Understanding Grasses
Members of the grass family dominate so much of our world. They make up the bulk of major plant communities in most habitat types and on most soils, from meadows and roadsides to lawns and parks, grasses are all pervading. We even rely on them as crop plants (wheat, barley and maize, for example) and use them as ornamentals in our gardens. Grasses may be tiny plants that are missed with the blink of an eye, or they may be stout bamboos which can reach 20m or more in height!
It is not always realised that grasses are flowering plants. Their flowers are highly specialised in having no petals or sepals but in having a series of bract-like structures arranged in order along narrow stems. The arrangement of these bracts and their branches help to make up the flowering spike that can be an important part of the identification process. Because these flower structures are different to other plants, their parts require their own names to identify them, so grasses have their own terminology and this it what often puts people off them. But the terminology can be learned reasonably easily with a little help from this page.
This page follows a standard sequence, starting with the vegetative parts, from the bottom up, followed by the parts of the flowers, working from the outside in. Not all parts are required for the identification of every species, but a combination of as many features as possible will certainly make the task easier. You can work your way through the page, or you can jump straight to a section by using the links on the left.
Roots & Shoots
The roots of a grass are, of course, largely out of sight underground and are not in themselves that useful for identification in most instances, but a quick scratch around at the base of a plant can sometimes reveal important details. Is your plant growing in a tussock (i.e. with short, fibrous roots only) or is it forming spreading or creeping patches by means of stolons or rhizomes? Is it annual or perennial?
Annual plants will typically only have fibrous roots, which tend to be relatively short and emerge in a cluster from the base of the plant. If your plant has longer extensions to the roots which allow the plant to creep through or across the soil, it is probably a perennial species. Rhizomes are part of the main stem of a plant, but can be mistaken for a root as they tend to be white (without green chlorophyll) and creep horizontally through the soil. Rhizomes have scale-like leaves and short internodes and can send out roots and shoots periodically along their length from the nodes. Stolons are side shoots from an existing stem that originate above ground level. They have long internodes and typically bear more or less normal-looking leaves. Being able to tell a rhizome from a stolon can help in identifying some grasses, such as the bents.
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Stems
As with any plant, the stems are the above-ground parts of a grass that carry the leaves and flowers. Grass stems differ from the stems of sedges and rushes in having nodes - an important and useful difference to tell these groups apart. Nodes are essentially joints (and look a bit like knees), with the sections of stem between the nodes being known as internodes. Nodes have a few features that can be useful in identification; they may be hairy or smooth and bent or straight, for example.
Grass stems come in two different types; they may be vegetative stems that carry only leaves, or they may terminate in a flower spike. Stems that carry only leaves are known as tillers and usually have short internodes with several to many leaves. Tillers are typically shorter than the flower-bearing shoots because the flowers are wind-pollinated, so they function best when carried above and out of the way of the leaves. Stems that bear flower spikes at their tips are known as culms. The culms tend to be stiffer and stouter than tillers so that they can support the flower spike. When describing the nodes of grasses, it is usually the nodes of culms that are being described. A small number of grass species develop bulbous bases to their stems which become storage organs for food, to sustain them during periods of weather that inhibit growth (typically either winter cold or summer drought periods).
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| and many leaves |
and few leaves |
and few leaves |
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Leaves
The leaves of grasses are highly variable in appearance from species to species and their features are often important for identification. Grass leaves are divided into two parts, the outer part (that which is furthest from the stem) is known as the leaf blade and is the most obvious part of the leaf, often standing out, flag-like, at ninety degrees to the stem. The leaf blades can be flat and relatively broad, or they may be inrolled from the edges to appear very narrow and even tubular in the most extreme cases. The leaves generally taper to a point, but the very tip may be sharply pointed, or bluntly rounded or, in some cases, keeled to give a boat-like tip.
The inner part of a leaf is known as the leaf sheath. This part of the leaf is less obvious as it is tubular and wraps closely around the stem, but it can usually be seen simply by pulling the leaf blade gently away from the stem. The leaf sheath may be rounded or it may be keeled on the side immediately beneath the leaf blade. As with the blade, the sheath may be smooth, or hairy in a variety of ways.
where the leaf sheath meets the blade, there is a raised feature known as the leaf ligule. This feature may be a whitish membrane (and which may be blunt or pointed, smooth-tipped or ragged-tipped) or it may consist of a line of hairs or bristles. At the bottom corners of the leaf blade, where it wraps around the stem and joins the leaf ligule, there may be small projections known as auricles ('ears'). If present, these are usually rather small, but in some species, they grow out into pointed tips and sometimes cross over to envelop the stem.
