Triggerplants
When prowling the forest in search of insects this month, the pink-flowering triggerplants have been near the top of my checklist. Small nectar-seeking bees are regular visitors – and they are surprisingly tolerant of the toll demanded by the plant! I decided to take a closer look at the workings of these fast-moving little flowers.
Flowering spikes of two adjacent triggerplants (Stylidium graminifolium)… with bee.
The Grass Triggerplant (Stylidium graminifolium) is a well-known Australian endemic. It grows across a wide region, from Queensland to South Australia and Tasmania. The species favours low nutrient soils and is common in dry sclerophyll forest … like ours here.
Stylidium graminifolium is a perennial species, but easily overlooked for most of the year as the leaves form a low tuft that looks rather like grass. The flowers appear in Spring and Summer, each spike blooming for several weeks.
The species is popular among gardeners of native plants. It’s pretty, hardy and grows well from seed. In addition, the trigger mechanism and sticky hairs appeal to collectors of carnivorous species.
Try a web search for the species name – half the hits are for online seed sales!
Stylidium graminifolium, October 2020. Each plant produces a single spike bearing numerous flowers.
The triggerplants have come back well, post fire. Scattered clusters and some isolated plants, more than we see in a ‘normal’ Spring. Some may be from seed, others from the underground bulbs of previous years.
A floral trap for bees
Compared to the grasses Paul has been working with, Stylidium flowers are almost conventional. At least they have recognisable petals! Beyond that, however, things do get a bit tricky.
Stylidium graminifolium flower, with column in set position.
The stamens and style are fused to form a strap-like column. Two, bilobed anthers flank the stigma at the tip of the column.
There are five petals, but only four are obvious. The fifth is small and thickened with lateral processes – the labellum.
The petals are fused basally to form a floral tube. Around the throat of the tube is a series of erect appendages. These may serve as pollinator guides or perhaps to block access to non-pollinating insects (Ref. 1).
The role of the silvery filaments beneath the anthers remains a mystery. To me, anyway.
The trap is set when the column is bent back against the labellum. The lightest touch applied to the base of the column causes it to flip over. The angle of the bend is reversed. The labellum plays no role in the activation.
The trigger response is best demonstrated by a bee …
The structure and movement of the column has been well studied (Ref. 2,3). When triggered, the cells at the bend suddenly lose pressure and the head of the column is rapidly swung toward the posterior petals. It takes just 10-30 milliseconds (1/100 to 3/100 of a second) … no wonder I’ve not managed a photo, mid swing!
Each flower can be triggered repeatedly, but there is a latency period (Ref. 3). After firing, the column can take several minutes to return to the set position. Even when set and again sensitive to touch, it takes about an hour before it can fire effectively. Bees can visit such flowers, probe and depart without the slap. A bit of a lottery for the bee.
Bees such as Exoneura and Lasioglossum are the recognised pollinators of Stylidium graminifolium (Ref. 5). Indeed, Exoneura have been the most common visitors to our triggerplants this month.
I see them land, get ‘caught’, wriggle free, and then move to another flower.
They do not seem at all bothered by the slap on the back. Sometimes they remain on the flower for a few seconds, grooming. They then fly to a nearby flower higher on the same plant or to a Stylidium plant nearby.
Anthers first, then the stigma
In order to reduce the chance of self-pollination, many plants separate the time of pollen release from the time of stigma receptivity. In Stylidium, the anthers open first. At that time the stigma is concealed and not receptive. Later, when the anthers are largely empty and desiccated, the stigma emerges and swells.
This excellent table by Willis & Ash (1990, Ref.5) describes the stages and their relative timing.
Note: the gynostemium is the column - the fused style and stamens.
Extract from Willis & Ash (1990, Ref.5), p.220.
The stages are easily recognised in the high magnification images below.
The flowers in the panel above were all on the same spike, and the photos were taken at the same time.
Bottom to top
The flowers on a spike develop and open sequentially from bottom to top. Therefore a single plant can be opening buds, shedding pollen, presenting receptive stigmata, and forming seeds … all at the same time.
Floral developmental is acropetal … upwards from the base.
So how does the plant avoid self-fertilisation?
The clue is in the behaviour of the pollinating bees. When moving from flower to flower, they move upwards!
Why? I really have no idea.
But it works well for the plant. Consider the sequence of events …
Visiting bees always seem to arrive carrying pollen on their backs, apparently collected when feeding on Stylidium elsewhere. If the first flowers they visit are at stage 3 or 4, and if the trigger is ready to strike, job done!
Then, as the bee moves up the plant, the flowers will be at earlier stages. It will collect pollen from Stage 2 flowers and carry it away to other plants.
Evolution … a masterclass in design!
Not every bee is a pollinator
On several occasions I’ve seen shiny black male masked bees (Hylaeus sp.) perching on flowers, but they never get slapped by the plant. Now I know why! It’s nothing to do with their small size or light weight. It’s that they don’t feed, so they don’t contact the touch-sensitive part of the column inside the floral tube.
So what are they doing? I suspect they’re on the lookout for nectar-seeking females. They certainly seem territorial, challenging other bees and bossily posturing.
I’ve yet to see a female masked bees visit the triggerplants, but I’ll keep looking.
Finally, are triggerplants carnivorous?
It is tempting to think that Stylidium might be carnivorous. The flowering stem is covered in sticky hairs that secrete protein-digesting enzymes. They typically grow in nutrient-poor soil, like the carnivorous Sundews (Drosera). However, recent research suggests that trapped insects do not make a significant contribution to the nitrogen needs of Stylidium (Ref. 4). Perhaps the hairs simply serve as defence, deterring hungry caterpillars and sap-sucking bugs.
References
Carolin, R.C. Stylidiaceae, 2007. pp 614-619, in Kadereit, J.W & Jeffrey, C. (Eds), Flowering Plants : Eudicots : Asterales. Springer-Verlag, Berlin, Heidelberg.
Findlay, G.P. & Findlay, N. 1975. Anatomy and movement of the column in Stylidium. Australian Journal of Plant Physiology, 2, 597-621.
Findlay, N. & Findlay, G.P. 1989. The structure of the column in Stylidium. Australian Journal of Botany, 37, 81-101.
Nge, F.J. & Lambers, H. 2018. Reassessing protocarnivory – how hungry are triggerplants? Australian Journal of Botany, 66. 325-330.
Willis, A.J. & Ash, J.E. 1990. The breeding systems of Stylidium graminifolium and S. productum (Stylidiaceae). Australian Journal of Botany, 38, 217-27.