Jumat, 12 Juli 2013

To self or not to self - the story of Drosera capillaris

What does it mean for a plant to "self?".  The expression is short for self-pollination. It's when the pollen of a flower lands on its own stigma or the stigma of a flower on the same plant, resulting in fertile seeds.  It's a feat only plants can pull off as they typically bear both male and female organs on the same flower. However it is not generally recommended even for them.  Most plants take measures to avoid self-pollination.  In some, the stigma is not receptive when pollen from the same flower is released. In others, the stigma recognizes its own pollen and rejects it. There are dozens of different ways to prevent selfing. Why? Because in general, populations that get into that habit become genetically, weak and eventually die out - they are evolutionary dead ends.

There are some times when it pays off , however. It is a way, similar to vegetative propagation, of quickly increasing the numbers of a population, and is especially useful in very transient habitats.

The carnivorous Drosera capillaris is seldom more
than an inch in diameter, and lives only in moist,
acidic, sandy soil.
Enter Drosera capillaris, a tiny carnivorous sundew common in Florida. Their sticky leaves trap tiny insects, supplementing their supply of nitrogen and other minerals in wet and nutrient-poor soils. Almost 200 species of this genus exist, mostly in Australia, southern Africa, and South America, but we have 5 species native to Florida.

These tiny plants appear by the millions in damp soil around swamps and freshwater marshes, seemingly out of nowhere.   Evidently the tiny seeds wait out dry periods in the sandy soil, sprouting when moisture returns.  I have even seen progressions of seedlings appear as the shoreline of a small pond receded over several months of a dry spell.

A few years ago, I became curious about how Drosera capillaris reproduced.  Lifespan can be pretty short for the moisture-loving plants as wet meadows come and go and shorelines recede.  They plants pretty much die when the soil dries out.  They can, however bloom within a couple of months of germination, while only about half an inch in diameter. In more stable moist areas, they can live for several years and get to be around 1.5 inches in diameter.

The flowers are small and white or pinkish, most commonly appearing in early spring.  A successfully pollinated flower can produce dozens of tiny seeds.  Because of the thousands of seedlings that can show up when the soil moistens, I assumed that pollination, presumably by some small bees or flies, was pretty common.

The flowers of Drosera capillaris, lifted well above
the sticky traps, stay open only for
a couple of hours, and only if the sun is shining.
I convinced a few of my botany students to spend a few hours in a large population of sundews during their blooming period in the USF Ecology Area.  Fortunately, it is relatively short.  Flowers typically open up in mid-morning, but only if the sun is shining, and close up by noon.  After several days of observation, one student caught a single small bee that might have been pollinating a flower.  Other than that - nothing.  We saw very few insects come near the flowers, and that has been my experience whenever finding these plants in the field.  So where do all the seeds come from to constantly regenerate the populations of sundews?

An accidental experiment suggested the answer.  I had started some sundews from seed in a pot, and eventually I had a single flower on one of them.  This was miles from any other sundews.  The flower closed up around noon and I figured that was the end of it.  A few weeks later, however, I noticed that a seed pod had formed, and eventually I had a crop of seeds from this single isolated flower.

That seemed to confirm my growing suspicion that these little plants self-pollinated.  Looking again at the flowers, it was clear that as the flowers closed, the stigmas and stamens would be pressed together, resulting in pollination.  It has not been verified by rigorous experiment, but it seems that this is the secret to the great fertility of these small plants and their success at inhabiting a fleeting habitat. Every so often, perhaps, an actual cross-pollination might occur, mixing different genotypes and boosting the genetic diversity of the population.




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