Grasses (Poaceae / Gramineae)

Grass

What grass is this?

Grasses can seem daunting to the novice, when it comes to trying to identify individual species. To begin with their anatomy seems so very different from that of other flowering plants. Grasses can be annual or perennial. Few are deciduous, though in many some or all of the above-ground vegetation dies away over a hard winter.
Tussock-forming grasses perennate (survive from season to season for an indefinite number of years) by producing dormant buds just above ground level. Stoloniferous and mat-forming grasses produce perennating buds at ground level. A stolon or runner is a horizontal connection between genetically identical individuals forming a colony of clones. In flowering plants, stolons are horizontal stems, growing at or just beneath the soil surface and producing adventitious roots at their nodes, which may develop into new individual plants. In rhizomatous and turf-forming grasses, the perennating buds are borne on an underground stem called a rhizome. Rhizomatous plants are usually perennial.

Grasses are generally very resistant to grazing and produce new shoots (
innovations) to replace any lost shoots. Tillers grow from a ring of tissue which is produced at each node and is called the intercalary meristem. Stems which fall to the ground can also develop addventitious roots at any node in contact with the soil and resume growth in a vertical direction. (Adventitious roots are roots which are produced in an atypical location, such as part way up a stem).

Culms are stems bearing an inflorescence (cluster of flowers borne on a stem). Tillers are stems which do not bear an inflorescence (though may do so in the future). The leaves are arranged in two alternating rows and each consists of a sheath and a blade. The sheaths envelope the stem and are often essential to give it strength. The blade grows from a meristem at its base called the plate meristem. On the adaxial side (the side facing the stem axis) where the blade meets the sheath is the ligule. If you bend back the leaf slightly, parting it from the stem, then you will see the ligule as either a thin tissue-paper like membrane or as a series of hairs. It has been hypothesised that the ligule helps stop insects and water from entering the sheath base and collecting there. It is also known to be a secretory organ: secreting mucilage which might help waterproof the seal but has also been hypothesised to lubricate elongation of the stem, reducing friction between the stem and leaf sheath. The shape and form of the ligule is characteristic of the species and aids in identification.

Another way to identify grass species, with some certainty, is to examine their floral parts under a microscope. The inflorescence is a modified shoot system in which the leaves subtending each shoot have been reduced. The inflorescence is a raceme: a monopodial axis (see plant architecture) which may end in a flower, bearing side (lateral) branches which often end in flowers also. If the lateral branches are themselves branched then the raceme is a panicle (complex raceme). If the flowers on a simple raceme have short stalks (pedicels) then the raceme is a spike. The basic inflorescence-branch unit is the spikelet, e.g. spikelets may be borne on one side of the main axis (the main axis is also called the rhachis)
in a simple raceme.

Each spikelet consists of a central axis (the
rhachilla) bearing a series of scale-like bracts (reduced leaves subtending a flower or group of flowers) arranged in two alternating rows (distichous arrangement). The spikelet is usually borne on a stalk or pedicel. Two lowest scale-like the bracts, the upper and lower glume, protect the developing spikelet (one or both of these glumes may be reduced or absent). Above the glumes are a series of pairs of alternating bracts; one of each pair is called the lemma and the palea. This pair of bracts encloses a single flower; the whole unit (flower + lemma + palea) is called a floret.

The flower inside the lemma and palea consists of two reduced scales (reduced petals?) called the lodicules, 3 stamens and an ovary (with a single chamber or locule) bearing two plumose (feathery) stigmas. The lodicules swell to allow the flower to open for the anthers to emerge and hang down (to catch the wind which disperses the pollen as it forces the inflorescence to wave about) and then allows the flower to close again until the stigmas are ready to emerge. This pattern of male parts maturing before female parts (protandry) reduces the odds of self-pollination. In other species the female parts may mature before the male (protogyny) for the same reason. Cleistogamous florets do not open and are
modified to favour self-pollination.The long flexible filament of the stamen supports the anther which dehisces (dries and opens) either by a longitudinal slit or a terminal pore.

