The most beautiful monocot?

…or at least the most intriguing.  My last blog about monocots leads nicely into this one on orchids, prompted partly by Leif Bersweden’s book, The Orchid Hunter and also by excitement about an imminent trip to Cyprus, which I hope will have plenty of potential for orchid hunting. I do have a bit of a thing about orchids and am lucky enough to live in a limestone-rich part of the country where there are lots to be seen, though I’m all too aware that I’ve seen only a fraction of the 50 plus species in the UK. I’ve also been lucky enough to see orchids in other parts of Europe and started off thinking it would be interesting to try and list all the wild orchids I’ve seen over the last ten years or so but then, as usual, I got side-tracked.  No trying to fit everything into one season, or one blog post, for me!

Orchids, being monocots, have flower parts arranged in characteristic sets of three, though sometimes this is difficult to see.  They have three sepals and, in front of these, three often very different-looking petals.  The middle petal (labellum) is usually enlarged and acts a landing stage for the insects which generally fertilise orchids.  In many orchids it is also elongated into a long, nectar-producing spur at the back.

Kew orchid festival 2015.  Flowers have three pale sepals with three darker petals in front. The labellum is the central one.

As monocots, you’d expect orchids to have three stigmas too, but one is modified into a rostellum, which separates the pollen masses on the anthers from the stigmas and reduces the chances of self-pollination occurring. The pollen itself is held in pollinia – packets of pollen grains, held together by elastic threads, on a stalk or caudicle (shown in F in the figure below).

Early-purple orchids (Orchis mascula) and Pyramidal orchids (then called Orchis pyramidalis) are two of the first species Darwin discussed in his snappily-titled book, ‘On the various contrivances by which British and foreign orchids are fertilised by insects, and on the good effects of intercrossing’.  Observing how the wild orchids growing around his beloved Down House were adapted to their insect pollinators, and vice versa, prompted Darwin to propose the then-novel idea of the coevolution of plants and insects. The fantastic ‘Darwin online’ project (http://darwin-online.org.uk/) means anyone with internet access can look at his original work.

Darwin describes how the two pollinia (labelled p below) are completely enclosed by the rostellum until the flower opens.  When an insect lands on the labellum and tries to reach the (fake) nectary at the back of the flower, the rostellum ruptures horizontally and exposes the pollinia.  The change in orientation forces the sticky (viscid) discs at the base of the of one or both of the pollinia (d below) against the insect’s head and glues them firmly in place.

Woodcut of Orchis mascula from Darwin’s book (all sepals and petals except the labellum removed) and an inflorescence from Thrislington NNR, June 2016.  a. anther; r. rostellum; s. bilobed stigma; l. labellum; n. nectary; p. pollinium or pollen-mass; c. caudicle of pollinium; d. viscid disc of pollinium

The really clever bit, though, is that as the sticky disc dries out it contracts, changing the orientation of the pollinium so that when the insect reaches the next flower, the pollen mass will be in the correct position to be deposited on the second plant’s stigma.  Darwin showed this simply, using a sharp pencil to mimic the insect’s body.

1862_Orchids_F800_fig02

A. Pollen-mass of O. mascula, when first attached. B Pollen-mass of O. mascula after the viscid disc has contracted

The stigma is sticky, but not so sticky as to pull the whole pollinium off the insect’s head.  Instead, the elastic threads holding clumps of pollen together break and some packets are deposited on the stigma.  The remainder will be deposited on flowers the insect visits subsequently, ensuring the pollen is dispersed as widely as possible to enhance genetic diversity.

Hawk moth attempting to sup from Orchis mascula

If Early-purple orchids are a favourite, for their early appearance, Pyramidal orchids are another, for their cosmopolitan distribution.  I’ve seen their dense pink spikes as far afield as the dry maquis of Eastern Crete and as close to home as the distinctly brown-field site of South Gare at Teesmouth, round the mothballed Redcar steel works. The pyramids begin to look more like cylinders as more flowers open.

