Monday, 1 September 2014

Anyone for a cuppa?

A bud about to burst open

Cobaea scandens is a vigorous climber from the tropical forests of Mexico. It climbs using hooked tendrils which spin round in spiral movements to grip onto supports. In its natural environment it grows as a perennial, but in the temperate zone it is usually grown as an annual, where it will put on a lot of growth in a short period of time, and flower at the end of the summer.

Each flower is borne singly, on outstretched stems, held above the foliage. They start out as a green, flanged, pod-like calyx which slowly opens up to reveal the closed petals which resemble an egg, and are whitish green. Once the flower has opened it starts to look more bell-shaped, and with the calyx sitting underneath the bell, it looks like a cup and saucer, which is the common name for this plant. As the flower matures it gradually changes colour from white to dark purple, although it retains some white stripes which radiate out from the centre.

A creamy white, newly opened flower
Protruding from the back of the petals there are five stamens, each carrying a pair of pollen parcels, or anthers at their tips. These parcels need to open up to release the pollen that they hold, and once this has happened the anthers are pushed forwards and held up ready for a pollinator. The pollinator of choice for a Cobaea scandens is a bat. This is significant because it explains some the flowers' morphology, and why they open their petals during the night. The size and shape of the flower is perfect for a small nectar-eating bat to sup from, while brushing its belly against the stamens to extract the pollen, and then to deposit it neatly onto the style of another flower.

The flower begins to turn purple
One of the major advantages of bat pollination is that pollen can be transported across further distances, and in significant quantities, providing plants with a bigger genetic neighbourhood, while reducing genetic subdivision between populations. Another reason why bats are good pollinators is that they carry pollen from several different potential fathers to the same stigma, which increases the genetic variability of the population.

A mature flower
Virginia Vargo, Trainee Botanical Horticulturist.

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