Plant of the Week – 2nd August 2021 – Wall Lettuce (Mycelis muralis)

This plant belongs to the Daisy family,  the Asteraceae (formerly called the Compositae). It is a perennial plant, over-wintering with just a few green leaves and sprouting new leaves in the late spring. The Asteraceae have many tiny flowers (‘florets’) grouped together to form a flower head (‘capitulum’) that appears and smells attractive to pollinating insects. These florets are often of two sorts, the outer ‘ray florets’ that look like petals and the inner ‘disc florets’ that are tubular. Think of the common daisy Bellis perennis: the outer white ‘petals’ are not really petals but ray florets and the yellow middle of the daisy-flower is a collection of disc florets. This daisy-flower structure is relatively ‘modern’; it evolved in the late Cretaceous (76–66 million years ago) whilst the much simpler buttercup-structure is found in the early Cretaceous, 126 million years ago. The Asteraceae have been very successful. There are many of them (32,000 species) occupying all parts of the world.

In most members of the family, including daisies, sunflowers and asters, you can clearly see the ray florets and the disc florets. But lettuces have lost their disc florets and they have only ray florets. There are so few florets per capitulum that you might be forgiven for thinking this flower comes from quite another family. You might think it’s in the cabbage family Brassicaceae which often has small yellow flowers (but always with four petals, don’t forget to count the petals).

Mycelis usually has five ray florets looking like five petals. Also, five anthers protruding from the florets.The capitula are arranged in large open panicles, as you can see in Richard Milne’s photograph below. Each stem has a few hundred capitula. If each produces five seeds then one plant is capable of making at least a thousand offspring.

Like other lettuces, its seeds are dispersed by means of a parachute-like structure of fine hairs (‘a pappus of hairs’) which I’ve tried to show in the image below. When I poked the structures, they flew away in a random fashion, carried by the motion of little eddies and convection currents, even when it seemed like a calm day.

This is a perennial plant that has spread since the year 2000. In Britain it is a southern species, described by several authors as ‘rare’ in Scotland and Ireland (see Braithwaite 2013). However, it is now colonising urban spaces in all low-lying areas of Scotland and much of Ireland. Is this yet another signal of warming? Not necessarily; it is rare in the really warm parts of southern Europe and North Africa and it extends to colder climes in Norway and Finland. It therefore has a broad temperature tolerance. Some people like to correlate distribution patterns with temperature patterns. The northern limit is at a January cold-limit of -7 ^oC and its southern limit appears to be 25 ^oC July temperature. Average Scottish temperatures are around 5 ^oC in January and 14 ^oC in July, so Scotland is well within the climatic envelope of the species.

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It seems strange that it isn’t more widely distributed, considering the copious seed production and the ability of the seeds to be dispersed by only the slightest breeze and carried far. But impressions can mislead. I’ve seen many seedlings around the base of plants, telling me that many, perhaps most, seeds were dispersed no more than 2 metres.

It is a Native in England and much of the rest of Europe, but probably it is not native in Scotland and Ireland. We cannot be sure of its true status in Scotland and Ireland. In Scotland its first record seems to have been 1839 whilst in Ireland it was first found in 1726 (Braithwaite 2013, Clabby ad Osborne 1999). Like many British wild flowers, it has travelled overseas to former colonial territories eg New Zealand and both the west and east coasts of North America but strangely not Australia. Very likely it does its long-distance travelling on boots and shoes, stuck in the mud and debris in the tread. Travellers to New Zealand are now required to hand in their boots at Immigration Control to have them sterilised with steam to destroy all life on the boot.

It’s a lettuce, it has the trademark milky latex and little yellow flowers, so why is it not in the genus Lactuca with the other lettuces? Well, it was indeed placed in Lactuca in all earlier floras (Bentham and Hooker 1920); and Stace (2019) thinks it might be time to restore it to Lactuca. Mycelis and Lactuca are separated on the basis of the pappus of hairs but the difference is quite minor: both have two rows of hairs but in Mycelis the inner row has longer hairs than the outer. Molecular genetics hasn’t yet turned its attention to the Mycelis/Lactuca split so it would be premature to decide one way or the other.

Of course the wild lettuces look nothing like the cultivated cousins. The ‘heart’ of the cabbage lettuce is the result of centuries of selection, but the similarity is clear once the cultivated lettuce reaches its flowering stage. Also we see the tendency to produce the red anthocyanin pigmentation (in red varieties of cultivated lettuce and in Mycelis muralis as well as Lactuca virosa).

Earlier authors say this is a woodland plant, and in the National Vegetation Classification it is recorded in W12, Beech Woodland, where it is described as a ‘stately’ plant up to a meter tall. In the rough and tumble of urban streets it isn’t usually stately, often ragged and frequently stunted. It is associated with soils and substrates of high pH. Hence, it colonises the mortar of walls, and it finds a home in the crevice between pavement and wall. It occurs frequently in cemeteries, especially on the north facing surfaces. Is doesn’t like too much light, presumably as it is adapted to woodlands, and so it appears preferentially on north-facing walls. Botanists from Dublin have drawn attention to its presence in the limestone pavement of the Burren in County Clare, Ireland. This is a natural geological formation – a type of Karst. It has deep fissures (‘grikes’) where Mycelis muralis can be found, sheltered from bright sunlight and salt-spray (Silvertown 1982). This astonishing site has a rich and colourful flora which has inspired generations of students and continues to attract visitors (Cabot and Goodwillie 2018).

I suspect Mycelis muralis is ‘on the move’. I’ve looked for it in South Ayrshire but can’t find it. Yet there are plenty of suitable sites. The Irish distribution is fascinating, suggesting a gradual migration from west to east, following the prevailing wind direction. And why is the northern border of its English distribution so sharp and co-incident with the Scottish border? These are questions for further research.

References

Bentham G and Hooker JD (1920) The British Flora. Reeve & Co., London.

Braithwaite ME (2013) The status of Mycelis muralis (Wall Lettuce) in Scotland. BSBI News 19-22.

Cabby G and Osborn BA (1999) Mycelis muralis (L.) Dumort. (Lactuca muralis (L.)
Gaertner). Journal of Ecology 87, 156-172.

Cabot D and Goodwillie R (2018) The Burren. New Naturalist Series, Collins.

Silvertown JW (1982) Measuring plant distribution in limestone pavement. Field Studies 5, 651-662.

Stace CA (2019) New Flora of the British Isles. C&M Floristics.

John Grace