The curious incident of the primrose that wanted to be a polyanthus ...

What's the difference between a primrose and a polyanthus? They're both part of the primula family, but I'd always thought that primroses had one flower per stalk, while polyanthuses had long stalks with a cluster of flowers on top. And yet year after year, I've found that after a few weeks of producing compact, round heads of single blooms, some of the plants I'd thought to be primroses would suddenly schizophrenically shoot out a long stem with numerous flowers, breaking the symmetry of the plant.

And lo and behold when I went out on the balcony today to check if anything needed watering ....

Clearly a primrose with ambitions.

Or is it? Did I buy polyanthus thinking they were primroses? In which case, why were the original flowers single stemmed?

Browsing the web I came across this explanation on www.aboutflowers.org :

It happens, however, that primroses are produced in clusters, as polyanthuses are, but they appear to be produced singly, because the stem that carries the cluster is very short, and the secondary stem, or peduncle that carries the flower, is very long. Now and then a common primrose determines to explain the case to the young botanist, and then we see a stout stem bearing on its summit a cluster of primroses.

Leaving aside the fact that no way do I come under the heading of a "young botanist", the rest seems accurate. The offending stem is definitely stout, and peering down into the stems at the bottom of the plant, well yes - they do seem to come from a cluster. It's difficult to see though without harming the plant.

It's not clear who wrote the article, nor how authoritative it is, and I can't find the same explanation anywhere else. But then I can't find any explanation anywhere else. Does anyone know?