Pyramidal orchids, Anacamptis pyramidalis, Crete and Teesmouth

Whilst the labellum of Early-purple orchids has side lobes shorter than the central, notched, lobe Pyramidal orchids have three more or less equal lobes.  Darwin describes A. pyramidalis, another species pollinated by moths and butterflies, as ‘the most highly organised species’ he has seen.  It has the same kind of arrangement of a pair of pollinia but this time they are attached to a single, saddle-shaped viscid disc.  The flower also has a pair of prominent ridges running down the labellum, guiding an insect’s proboscis towards the nectary. When a proboscis is inserted into the flower, and then removed, the saddle-shaped disc will contract rapidly as it dries and wrap itself around the proboscis, with the pollinia sticking upwards like the pom-poms on a teeny-bopper hairband.    This time though, when the disc contracts, the two pollinia end up at an angle of nearly 90° to one another (as in G, below).   When the proboscis carrying the pollinia is guided by the ridged labellum on the next flower the insect approaches, the two pollinia will be perfectly positioned to strike both its two stigmas. No wonder Darwin was prompted to think about co-evolution!

1862_Orchids_F800_fig03

Woodcut of Orchis pyramidalis (now Anacamptis pyramidalis) from Darwin’s 1862 book. A needle was used to collect the pollinia in F and G.   Key: a. anther;  s. stigma; r. rostellum; l. labellum; l’. guiding plate of the labellum; n. nectary.

Marsh- and spotted-orchids also used to be part of the genus Orchis, though they are now known as Dactylorhiza species. In this genus the labellum is around the same width as it is long.  Several of these species hybridise fairly freely with one another, making identification really tricky for the non-expert.

Heath- and common-spotted orchid flowers are pale pink or white and marked with darker dots, streaks or loops.  They can be reasonably easily distinguished by looking at the middle lobe of the three-lobed labellum.  In Heath-spotted orchids the lobes are very shallow, whilst in Common-spotted ones the lobes are well-separated and pointed. The habitat is also a clue – like most of our UK orchids, D. fuchsii  prefers chalky soil but D. maculata  grows in acid heaths and bogs.

Common spotted-orchid, D. fuchsii (top); Heath spotted-orchid, D. maculata (bottom)

It’s the spotty leaves which gives both these orchids their common name.  The leaves of Common spotted orchids are often tiger-striped with transverse purple spots, most obvious when the leaves are still in a small, basal rosette.  Early purple-orchids also sometimes have spotted leaves but in this case the blotches run lengthwise along the leaves.

Common spotted-orchid leaves,  D. fuchsii

Most of what I see around County Durham are D. fuchsii, but one of last year’s botanical pleasures was swathes of D. maculata growing on the marshy ground at the east end of Ennerdale Water.

The marsh-orchids, also abundant on my regular Crowtrees botanising patch, usually have unspotted leaves and more strongly coloured flowers (hinted at by their specific names).  They are even more variable!  I suspect there are both Northern and Early marsh-orchids but am more confident about the former. The deep rich purple of the Northern species’ flowers contrasts well with the flesh-coloured Early marsh orchids I’ve seen in upper Teesdale, though.  Whilst D. purpurlella flowers are covered in crimson streaks, the flowers of D. incarnata have U-shaped purple loops surrounding patched of dots.

Northern marsh-orchid, D. purpurella (left) and Early marsh-orchid D. incarnata spp. incarnata (right)

At the risk of sounding too much like a twitcher, I reckon that is six of our native orchids down – only nearly 50 more to go!

 

4 thoughts on “The most beautiful monocot?

  1. Pingback: More beautiful monocots – Ophrys species | heatherkellyblog

  2. Pingback: Yet more beautiful monocots – some other UK orchids | heatherkellyblog

  3. Pingback: Crowtrees LNR, July 2018 Part 2 – Gentians to Grasses | heatherkellyblog

  4. Pingback: Raisby Hill Grassland SSSI, June 2019 | heatherkellyblog

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