Plant Story - More Reasons to Like Primroses, Primula

primrose, Primula

Primroses, genus Primula, are well-known European spring garden flowers. And wildflowers, since the common English name cowslips is probably from cowslops, --the common primrose, Primula veris, grew (and grows) well in English cow pastures. The English liked them as a sign of spring. (Link to previous post).

Which is probably why Charles Darwin and botanists before and after him, took a close look at the flower and discovered that there were two kinds of flower. Not two species, but two different flowers within a primrose species.


One had an obvious dot in the center as you looked in (red flower above), the stigma, onto which pollen was deposited. The other did not, in fact it had a ring of anthers at the top as you looked into the flower. Somebody called the two types "pin" and "thrum" respectively.

primrose pin and thrum flowers

When I write "two kinds of flower in the species" it sounds exotic. But we have "two kinds of people within humans", as a plant might describe us--male and female. 

In humans and other mammals, and birds, there are two sexes, one of which specializes in carrying the developing young. One consequence of this system is that it requires outbreeding--two different parents for the young. It sounds weird but there are any number of ways that a single individual can reproduce without a mate. 

However, species that are outbreeding are so common in the natural world that evolutionary biologists concluded that it must be advantageous over the long run. Presumably, those families with genetic diversity among their descendants are much more likely to survive changes in their environment than uniform families. 

Most plants can be both mother and father to their offspring. They make pollen which contains sperm and they have eggs which develop into seeds. Because of that, selfing--pollinating one's own eggs--is very possible. Some species self all the time, but it is a minority breeding system in plants. 

Plants have various mechanisms to prevent selfing. In some the pollen is released from a particular flower a day or so before the eggs are receptive to pollination, in others there are complex biochemical barriers to self-fertilization. Primroses represent a third approach.

The two flowers in the diagram above cross with each other when pollinated by a bee, fly or butterfly, but not with themselves or with flowers of the same structure. Pin crossing with thrum and thrum with pin works easily, pin with pin and thrum with thrum does not.

Primula eliator, pin

Primula eliator, thrum

The bee or butterfly, reaching into the pin flower to drink nectar at the bottom of the flower, reaches past the stigma and gets pollen on its face or at the tip of its tongue, depending on the pollinator. When it flies to a thrum flower, it gets pollen part way up its body as it is reaching down to get more nectar and will likely brush pollen off onto the stigma (not shown well in my picture) way down inside the floral tube. Conversely, pollen taken from the anthers of the thrum flower will go easily onto the stigma of a pin flower but not easily onto the stigma of other thrums. 

primrose, Primula

The pin/thrum system is another reason I like primroses. I was delighted by the concept. Animals, including humans, must find a mate of the opposite sex to produce offspring, but there is a complicated division of labor between males and females. Primroses built in the part about needing to find a mate of the "other type" to produce offspring, but didn't go for the division of labor. Every primrose can produce seeds, if its eggs are pollinated. So two features that I had always thought of as linked, are not in primroses.

This system, is called heterostyly because the styles--the stigma sits at the end of the style--of pin and thrum are in different positions.  (Or, specifically, distyly because there are plants with three style morphs).

And, I'm always thrilled when I see something from the literature for myself. Checking my newly flowering primroses was a treat! Pins and thrums!  YES!

Try it! Look for yourself at primroses to see the pin and thrum structure. Looking in from the top (see pictures below) should show you the differences. To see where the anthers are on a pin or the stigma on a thrum, gently slit the flower open (photo with labels, above). Of my three Primula elicitor plants, two are pin and one is thrum. Likewise two of my three of my Primula  hybrids plants are pin and one is thrum. 

primrose, Primula hybrid, pin

primrose, Primula hybrid, thrum

Other plants you might check out for heterostyly include Linum (flax), Lythrum (purple loosestrife), and Plumbago (plumbago). If you are in the tropics, check out Ixora (West Indian jasmine) and Turnera. For a list see Fred Ganders' paper link .

Primroses are small compact plants with showy flowers in the spring and a unusual system for ensuring crossing between different individuals. It is always a treat to see a character from the textbooks for myself. And, this year, I attained an ambition to grow them. Nice plants!

Comments and corrections welcome.

Footnote: Google is forever trying to fix my botanical words. It corrected primus to primes, vulgaris to vulgarism and distyly to distally. I think I caught them all.

References

Ganders, F. R. 1979. The biology of heterostyly. New Zealand Journal of Botany. 17: 607-35. paper online

What is Heterostyly?  Youtube link Video that describes heterostyly (I'd pronounce the y as "i" not "e") The graphics showing how the pollination system works are delightful

Kathy Keeler

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