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Standing out in “blue” - Common Day-flower

Aug 2022
Author : Green Power

Statistics tell us that blue flowers account for only 10% of all flowers found in nature. Common Day-flower (Commelina communis) is one such special plant; and—albeit its flowers are tiny—easily catches the eyes with true blue blossoms!

Common Day-flower is an annual herbaceous plant that favours moist environments such as wetlands. The plant can grow up to a metre long, but often appears short as the lower part of its stem sprawls along the ground.

Common Day-flower is distinguished by the blue flowers. Each flower is made up of three petals; one is white and the other two, larger ones, are blue. The colours show up because of the pigments in the petals, which can be broadly classified into carotenes and anthocyanins. The former are red, orange and yellow. The latter appear red, purple or blue according to the acidity/alkalinity. In alkaline conditions, the colour blue shows up.

So, why is that there are few blue flowers? This is because anthocyanins are unstable. Furthermore, when a plant photosynthesises, the blue-purple light from the sun is absorbed and transformed into chemical energy required by the plant. During the process, blue light is largely absorbed and will not be reflected.

Although blue flowers are rare, Common Day-flower, as its name suggests, is actually a common plant and its flowers were widely used as blue dye. However, because the anthocyanins are water soluble, the dye on clothing does not last long. Blue-and-white paper, on the other hand, is more enduring.

The “grass stamped on by ducks”

Interestingly, the flowers of Common Day-flower have no nectar. They produce only a small amount of pollen. Effective reproduction relies on the unique structure of the stamens. Each flower contains six stamens, standing in three rows. The row closest to the petals has three stamens, with bright yellow anthers on the tips that call for attention! The three stamens have very little pollen on them, but it’s sufficient to attract hoverflies and honeybees. The only stamens in the second row have much more pollen to keep the insects in. Meanwhile, the other stamens in the third row—which are longer and point forward—can easily attach pollen onto the backs of the feeding insects. What a calculated way of pollen spreading!

Another characteristics of Common Day-flower is its brief blossoming period. Sometimes, the flowers bloom in the morning and wither by noon. If no insect comes along before the flower fades away, its stamens and pistils will carry out self-pollination by turning inwards. Propagation matters!

The English name Day-Flower vividly describes its daytime flowering behaviour. As for its Chinese name, the origin is more ambivalent. One story is that plants growing in the wetlands are often trodden on by ducks. Hence the name translating as “duck-stamped-grass”.

Flower of Common Day-flower
© Peggy Chung
The six stamens are divided into three groups, each fulfilling different functions.
© Peggy Chung
Without the eye-catching blue flowers, Common Day-flower is no different to other weeds.

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