The fruit is a
caryopsis if:  the thin fruit wall (pericarp) is firmly adherent to the enclosed seed. If the pericarp is soft ,and so easily separated from the seed, then the fruit is a utricle. If the pericarp is hard but easily separated from the enclosed seed, then it is an achene.

Phragmites australis (Common Reed)

Some grasses are very distinctive and hence a good place to start. The Common reed, Phragmites australis is unmistakable as it produces large stems up to 3.5 m tall and occurs chiefly in lowland wetland habitats, such as beside rivers, lakes, ponds, in ditches, marshes etc. In this instance it was found in abundance, and at high density, on the uppermost zone of a saltmarsh formed from a tidal river. It grows best in shallow water.

Phragmites australis is very widely distributed in temperate zones around the world. Its large and extensive system of rhizomes may penetrate to a depth of 2 m and the plant may also put out surface runners up to 10 m long ( see review by Cope and Grey, 2009).

Phragmites australis is thought to reproduce mainly by vegetative means: fragments of shoot or rhizome may be carried on water to root at new locations. Each seed has a long tuft of hairs to assist in effective long-range wind dispersal. (For the rhizome see: https://www.greatlakesphragmites.net/blog/20200910-rhizomes-getting-to-the-root-of-it-all/).

Some other grasses are striking enough to be immediately recognizable without a hand lens. one of my favorites are the quaking grasses (Briza). Many, however, require a closer look at their component parts.


Dactylis glomerata (Cock's-foot)

Cock's-foot

Cock's-foot

Cock's-foot

The grass below had almost all the features of Dactylis glomerata (Cock's-foot) and was found growing on a roadside (a typical habitat for this species). The only oddity were the glumes, which were variably subequal and both generally with a single vein (rather than definitely subequal with one glume having three veins). However, this grass is noted as being very variable.

Above and below - a cluster of spikelets.

Below- a single floret. The lemma is larger than the palea and has a short awn (spike-like extension) and a comblike row of hairs on its keel. Note the short (and hardened) stalk/rim bearing the floret, this is called the callus.

Below: left - a pair of florets; middle and right - glumes, which although almost equal in this case were sometimes more unequal in size on some spikelets.

Above and below: glumes, which although almost equal in this case were sometimes more unequal in size on some spikelets.

The pair of small translucent lodicules, surrounded by the three stamens.

Above and below: The ovary with its feathery stigmas (considerably hairier in some florets, presumably when the ovary is more mature).

Below: the tiny, soft and fleshy prophyll in the adaxial axis of a spikelet.

Above: lemmas.

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Above: left - a glume, lemma and the three stamens; right - the ovary amid the three stamens.

Poa pratensis

Poa is the genus of 'meadow grasses'. Poa pratensis is a perennial grass with subterranean rhizomes with culms (stems) up to 1 m tall. It grows in tufts. This species is a complex of apomictic and sexual races with a variable number of chromosomes. In this context, apomixis means 'asexual seed production'.

Apomixis involves a genetic modification to normal sexual reproduction, namely, the loss of a meiotic reduction division in egg cell development. The adult plant (sporophyte) in general has two sets of chromosomes (it is diploid, with 2n chromosomes) - a maternal and a paternal set.

Meiosis is the standard mechanism of cell division in the production of gametes, but since plants exhibit alternation of generations, meiosis of the diploid megaspore mother cell produces a haploid group of cells, the gametophyte, which is the embryo sac. One of the embryo sac cells is the egg cell. Meiosis ordinarily halves the number of chromosomes, resulting in a haploid embryo sac and hence a haploid egg cell (with n chromosomes).

In apomixis, however,the embryo sac develops from the megaspore mother cell by a series of mitotic cell divisions. Mitosis is not a reduction division since it maintains the number of chromosomes. The diploid embryo sac results in a diploid egg cell.


Above: a model of apomixis in Poa pratensis (based on Matzk et al., 2005). Details in main text.

