Elucidating the mechanism of twisting action in black speargrass awns

Below is a report about my observations of how black speargrass (Heteropogon contortus) twists when wet. It is an intriguing behaviour, and I don’t believe there are many scientific papers on the topic.

 

Elucidating the mechanism of twisting action in black speargrass awns

By Joel Johnson

ABSTRACT

Black speargrass awns were found to increase in length by an average of 2.2 ± 0.9 mm, but not in diameter. The awns consisted of a number of thin strands, spiralling around a hollow centre. The angle that the strands made with the length of the awn decreased significantly, from 16.4° to 1.8°. These findings support the hypothesis that the mechanism of the twisting action is the ‘untwisting’ of the awn.

INTRODUCTION

The native grass Heteropogon contortus (black speargrass) covers approximately 29 million hectares of land in Queensland (1), and supports 3.5 million head, or around one third of the total cattle population (2). A large amount of its dominance is due to its successful strategies for dispersion and germination. It is well known among farmers that the barbed awns are “hygroscopic and twist when wet”; this behaviour is thought to help screw the seeds into the ground (1). Here, the twisting behaviour of black speargrass is examined, with the intention of determining the mechanism responsible.

METHOD

A single clump of black speargrass was collected from the Central Queensland property “Pennyroyal”. Individual awns, including the seed heads, from this cluster were measured before and after immersion in water for 2 minutes. After this period of time, virtually all movement of the awn had ceased. Length was measured with a ruler (extrapolated to the nearest 0.1 mm) and diameter was measured with a micrometer.

RESULTS

The results are summarised in the table below.

  Mean before (±SD) Mean after (±SD) % change
Length (n = 10) 83.5 ±12.4 mm 85.9 ±13.1 mm +2.8 ±0.8%
Diameter (n = 5) 0.34 ±0.05 mm 0.34 ±0.04 mm +0.3 ±10.0%
Strand angle (n = 2) 16° 1.8° -88.6%
Strand period (n = 1) 3.3 mm 15.7 mm +381.6%

The speargrass awns were an average of 83.5 mm long and had a diameter of 0.34 mm near the seed head. After 2 minutes in water, the awns had not increased in diameter, however they had increased an average of 2.4 mm (2.8%).

The awns all rotated counter-clockwise viewed from the tail, for an average of 9.6 ± 2.2 rotations (n = 12).

ANALYSIS

As there was no increase in diameter, it is likely that the awns do not swell significantly, or that if they do swell, then some of their constituent matter is distributed lengthways. Observations under a microscope revealed that the half of the awn ‘tail’ nearest the seed head consists of spiralling strands (see Figure 1), with the rest consisting of straight strands.

It was hypothesised that the twisting of the awn corresponded to the ‘untwisting’ of the awns, and that the awns consisted of a solid core surrounded by thin, hydroscopic fibres. This would explain the lengthening of the awns.

Figure 1 – A speargrass awn

Awn 11 - before (1).JPG

Figure 2 – The same speargrass awn after immersion in water

awn-11-after

Further observations revealed that the awn is hollow, consisting of somewhere between 30 and 50 individual strands, each approximately 30 µm in diameter. The strands formed an angle of 16.4° with the length of the awn, on average (n = 3). After immersion in water for 2 minutes, the angle was reduced to an average of 1.8° (n = 2). In addition, the period of the strands (the distance for 1 complete revolution of a strand) increased by 382%, from 3.3 to 15.7 mm (n = 1). These data, along with the increase in length, confirm that the spiralling action is due to the ‘untwisting’ of the awn.

Further trials showed that neither the head nor the tail end of the awn increase significantly in length; only the section of the awn nearest the seed head lengthened, by 3.4 ± 0.7 mm (n = 2). Therefore it appears that the entire twisting action, and therefore the lengthening of the awn, comes from the half of the awn closest to the seed (approximately 30 to 40 mm in length).

Assuming that the spiralled strands entirely untwisted, calculations using the average diameter and strand angle suggest that the twisted section could increase in length by 4.2%. This is fairly close to, though slightly higher than the 2.8 ± 0.8 mm observed in this experiment. The difference is likely due to the fact that not all of the awn is twisted; only around half of it untwists. However, if this is the case, then the lengthening observed is probably more than is theoretically possible. Therefore, the strands may also swell, and increase in length that way as well. Further work is required to determine if this is the case.

There is much room for further research in this area, including determining how successful this twisting action is in embedding speargrass seeds into the ground, and the factors that lead to the formation of clumps of awns. Also not covered in this work was what type of hygroscopic matter the strands were made up of, and how they actually change shape when they absorb water.

CONCLUSION

Microscopic observations and the data from this experiment support the hypothesis proposed for the twisting mechanism of black speargrass awns, namely, the untwisting of the hollow awns.

References:

  1. Meltzer, R & Plumb, J 2007, Plants of Capricornia, Capricorn Conservation Council, Rockhampton, Qld.
  2. Anderson, E 2003, Plants of central Queensland, Department of Primary Industries, Brisbane.

 

Some other graphs of my results are below. A PowerPoint of my report is also available, and it contains some information about the mathematics of how much the awns should theoretically increase in length.

speargrass-3speargrass-5speargrass-6speargrass-2speargrass-4speargrass

Some interesting patterns. It makes me want to do some more experimenting, anyway!

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