A simple guide to Buttercup ID

And a note on glossy yellow petals............

The Ranunculaceae are a large family with about 6000 members across the world. The genus Ranunculus that comprises the buttercups has 600 members worldwide but only 20 occur in the UK.
It makes it easier, from the point of view of plant ID, that there are only three kinds of buttercup to be found in Leazes Park. So as long as you can recognize a buttercup from the shiny yellow petals, recognising which species you are looking at isn't too difficult. The three types of buttercup are bulbous buttercup Ranunculus bulbosus, meadow buttercup Ranunculus acris, and Creeping Buttercup Ranunculus repens. 
It also helps that bulbous buttercup gets going early which means that the buttercups we see in early spring are almost certainly of that kind. However, a more formal system of ID is useful for later in the season. On the basis of four characteristics (three would be sufficient), you can easily separate out the different varieties of Buttercup that are to be found in the park. They are the appearance of the sepals - reflexed or not, the presence of a ridge along the stem that is more easily felt than seen, the shape of the leaves and finally the presence of runners or stolons along the ground. These features are summarised in the table below and illustrated in the underlying photographs.


Bulbous buttercup (Ranunculus bulbosus) showing reflexed sepals and ridged stem.

Meadow buttercup (Ranunculus acris) showing non-reflexed sepals and smooth stem.

Three-lobed bulbous buttercup (Ranunculus Bulbosus) leaf.

Left- Creeping buttercup (Ranunculus repens) leaf and Right- Meadow buttercup (Ranunculus acris).  Note the slightly whitish spots in the sinus' of the trifoliate creeping buttercup leaves (white arrow)  that can help distinguish it from bulbous buttercup when  no flowers are visible.

 Summary
Bulbous buttercup Ranunculus bulbosus - Ridged stem, Tri-lobed leaves and reflexed sepals
Meadow buttercup Ranunculus acris       - Smooth stem, deeply cut 5 lobed leaves 'normal' sepals
Creeping buttercup Ranunculus repens   - Ridged stem, tri-lobed leaves 'normal' sepals

Glossy yellow petals

Glossy petals of a meadow buttercup

A feature of buttercups, that helps to distinguish them from other plants with small yellow flowers, is the glossy and reflective appearance of their petals. This is unusual in the plant world. It is not even found in all members of the Ranunculaceae, the family to which buttercups (genus Ranunculus) belongs. For example, marsh marigold (Caltha palustris) which will be discussed in a future blog has showy yellow flowers like buttercups that lack the glossy appearance. On the other hand,  lesser celandine (Ficaria verna) another member of the Ranunculaceae, does have the same glossy petals. Ranunculaceae is a phylogenetically old lineage that split off from the main pathway leading to other plant families a long time ago. They have been 'going their own' way since and many, but not all members of the family, have retained 'primitive' features such as radial symmetry, unfused petals, and multiple unfused male and female sex parts. It is thought that the ancestral state of this family was to have glossy or reflective petals, but somewhere along the evolutionary line glossiness was lost in some genera but not others.
Does the glossiness serve any purpose? We can only speculate but it seems likely that flowers comprised of reflective petals would be more visible to potential insect pollinators who may perceive a flash when flying overhead. Another purpose may be to increase the temperature of the flower parts to speed up the maturation of pollen and seeds. And furthermore, make the flower a more 'welcome' and warm place for pollinators. The petals function as a parabolic reflector that reflects light towards the centre of the flower. As the heliotropic flowers follow the sun across the sky it is easy to imagine how this might heat the central parts of the flower.
The structural mechanisms that give rise to the glossy reflective petals are outside the scope of this blog. If you want to know more a good place to start is a paper published by the Royal Society Functional Optics of Glossy Buttercup Flowers.

Light reflected onto the centre of the flower


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