Violets - Viola spp. - Violettes

I noticed recently that the violets are starting to bloom. There are quite a lot of Viola sororia (wood violets) in the park near our apartment.

V. sororia in bloom

V. sororia is a woodland perennial herb native to Canada and the US (range map here - note that although this range map indicates that the species is not native to these provinces, the Plants of Canada database includes Manitoba, Nova Scotia, and Newfoundland & Labrador in the plant's native range). I was not able to find any conservation information for the species in the US, but V. sororia is secure in Canadian parts of its range except in Saskatchewan and Newfoundland & Labrador, where is listed as 'may be at risk' [1].

V. sororia flower - note the hairs, which ensure the distribution of pollen over the back of visiting pollinators

I also spotted a reasonable number of Viola pubescens (downy yellow violet) on my wanderings, including a few unfortunate individuals that were growing on somebody's lawn and have since been mown into oblivion. This woodland perennial herb is native to eastern North America (range map here) and is listed as secure globally [2], but is a species of special concern in Rhode Island [3]. The species is secure in Canada except in Saskatchewan, where it may be at risk, and in Prince Edward Island, where it is sensitive [4]. V. pubescens' conservation status is unknown in Nunavut [4].

Viola pubescens

The type of flower in the above photos is chasmogamous, meaning that it has exposed reproductive parts which are available for sexual reproduction via some form of pollen vector, whether it be wind, animal, or other. V. sororia and V. pubescens, along with many other Viola spp., also produce cleistogamous flowers later in the season, which are the opposite: the flowers never expose the reproductive parts to any pollinator (they don't open), and simply self-pollinate.

V. pubescens chasmogamous flower

So why would any plant produce a structure for sexual reproduction (a flower), and then not use it to exchange genes with others? If one isn't exchanging genes, why not propagate vegetatively (clone)? And if one is producing flowers, why not exchange genes?

V. sororia chasmogamous flower

The answer to these questions is a bit complicated, but the essential idea is that, unlike with cloning, there is still some genetic variation occurring with self-fertilization, and that cleistogamy requires less energy than chasmogamy. So the can still preserve some of the advantage of sexual reproduction (genetic variation), while not putting in as much energy as with proper flowers, which require fully-developed petals, nectar, and lots of pollen. The flower can produce no nectar, no real petals, and very little pollen and still make seed. This is a useful adaptation particularly where resources are extremely scarce or conditions particularly hostile [5].

The primary disadvantage of such a reproductive strategy is of course that it restricts genetic variability enormously, which can be seriously damaging to the health of a population.

The V. sororia population I saw was quite extensive. It is rather unassuming, but quite pretty if you give it a chance:

V. sororia population in the park