Plant Cuttings

Flowers (it’s what angiosperms are all about!)

Although one shouldn’t, it is easy to accept that flowers (the defining feature of the angiosperms, the flowering plants) are ‘just there’ and get on with life in their quiet, seemingly unremarkable way. If one subscribed to that view, hardly any study of floral biology would be carried out, and we’d miss a lot of really interesting stuff. To demonstrate just what we might have been missing, this item showcases several insights into aspects of the biology of flowers that have surfaced so far in 2018.

A poster with twelve flowers of different families.
A poster with twelve flowers of different families. Image: Alvesgaspar / Wikipedia

The intimate association between flowers and their pollinators provides many examples of ways in which flowers are adapted to specific pollinating organisms. In a study of field bean (Vicia faba, faba bean), Emily Bailes et al. investigated – among other aspects – the ‘operative strength’ of a flower. The operative strength is equivalent to the force a pollinator needs to exert to ‘trip’ a flower so that it can gain access to the pollen-containing interior. For the bean lines studied it ranged between 17.1 and 20.1 mN. Although those values might not mean all that much to the uninitiated it does imply that only certain insects will be powerful enough to open the flowers and therefore act as pollinators of this species. So, while this flower-accessing feat should be easily achieved by bees such as Bombus spp. (bumble-bees) – which can exert over 200 mN force – it might prove problematic for weaker individuals of Apis mellifera (the honey bee) which can only generate approx. 26 mN of force, and other smaller – and less powerful – bee species. This analysis therefore introduces another factor to bee (yes, ‘typo’ intended…) considered specifically in breeding field bean lines and varieties to suit available pollinators to maximise crop yield. It also has relevance more generally for other crops where insects must physically open the flowers to participate in pollinating activity.

But, having allowed a suitable pollinator to access the flower, what’s the best way to ensure the visitor gets coated with pollen, the better to pollinate the next flower it visits? Callin Switzer et al. examined this phenomenon in mountain laurel (Kalmia latifolia). This plant releases pollen in an explosive fashion when the anthers are triggered by appropriate insects. Although the pollen moves at only approx. 8 mph, its acceleration to achieve that is 400 times the acceleration due to gravity (!). Importantly, pollen-release appears only to be activated by insects such as bumble and honey bees, which are able to effectively transfer that pollen to other flowers. Combined with other aspects of the investigation, this study appears to settle the question of whether this pollen-release mechanism is for insect pollination (yes) or wind-dispersal of the pollen (apparently not).

Once flowers have been pollinated, and fertilised, and seed formed, there’s the issue of how to jettison the seed so it lands sufficiently far away from the parent to have a chance of establishing itself as a new individual. To achieve this feat, the wild petunia (Ruellia ciliatiflora) also employs an explosive release mechanism to launch its seeds at velocities exceeding 30 mph, and which land up to 7 m away from the parent plant. But there’s more to this phenomenon than that, as Eric Cooper et al. have revealed. In particular, using high-speed video of the seeds’ flight, they show that the seeds spin at 1600 revolutions a second. This ‘backspin’ stabilises the flight of the seeds in such a way that it reduces the energy costs for their dispersal by up to a factor of five. Not only that, but the spinning reduces drag enabling the seeds to travel further from the parent plant than if they didn’t spin.

Finally, we must appreciate that flowers are such a precious and all-important part of the angiosperm’s life-cycle that they need to be protected from those organisms that would eat them. We began this item with flower opening; we come full circle now and end with an example of floral closure (and, coincidentally, a third example of rapid movement in a plant related to floral biology). Investigating Drosera tokaiensis – a sundew, which group of insectivorous plants is probably better-known for their mechanically-stimulated tentacles and leaves whose movements help to trap and wrap insect preyKazuki Tagawa et al. report that its petals close rapidly in response to mechanical stimulation. Petal closure was recognised and brought about artificially – by humans touching the flower with a pair of tweezers. However, the authors speculate that this phenomenon may function in nature as a defence against specialist florivores (organisms that consume flowers prior to seed coat formation) that would eat the flowers rather than play a role in their pollination. Whether any such specialist sundew flower-eating organisms exist was not mentioned in the paper, but this remains an intriguing hypothesis that is ripe for testing. It could be further speculated, that sudden petal closure might startle or dislodge the florivore so that it falls off the flower on to the equally mechano-sensitive insect-trapping leaves of the plant and ends up as lunch itself; the would-be plant-predator plummets to be predated by the plant. Flowers, much more than meets the eye.

[Ed. – Mindful that there might be ‘communications’ complaining that what’s described above doesn’t reflect any single flower that’s known to exist in nature, we would like to emphasise that the above account is not based on a flower of any known single flowering plant species, but is a compendium of insights into aspects of floral biology from several different species (as indicated by the different taxa specified).]