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| stem to show parts |
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Flower Spikes
Although much can be learned from the leaves and stems of a grass, it is really the flowers that define each species and, certainly if you are at the learning stage, it is often best to wait until a grass is in flower before trying to identify it, barring a few highly distinctive species. The flowers of grasses are really rather different to those of what we might call 'typical' flowering plants, to the point where words have been coined to describe their parts which are very specific to grasses. This means learning new terminology, but it's not too difficult and the general format is the same in all grasses - they are just variation on a theme. This section covers the overall flower spike and how the parts are arranged, so that you can find them when reading the species descriptions. The next section covers the individual flowers (or florets) and their parts.
Grass flowers are typically rather small, but they are arranged in groups in often quite large heads. The arrangement of these groups is specific to each genus and species of grass. The flowers will be carried either as a raceme or as a panicle. In a raceme, all of the flowers are individually attached to a single, central stem called a rachis. In a panicle, the flowers are attached to side branches of the main stem. These side branches may be long or short (sometimes even shorter than the flowers themselves so they need to be searched for!) and may be densely packed, upright, spreading widely or drooping. The flowers themselves are borne upon the main stem in a raceme, or on the side branches in a panicle. They may be carried singly or - more often - in small clusters called spikelets.
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| purplish spikelets on one side of a green rachis |
spikelets on alternate sides of the rachis |
a cat's-tail |
a barley |
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| broadly overlapping flowers |
with few branches |
with many branches |
open branches but spikelets many and tightly packed |
Flowers
The flowers of grasses differ in a number of ways from the more traditional idea of a flower, such as a rose or a buttercup. Generally, they have no brightly coloured parts because they are not trying to attract insects for pollination. They have no petals, but instead have a series of scale-like bracts, arranged in a regular sequence on a short stem. The flowers may be solitary or arranged in small clusters and, whether solitary or in clusters, these are referred to as spikelets. Many spikelets cluster together to make the flower spike.
Spikelets consist of a series of scale-like bracts along a stem and these bracts always appear in the same order - though sometimes one or more may be missing! The first two bracts at the base of the spikelet are the glumes and there are usually two of them, one each side of the stem and one slightly above the other (remember the parts alternate up the stem); these are the lower and upper glumes. The glumes may be tiny and hard to spot or they may be so large that they enclose the rest of the flower or even the entire spikelet. Above the glumes come the flowers themselves, or florets as they are often known. If there is more than one floret, they will be arranged alternately, one above the other. Each floret consists of a lower bract called a lemma and an upper bract called a pallea. between these two sits the actual floret, with two stigmas and three stamens. The lemmas can be relatively large and are typically the most obvious part of the spikelet, often forming the appearance of a series of overlapping bracts with the florets hidden beneath. The lemmas often have spike-like appendages called awns. Palleas are often tiny and hard to spot without dissecting the spikelets.
Something to bear in mind is just how different a grass flower can look when the flower is closed compared with when it is open. Typically, each flower is closed when young and the stigma and stamens are hidden from view and protected by (usually) their lemma or the glumes. At anthesis (the point when the flower disperses its pollen from the anthers), the flower opens out the lemmas and the stamens and stigmas protrude to disperse and receive pollen. The pictures of False Oat-grass and the two rye-grasses immediately below show how different the florets can look when open or closed. After anthesis, the flowers typically close up again while the seed forms.
Common Millet is shown below as an example of a species with flowers that don't follow the rules, until you look very closely. The flowers are small and globular and tightly packed. Each spikelet appears to have two glumes (one unusually large and enclosing the entire spikelet) and two lemmas surrounding a single flower. However, the secret is that there are two flowers (hence two lemmas) but one of the flowers is sterile, so bears no stamens or stigmas.
Remember, the sequence of the spikelet parts is always the same, but the size and shape of the parts vary to the point of being ideal for telling one species from another. Thus, attention to these flower parts is the most important part of grass identification. Oh, and don't forget a small ruler!
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| eight florets |
two large glumes and 14 strongly overlapping lemmas |
with single florets closed |
with single florets open |
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| Millet with exposed anthers |
with large glumes hiding the lemmas |
florets closed |
florets open |