Photo by twiddleblat licensed under CC BY-SA 2.0
There is a common morphological thread among herbaceous plants growing in the colder regions of the world. Most grow small and take on a cushion-like habit. For these species, it is all about getting sensitive tissues out of the chilling winds and into an insulated microclimate. This convergent morphology seems to have been entirely lost on a cohort of plants native to the sub-Antarctic islands of New Zealand. The aptly named "megaherbs" are characterized by their large size an the often gaudy dark coloration of their blooms. Why would an entire guild of plants growing in such cold, dreary, harsh conditions converge on a strategy that, for most plants of their size, spell certain death?
The answer to this mystery is heat. In such a harsh environment any advantage, no matter how slight, can make a huge difference. What's more, whereas smaller neighboring species largely reproduce asexually, these bizarre behemoths seem to have sexual reproduction all to themselves. The key lies in their large size and extravagant coloration. A team of researchers looking at six different species of megaherb found that the thick, hairy leaves and dark colored flowers were able to take advantage of the rare occasions when the sun poked through the thick, grey, sub-Antarctic clouds.
Photo by twiddleblat licensed under CC BY-SA 2.0
On average, leaf and inflorescence temperatures of these megaherbs were significantly higher than the ambient conditions. For instance, in the Campbell Island daisy (Pleurophyllum speciosum), leaf and flower temperatures were consistently 9 and 11 degrees Celsius warmer than their surroundings during periods of sunshine. Because of their large size (think surface area to volume ratio), they were able to hold on to this heat much longer than smaller plant species in the same habitat. In essence, they are creating a glasshouse effect.
This means more than just a warm microclimate for these plants. Insects in this environment seek out these plants for warmth and shelter. In a region with such a sparse insect community, concentrating pollinators in and around your leaves means a higher chance of pollination, a win-win for both sides. As if this wasn't enough, higher temperatures can also facilitate seed production, adding yet another layer of benefit to growing large and darkly colored.
Further Reading: [1]