Watching grass grow
As we enter Summer, the undergrowth has taken on a new look. The dazzling colours of Spring-flowering forbs have receded, to be replaced by a sea of rapidly growing grass stems.
While the grasses may lack the bling of the forbs, their flowering heads display a diverse array of interesting forms - from the extravagant Foxtail Speargrass to the weird Kangaroo Grass to the modest Reed Bent.
“To try and catch the wind” (apologies to Donovan)
The flower-bearing grass stems or culms tower over most of the vegetation in the undergrowth - even relatively tall forbs such as Tall Bluebells, seen here.
At 190cm tall, it’s not often that I find myself staring at a grass seed head when standing upright.
But that’s the case for the culms of one of our clumps of Redanther Wallaby Grass (Rytidosperma pallidum).
Redanther Wallaby Grass with scale bar
Why do grasses produce such tall flowering stems?
Grasses are pollinated by the wind, rather than insects. To catch the wind it helps to be taller than the rest of the undergrowth.
Downs and Krizek (ref. 1) report that an air flow speed of 2.5m/s measured at two metres above a bean crop drops to 0.9m/s at the top of the crop and less than 0.25m/s near the middle of the crop.
Watch the culms of several different grass species using the wind in this video.
The exaggerated height of the flowering stem of grasses contrasts with the modest height of their non-flowering, vegetative stems, which bear only leaves. These are often much lower than those of other plants, which is partly why grasses seem to ‘shoot up out of nowhere’ when they flower.
The low position of the basal meristem of grasses enables them to keep growing as their leaves are cropped by herbivores (and lawn-mowers).
The grass flower - evolution goes to town
The fact that grasses rely on wind for pollination also accounts for their somewhat drab, but very unusual flowers.
To appreciate the special character of the grass flower, it’s helpful to revise the structure of a 'normal’ flower. I’ll use the Heath Teatree Leptospermum myrsinoides for this purpose.
The green cup in the middle of the flower is filled with nectar, to attract insect pollinators.
There is a whorl of five petals and a separate whorl of five triangular sepals, which sit between and beneath the petals.
Like most plants, the teatree has bisexual flowers, with both male and female reproductive structures. The male sex cells, the pollen, are housed in the anthers, the small brown dots sitting on the end of a white stalk, the filament. The filament/anther combination is called a stamen.
The female sex cells, the ovules are housed in the ovary, which is hidden beneath the green cup in the teatree flower. A single green rod, the style, projects vertically from the ovary and ends in an enlarged round cap, the stigma.
Pollen cells land on the sticky stigma - the act of pollination - and form a long tube, which grows down the style. This tube fuses with an ovule and fertilisation, the meeting of the male and female nuclei ensues. This marks the beginning of a new teatree plant.
So how does the grass flower differ?
I’ll use the flower of the Redanther Wallaby Grass (Rytidosperma pallidum), one of our forest grasses, for this comparison.
The flowering head or inflorescence of this grass is shown in photos A and B. Each of the green/purple, spike-shaped objects, is a spikelet. One of these are shown at greater magnification in photo C. The eponymous red anthers can be seen bulging out of the spikelet. Rather attractive. Who said grass flowers are drab?
Let’s unpack a spikelet
The two green/purple leaf-like structures seen at the base of the spikelet in photo C are the glumes. Enclosed within the glumes are four flowers (also called florets).
I’ve dissected out the flowers from a young spikelet and laid them side by side above the glumes in photo D. These are all bisexual (i.e. have both male and female parts).
Some grass species have just one flower per spikelet, while others have two or more.
Different grasses may have all bisexual flowers or a mixture of bisexual, unisexual and neuter flowers.
As you can see, grass flowers are totally different in appearance to ‘normal’ flowers like the teatree, lacking petals and a nectar-filled cup. As they employ wind pollination, grasses don’t need to attract insects with colourful petals and the promise of a feed of nectar.
In place of petals and sepals there are two greenish/white elongated flaps, the lemma and the palea, which enclose the reproductive parts in a tight bundle. (Recent studies of plant mutants and whole genome sequences of different plant species strongly suggest that the lemma and palea are modified sepals - see ref. 2).
The lemma has a long, spike-like extension, the awn, which attaches to a grazing animal, helping to disperse the grass seeds. The silvery hairs protruding from the surface of the lemma serve the same purpose.
Photo E shows one of the flowers from the spikelet. The palea has been pulled upwards, to display the reproductive parts.
Think of the palea and the lemma as the slices of bread in a sandwich, with the reproductive parts being the filling. (This flower is actually shaped more like a kebab, but you get the idea).
Photo F is a view of the same flower turned through 90°. I’ve removed the palea completely and spread the three anthers apart to reveal their long filaments.
The female parts of the flower are now visible - a pair of purple stigmata, connected to the ovary by short styles.
The anthers of this wallaby grass, like most grasses, are huge compared to those of regular flowers. They are packed with thousands of tiny pollen grains.
Redanther Wallaby Grass inflorescence
Grasses need to produce huge amounts of pollen because wind pollination is an inefficient process. There is only a tiny chance that a single pollen grain released from an anther will find its way to the receptive stigma of a flower in a different plant.
The odds are improved by allowing the stigmata and anthers to protrude from the spikelets into the passing breezes as the flowers mature (see photo G).
Photo G also illustrates the basic branching form of the inflorescence of Redanther Wallaby Grass. Single spikelets are borne either on stems that branch directly off the main axis of the inflorescence (e.g. # 1) or on sub-branches of such branches (e.g. #2, 3).
This branching type, found in many other grasses, is known as a panicle. Redanther Wallaby Grass is said to have a loose panicle - branches and sub-branches are spread apart rather than being bundled closely together.
