Friday, July 8, 2011

White out

Yesterday we went for a short ramble in the Malvern Hills. Parking at Berrow Down car park (SO765382, £3 all day), we made a short circuit up Hangman's Hill, over Broad Down, around British Camp reservoir and back via Shadybank and Castlemorton Common. There's a map on the informative Malvern Hills Conservators' website.

Our walk was puctuated by frequent halts due to rain (to shelter under trees), plants (to enjoy, identify and attempt to photograph without drenching the camera) and snacks (most important).

The views of the Malvern Hills and surrounding countryside were superb.
Malvern Hills looking north from near Hangman's Hill
Before I'd even gotten out of the car, I'd noticed there were lots of white marsh thistles (Cirsium palustre). Usually flowers are pink/purple; whites are rarer. 

White marsh thistle (Cirsium palustre)
I wonder why there are so many white-flowered marsh thistles, both on the hill slopes and the commons? Interesting! See Addendum.

Other plants seen included enchanter's nightshade (Circaea lutetiana), named after Circe, the enchantress, who turned her enemies into animals. Lovely.
Enchanter's nightshade (Circaea lutetiana)
And wood sage (Teucrium scorodonia). It's leaves only look like sage; it is bitter and astringent, so it isn't used in the kitchen (it's in the same family as sage though). It grows in a wide variety of habitats, not just woodlands.
Wood sage (Teucrium scorodonia)
And the small, woolly, oft overlooked marsh cudweed (Gnaphalium uliginosum).
Marsh cudweed (Gnaphalium uliginosum)
We sat on a small rocky outcrop to eat lunch and saw 2 (what I thought were) hummingbird hawkmoths ovispositing (laying eggs) on lady's bedstraw (Galium verum) (I'd seen HBHMs doing same at Park Gate Down in Kent last month). They were moving so fast it was hard to see anything but a blur. 

I looked with great care for any eggs on the tiny bedstraw plants. I found one almost immediately, but it was white and ridged, not green as I've seen previously (although those were freshly laid). I also found a teeny-weeny caterpillar with a little black 'horn'. I've no idea if these are HBHMs. I have put these shots on ISpot so will update if any news from there.
 Egg on lady's bedstraw
Caterpillar on lady's bedstraw
Sneezewort (Achillea ptarmica), a super plant closely related to yarrow, was just coming out. It's specific name is from the Greek ptairo, to sneeze. It used to be dried and powdered, and used as snuff to induce sneezing, I assume to clear the sinuses. According to Grigson (1958), one of its local names was old man's pepper box.
Sneezewort (Achillea ptarmica
The grazing contractors were watching me closely whilst I was taking photographs and looked less than impressed.
Grigson, Geoffrey (1958) The Englishman's Flora. Paladin

Addendum: Why are there so many white flowered thistles in some places?
White flowered marsh thistles are often more numerous on mountains and sea cliffs. Ecologists (see links to Mogford (1974) below) theorised that there may be some factor (or factors) that maintains both pink and white flowered plants in a population in exposed locations. Perhaps whites have a hidden advantage in some situations?

Climate (temperature/exposure) is likely to be a factor, but this may be indirect. There are fewer (and less diverse) pollinators in mountain areas. Experiments on these pollinators (bees) indicate that some of them preferentially visit white flowers, and this may facilitate the maintenance of white-flowered plants in the population. But, this alone may still not be sufficient to explain the abundance of white-flowered thistles in some places.

The pink/purple colour in ‘normal’ thistle flowers is due to the production of plant pigments called anthocyanins. Anthocyanins attract certain pollinators and are advantageous in many environments. But in more exposed situations with fewer, choosier pollinators, the relative advantage of being pigmented (i.e. pink) might be reduced. Anthocyanins, and their chemical precursors, have wide-ranging roles in plants; not just petal colour. And they are costly for plants to manufacture. Perhaps there are indirect costs associated with pigment production, which gives whites a selective advantage at exposed sites, over and above choosy insects?

Of course, these theories don’t satisfactorily explain why there are so many (anecdotally) white-flowered plants at the Malvern site, which is low-lying grassland, and which, I assume has abundant and diverse pollinators.

Bibliography for Addendum
Dr Phil Gates wrote a short blog on white marsh thistles in 2009.
Dierkes, LE and Galloway, LF (2008). PS 79-81: Maintenance of flower color polymorphism in the herbaceous perennial Tradescantia ohiensis: The importance of non-pollinator agents of selection. Paper presented at the 93rd ESA (Environmental Society of America) Annual Meeting. 3rd-8th August. Linkie
Mogford, D (1974) Flower colour polymorphism in Cirsium palustre 1. Heredity, Volume 33 (2), pps 241-6. Pdf is here
Mogford, D (1974) Flower colour polymorphism in Cirsium palustre 2: Pollination. Heredity,  Volume 33 (2), pps 257-63. Pdf is here

1 comment:

  1. Ah, lovely land where I learned to fish and ride! No glowworms then?
    Sneezewort looks a cool flower.