Deconstructing Coltsfoot

Thursday's Guardian Country Diary is all about coltsfoot.

With its radiant yellow petals, it's one of those cheerful flowers that's a signpost on the path to spring, all the more striking because the flowers appear long before the foliage. They spear through the soil unnoticed, then open as soon as the sun comes out.

The plant grows from a creeping underground rhizome that stores starch - the fuel for that rapid burst of growth in spring. When the rhizomes are fragmented even small segments regenerate into a new plant, which is why it persists so well in urban environments, carried around and endlessly redistributed in developers' soil and rubble.

Part of its appeal lies in its extraordinary tenacity and tolerance of inhospitable habitats. This crowd of flowers appeared between the massive stone blocks that armour the sea wall against the waves at the mouth of the river Wear in Sunderland.

Coltsfoot is one of those familiar wild flowers that's a pleasure to see in spring, but it's one that is rarely closely examined. There are two kinds of florets in the inflorescence. The central core of around thirty disk florets open in centripetal sequence over a few days and have no petals. The surrounding ray florets are far more numerous - perhaps as many as two hundred.

A closer look reveals that the central disk florets are hermaphrodite, each with a ring of stamens and a central stigma that grows up through them, acting like a piston that pushes pollen ahead of it. Presumably these are capable of self-pollination, as well as presenting pollen to a visiting pollinator for onward transfer to another flower. Some books claim that the disk florets are entirely male, but in the flowers that I've looked at they have a stigma and stamens. The disk florets are enclosed in sepals, that open to form a pointed star.

The surrounding ray florets are female only, with a longer, thinner stigma that divides into two near its tip, ready to receive pollen from a visiting insect. 

Here's s closer view of a central disk floret, with a pile of pollen perched on the tip of its stigma that has forced its way up through the surrounding tube of stamens. 
Stigmas of the female ray florets are visible on the left.

Closer still, and here you can see the receptive surface of the stigma of a disk floret, with its surrounding tube of stamens after it has forced its way up through them. In this case the pollen has been brushed off the stigma surface, perhaps by a visiting insect.

Here you can see there that stigmatic papillae - minute finger-like projections where pollen attaches when these stigmas are self-pollinated.

A single disk floret, removed from the inflorescence.

Pollen grains adhering to the surface of the stigmas of the all-female ray florets, after a visit by a pollinator carrying pollen.

Unpollinated ray floret stigmas of a newly-opened inflorescence, with their narrow, strap-shaped petals.

Comparison of a ray floret (upper left) with its stigma and petal and a disk floret (right) with its stigma and stamens. The slender structures at their base are the hairs that will grow after pollination and form the pappus - the parachute of hairs that will carry away the ripe seed on the wind.

One curious feature of these two forms of floret is that their stigmas and styles are so different - slender in the female florets but much more robust the the central disk florets where they perform a dual function, of pollen presentation and a site for self-pollination. Both appear to have ovaries at their base but I need to have another look after pollination to see if seeds are formed in both and whether, if they do, they are identical.

Coltsfoot is one of many examples of common wild flowers that are far more complicated that they seem. They are remarkably complex, intricate mechanisms that have evolved to compete for the few pollinators that are around in early spring - and to ensure that both cross- and self-pollination can occur in the same inflorescence.