This talk covered the grass family in general, the structure of grass plants and other plants related to grasses (sedges and rushes).
The grass family
This is a large family of wind-pollinated flowering plants
with about 10,000 species. Grasses are
found in nearly every habitat on earth, from the Polar Regions to the
tropics. In Britain there are many
introduced species and in total there are 160 species either native or
naturalised here. The major grasslands
worldwide are in the North American prairies, the South American pampas, the
African savannah, the central Asian steppes and the north and east of
Introduced species include bamboo on Harrow Weald
Common. Bamboos have nearly 1,500
species, can grow to 36m and are the only woody grass. The tallest grass in
Grasses are important because lots of animals like to eat them, hence the oft-quoted - All flesh is grass. Grasses also provide food directly for humans in that wheat, barley, oats, rye, maize, rice, millet, sorghum and sugar cane are all cultivars of grasses. It is, however, a little surprising how few species of grasses have been cultivated.
Structure of grasses
Grasses are monocotyledons, characterised by long, narrow leaves with flowers having 3 petals, though most appear to have more because they have identical petals and sepals. Dicotyledons in contrast have leaves of all shapes and sizes and generally 4 or 5 petals. Monocots germinate producing a single seedling leaf while dicots germinate as a shoot with two leaves.
Ligule and Auricle
The basic structure of a grass leaf is shown above. The leaf begins at a node on the stem. At this point it is a sheath which encloses the stem. This sheath extends upwards from the node to the formation of the blade when the leaf separates from the stem. The ligule is a flap at the intersection of the leaf-blade and the sheath. It is an important diagnostic feature, as in the rough and smooth meadow grasses seen above. The common reed has a hairy ligule, as do purple moor grass and cheese grass. Where the ligule joins the sheath and the blade, there is often a pair of rounded lobe extensions known as the auricles. Only a few grasses have auricles, these include perennial rye-grass and common couch grass where they are seen above to envelop the stem.
The different types of flower heads are shown below. A panicle is an inflorescence with flowers on small branching side-stems off the main stem, a raceme has flowers on individual side-stems off the main stem and a spike has flowers without a stalk which are attached directly to the main stem.
The basic unit of the grass inflorescence is the spikelet. This can have 1, 2 or more flowers within it, (this is particularly easily seen in the sedges).
Examples of panicles include barren brome, so-called not because it is sterile but because it produces very few leaves and there is not much for animals to eat, red fescue and creeping bent, which like other bents has only one flower in each spikelet.
Examples of Panicles
Examples of Spike-like flower heads
The structure of the flower.
At the base of the spikelet there are two glumes. These are scales which enclose the flowers, more accurately called florets. In some grasses the glumes have a bristle-like extension called the awn. Each floret is protected by two further scales known as lemmas. The upper lemma is known as the palea. The sexual part of the floret consists of male anthers and two female stigmas which are attached to the ovary. This ovary produces just one seed which is a property that has made grasses so important in their development for food production.
The awn mentioned above is the feathery extension which is so prominent in the ear of crop grasses such as oats, rye and barley.
Unlike other monocots which have a perianth consisting of petals and sepals required to attract pollinating insects, grasses and sedges have no petals and sepals. However at the base of the ovary there are two structures known as lodicules which may be remnants of the perianth. These lodicules perform a function once the floret matures. They force apart the lemma and palea to allow the stamens carrying the anthers to become exposed.
The rushes are at a phase between typical monocots and the grasses. Their florets have kept their perianth in the form of 6 inconspicuous segments around a superior ovary.
As explained previously, unlike grasses are wind pollinated, hence the need to produce far more pollen than would be required in flowers pollinated by insects. For example, one anther of rye produces about 19,000 pollen grains and each flower has 3 anthers so one flower produces 57,000 grains. Each ear of rye has 70 flowers so each ear of rye produces 3,990,000 grains and the number of grains produced by a whole field of rye is unimaginable. While this may appear to be wasteful, pollen is very small and most of it will not land where it can fertilise the plant.
Cocks-foot (Dactylis glomerata) is a good grass for pasture but it is rather
coarse and no good for lawns. Annual
meadow-grass (Poa annua)
is unusual in having a 6-week life cycle at any time of year.
Yorkshire fog (Holcus lanatus)
Timothy Sweet vernal grass Perennial rye-grass Cocks-foot Annual meadow-grass
Phleum pratense Anthoxanthum odoratum Lotuium perenne Dactylis glomerata Poa annua
Sedges and rushes
Pendulous sedge Rush Wood-rush Great Reed-mace or bulrush
Carex pendula Juncus Luzula Typhus latifooia
Sedges and rushes are related to grasses but grasses are hollow, sedges have edges and rushes are round and in-filled with pith. Sedges have triple sets of leaves, some have separate male and female flowers on different parts of the plant but others have flowers all mixed up. Rushes have what look like 6 petals but they are rudimentary and of no use. Sedges and rushes are not easy to identify.
The Great reed-mace or bulrush (Typhus latifolia) is not a rush but a separate family altogether (the reed-maces) and another plant also called a bulrush is actually a sedge.