Grass diversity - a selection from our forest
2020 has been a brilliant year for our home grasses. We’ve never before seen such vigorous grass growth.
This may be due simply to the high rainfall we’ve enjoyed this year. But it’s likely that other factors, resulting from the January bushfire, have contributed. These could include increased levels of nutrients in the ash-laden soil, extra sunlight falling on the forest floor following loss of the canopy and reduced competition for water and nutrients because of the absence of bushes (remnants of which show as black twigs in many of my photos).
Redanther Wallaby Grass is a stand-out example of a grass showing exuberant growth this season. We first sighted this grass 5 years ago - a handful of plants growing in a small patch of a few square metres. Then in following years it disappeared.
This year we were delighted to discover scores of flowering plants of this species growing across a large area of the forest.
Not only are there many more plants, clumps are larger and culms much taller than previously seen.
Red-anther Wallaby Grass
To date, I’ve discovered 18 native grass species in our forest. I’ve revised my identification of some of these as time has gone on - grass species ID is a tricky game for new players! You can see photos of all except Wiry Ricegrass (Tetrarrhena juncea) on our iNaturalist site.
These 18 species display a wide variation in both inflorescence and spikelet structure, which I’ll illustrate with a few examples.
Kangaroo Grass, Themeda triandra
Before this Spring, we’d only ever seen this attractive species, with its distinctive inflorescence, in a few isolated patches of the forest.
In early November we saw many new culms sprouting from the soil - presumably from seed.
It’s now growing widely across the forest, including areas where we’d never previously seen it.
A cluster of young Kangaroo Grass plants
Kangaroo Grass also has a panicle inflorescence, with spikelets connected to the main axis of the culm by branches.
However it differs to the standard panicle in that spikelets are grouped together in clusters called racemes. Each raceme has a leafy bract called a spathe at its base.
Racemes are in turn grouped into clusters of 2-3 racemes, which share a common connecting branch to the main axis. Each raceme cluster has a larger spathe at its base.
Photos H and I illustrate this arrangement of racemes in two different plants.
This grouping of spikelets into clusters could be viewed as just a minor modification of the basic panicle branching pattern.
However, something quite radically different to the norm is apparent when we examine the composition of spikelets in a raceme.
Each raceme has a total of 7 spikelets, but only one of these bears a fertile flower, with both male and female parts. The other spikelets have male (with just anthers) or neuter (with no reproductive parts) flowers. That’s quite a mixture!
Photo J shows a whole raceme, photo K two of its 5 male spikelets and photo L its fertile spikelet, viewed from both sides (anthers were removed during dissection). Note that the palea is missing from all of these flowers - yet another mod. Even by grass standards, this is one weird inflorescence!
Oat Speargrass, Anisopogon avenaceus
This grass has a very distinctive panicle shape, with 8 large, drooping spikelets. It’s unmistakable!
Apart from their large size, the spikelets are pretty conventional.
Each bears a single bisexual flower, which is enclosed in a pair of glumes. There are 3 awns, the central being much longer than the two lateral ones. The lemma is covered in short, silky hairs.
At the base of the lemma is a sharp, hardened structure, the callus. This is a common feature of grass flowers and assists attachment of the seed to a grazing animal. Have a look at it the next time you pull a grass seed out of your socks.
The seed is the fruit of the grass plant, which develops if the flower is successfully pollinated. This photo shows the Oat Speargrass seed. The three awns can be seen, along with the hairy lemma and callus. It’s undergone quite the transformation from the flower.
Buried inside the seed is the embryo, its development arrested until conditions promote germination of the seed.
Oat Speargrass seed
Fine-leaved Snow Grass, Poa meionectes
This grass is rather special for us, as it is a diagnostic species for our class of forest, Lowland Gully Shrub Forest. It is probably the most widespread, common grass in our forest. Yet, it is easily overlooked because its culm and branches are thin, its inflorescence sparse and open and its leaves short and very narrow.
Poa meionectes displays a classic pyramidal panicle form with erect culms. These are generally less than 70cm high - not much taller than the Tall Bluebells.
The spikelets bear 2-3 bisexual flowers, which are rather small - only around 3mm long. Like all species in the genus Poa, the lemma lacks an awn.
Reed Bent-grass, Calamagrostis quadriseta
This grass provides a nice example of one of the other major inflorescence types of grasses - the spike. (A nice simple word for a technical term for a change).
What separates the spike from a panicle inflorescence is that the spikelets attach directly to a single, unbranched axis - the continuation of the culm. No branching or sub-branching.
This becomes clear when you look closely at the spike of this grass. It consists of a mass of closely packed spikelets, each of which is attached to the main axis of the culm by a short stem (first photo in the panel below).
You can see that attaching stem if you remove a single spikelet, as shown in the second photo. It comprises a single bisexual flower enclosed by a pair of glumes, shown in the last two photos. Pretty standard stuff. As a variation, the awn arises near the base of the lemma rather than from its apex. Just another example of how grasses play with their flowers.
It’s not overstating matters to claim that human civilisation as we know it couldn’t exist without grasses. Their fruits provide the staple foods of virtually all societies around the world (think wheat, think rice).
Arguably, that provides a persuasive reason to get to know them better.
For me, their understated beauty and extraordinary diversity suffices.
References
Downs, R.J. and Krizek, D.T. (2016) “Air Movement” Chpt. 6 in Growth Chamber Handbook. Eds. Langhans and Tibbitts. North Central Regional Research Publication No. 340. https://www.controlledenvironments.org/growth-chamber-handbook/
Lomardo, F. and Yoshida, H. (2015) “Interpreting lemma and palea homologies: a point of view from rice floral mutants”. Frontiers in Plant Science 18. https://doi.org/10.3389/fpls.2015.00061