Hyman 1959. (Drawings only of British species, no text, but very useful.) C. Stace New Flora of the British Isles, 2nd Ed. CUP 1997 Floral details
Refer to the illustrations below while reading the text.
Fig. 1 Flowering shoot showing three inflorescences each forming a capitate umbel supporting numerous small flowers or florets. The terminal inflorescence is 23mm x 18mm and the lateral inflorescences are 15mm x 18mm. Arising from the node, where the two lateral inflorescences arise, is the 1- or 2-pinnatifid, spiny toothed leaf. Numerous leaf-like, spiny bracts surround the terminal inflorescence, which may be entire or have spiny teeth. Note the upper portion of the stem is markedly ribbed.
Fig. 2 L.S. of a portion of the capitate umbel. The capitate umbel is solid and fleshy, supporting numerous florets that mature from the bottom upwards. At the base of each floret is a small stiff and pointed bracteole. Note the large bract which exceeds well beyond the length of the maturest flower. This large bract reaches up to 10cm.
Fig. 3 Details of a hermaphrodite flower during early flowering. The inferior ovary is 5mm x 2.75mm in the drawing and is covered with a few fish-like scales. Surrounding the upper rim of the ovary is a calyx of five persistent and stiff teeth (sepals) reaching a length of 2.5mm to 3.0mm. In the centre of the flower there are two long style branches surrounded by five stamens, two of which are emerging to release their pollen. Five petals can just be seen alternating with the outer whorl of calyx-teeth or sepals.
Fig. 4 L.S. inferior ovary. Note the style branches arising out of the apical portion of the ovary. There is a prominent ridge circling the upper portion of the ovary to which the calyx-teeth (sepals) are attached. There is a 2-celled ovary with a pendulous ovule in each cell which measures 2.0mm x 0.5mm.
Fig. 5 T.S. ovary with subtending bracteole. Note each side of the two ovules there is the early stage of a dividing line which will eventually form a deep suture at a later stage during fruit development.
Fig. 6 Petal ventral view. The petals measure 2.75mm to 3.0mm in length and are grooved on the ventral (inner) surface. There are five petals alternating with the calyx-teeth (sepals).
Fig. 7 One of the five stamens as viewed at early flowering. The filament reaches up to 4.5mm in length and is dorsifixed (joined to the back of the anther). The anther measures 1.0mm x 0.2mm. The anther shape changes during the course of flower development, as well as becoming more upright.
Fig. 8 Fruit – the fruit is at first a two-seeded schizocarp that later splits into two one-seeded mericarps. In Eryngium there are two mericarps, as shown, and they appear on the capitate umbel in such a manner. Note the fish-like surface scales on the fruit wall. It is to be noted that the fruit at first is two-seeded.
Fig. 9 Single mericarp separated out ready for fruit dispersal. The fruit measures 3.0mm x 4.0mm in the drawing, but may vary to some extent. The surface fish-like scales are brown at maturity and are attached near the middle, rather than the base, this is known as peltate.