FEATURE
Francis tied up awns into a bundle and observed
that they twisted around each other
like a rope when hydrated, proving a ‘schema’
for overall helical movement. Drawing
on my childhood experience of building
self-propelled elastic cotton reel cars, I too
made a model to satisfy myself that I (partially)
know how awns work: for live action
watch https://youtu.be/Ua63OPVzOzw -
Version 2.0 will be larger and faster and be
activated by the slightest change in atmospheric
humidity.
awns of Themeda
o its constituent cells
the microscope the
ual cells was beautictually
seen a single
cope untwisting and
y hand approached
from its neighbourfied
bilayers of differnt
cells, the oblique
f striae in the cell
unequal contraction,
f the awn could also
n, revealing the celuctural
arrangement
and enables overall
hese mechanisms as
rostructure was cont
expensive imaging
0 years later (Elbaum
Not having Darwin’s
city, nor access to an
uring the COVID-19
ed to do was to rerred
(but maybe the
taken with my cellreomicroscope
that
a do have a distinct
hygroscopic region
eep groove contribhaps
by channeling
ntraction joint allowkage,
remains to be
derstand something
rking model of it. Sir
What is the ecological value of having
long, speedy awns? Well, the Australians
proposed that the longer awns that T. triandra
plants have in the hot, dry interior
of their country compared to the shortawned
plants on the wetter coast could be
an adaptation to enable seed to be quickly
transported far across the soil surface to
find a germination microsite before the
rain dries up (Godfree et al. 2017). I tested
whether their prediction that awn length
should increase with decreasing rainfall
(i.e., increasing aridity) by examining how
mean awn length changed with mean annual
rainfall across 16 sites in the midlands,
interior and uplands of KwaZulu-Natal.
The result was another surprise. Awns
tended to be shortest, though quite variable,
in the driest areas and were consistently
long in the wettest Drakensberg sites
(for full results see the preprint at: https://t.
co/hQamBqo4Sx). Perhaps poor rainfall restricted
awn development in some of my
driest sites or a long awn is best for transporting
a seed through the dense maze of
tufts in mesic grassland to find a good spot
to germinate away from competitive established
tufts?
Further research is also required to establish
what determines awn length as well as
other possible roles that awns could play,
such as twisting the seed out of inflorescences
or when trapped in the canopy (as
Darwin speculated), or to perhaps make
it difficult for predators to drag the nutritious
seed away. Watch this space for some
more awnsome findings.
Acknowledgements
This research would not have been possible
without the enthusiastic assistance
provided by Janet Taylor and Debbie Jewitt
who kindly organised the collection of
awns (and Colin Everson and Kevin Kirkman
who also provided samples) as well as
Anita Morris who helped with the experiments
and tolerated me droning on about
the wonders and mysteries of awns for a
few months.
c awn of Themeda triandra. Journal of the Grassland Society of
in seeds are enabled to bury themselves in the ground. Transac-
149-167.
of hygroscopic movement in plant seed dispersal. Plant Science
017. Empirical evidence of fixed and homeostatic patterns of
and heat stress. Royal Society Open Science 4: 170934.
04