First the technical bit: Heterostyly is a mechanism whereby a species produces flowers in two slightly different “types” or “morphs”.
This is designed to reduce pollen transfer between flowers of the same “morph” - either to prevent self-fertilisation, or to reduce the waste of pollen being delivered to incompatible flowers.
In Primulas, as in most flowers, the centre of the flower contains the stamens and pistils, which respectively disperse and receive the pollen. However, instead of always been in the same arrangement, Primula flowers have two "morphs" or “types” : flowers on some plants have long stamens and short pistils, while flowers on other plants have short stamens and long pistils.
In Primroses, the two morphs are called pins and thrums. Pins are where the pistil is long, looking like a large flat-headed pin – hence the name – and thrums are where the stamens are long, overtopping the pistil.
In the comparison picture below, if you look closely at the very centre of each flower, you can see that on the left-hand one, there is a bundle of anthers, and this is the "thrum". The word “thrum” , incidentally, comes from a weaving term for a fringe or tassel of short unwoven threads, occurring when the work has been removed from the loom.
This bundle of anthers start in the centre of the flower, looking like a tiny bunched-up fist: and then gradually open out.
Now the fascinating bit: in every naturally-evolving colony of wild primroses, there will be a roughly 50/50 split between plant whose flowers have pins, and those who have thrums.
So next time you are in the garden, or the garden centre, or out for a walk in the woods, go and have a close look at the Primroses: check out the centres of the flowers and see the pins and thrums for yourself, and make your own assessment of the 50/50 rule.
Mind you, like everything in Botany, the rules are not set in stone: this article is taken from one of my Field Guides, detailing how to identify Primrose, Cowslip and Oxlip, (because someone asked me the question, and was too tight to spend £2.67 on buying their own copy!!) and one person, who bought this Field Guide, wrote to tell me that, in parts of Somerset and the Chilterns, some Primula are "homostylous", which means they lack this differentiation altogether.
Thus, we live and learn.
Generally, though, you will find it to be true, and it certainly livens up a walk in the woodland!
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