The next modification needed for apomixis to occur is parthenogenesis: the development of an embryo without fertilization, or more specifically, a circumvention of fertilization-dependent embryo formation. Normally the embryo sac undergoes a double fertilization in flowering plants: each pollen grain carries two sperm cells, one of which fertilizes the egg cell to produce the diploid zygote, whilst the second fertilizes the central cell (which is itself a fusion of two haploid embryo sac cells) to produce the triploid endosperm initial cell, which gives rise to the endosperm which nourishes the developing embryo (right up to germination in this instance).

in this type of apomixis, pollen is still required to initiate seed development, but only the central cell is fertilized and this goes on to produce the endopserm. The egg cell then develops without being fertilized and hence gives rise to a normal diploid sporophyte. (If it was fertilized this would give rise to a triploid sporophyte which we would expect to have reduced fertility).

Matzk et al. (2005) proposed a model of the genetic control of apomixis in Poa pratensis. In this model, five hypothertical genes (whose existence is supported by experimental data) are crucial to the decision making which determines whether the embryo will develop by way of normal sexual reproduction or by apomixis.These genes are: Apv (apospory preventor), Ait (apospory initiator), Mdv (megaspore development), Pit (parthenogenesis initiator) and Ppv (parthenogenesis preventor).

Apospory is the production of the embryo sac without megaspore formation. Normally the embryo sac develops from the haploid megaspore which is produced from the megaspore mother cell by meiosis and goes on to produce the haploid embryo sac by mitosis. The chief functions of these genes should be self-explanatory with the help of the above diagram.

Poa pratensis ssp. pratensis is native to Europe and temperate Asia. Here it is growing along a path beside a tidal river. It is commonly found in meadows, pastures, road verges, in parks, on walls and on wasteland. In North America it is known as Kentucky Bluegrass and is invasive in some parts of the USA and Canada where it is becoming a dominant grass in the prairies of the Northern Great Plains.

In Poa annua (see next grass species) the inflorescence branches occur in pairs

There are several subspecies of Poa pratensis. The one examined in detail below (and probably those shown above which are from the same population) is ssp. pratensis (Smooth Meadow-grass) which has a ligule that dips down towards the margins (it is decurrent on the sheath margins). Poa pratensis ssp. angustifolia (Narrow-leaved Meadow-grass) has distinctly very narrow leaves, as the name suggests, and a ligule which does not dip down at the margins with the leaf-sheaf. In contrast, ssp. irrigata (Spreading Meadow-grass) has long rhizomes generally bearing culms at intervals and a ligule that is often hairy on the back.

An inflorescence branch of Poa pratensis subspecies pratensis (Smooth Meadow-grass).

The pair of glumes at the base of each spikelet are unequal: the smaller lower glume has a single nerve, whereas the upper glume has 3 nerves.

Above:a spikelet with the glumes opened at the base. Leftmost is the larger, 3-nerved, upper glume and the smaller single-nerved lower glume is on the right. Each lemma has a tuft of woolly hairs at its base (on the callus or attachment point of the lemma).

Above:a pair of florets (the glumes having been removed) with the lower floret open with the lemma to the right and the more membranous palea to the left.

Poa annua (Annual Meadow Grass)

A common grass found in arable fields, gardens, waste-land, paths and walls, and grassland which is heavily grazed or trampled. Renown for flowering throughout the year, this specimen was one of several found in flower in mid-December.

The ligule is characteristically membranous, blunt and taller than wide.

  A spikelet of three florets

The outer lemmas of each floret can be clearly seen.

Above: The palea.

Above though hard to see in this photograph, the lodicules are present inside the base of this detached palea.

Above: The ovary with two feathery stigmas.

Above and below: the maturing fruit and enclosing palea.

 

Arrhenatherum elatius (False Oat-grass)

This grass is a loosely tufted perennial up to about 150-180 cm high (5 to 6 feet). It is often abundant in rough grassland, road verges, hedges and riverbanks and on waste ground, e.g. calcareous slopes, as an arable weed.

A floret of Arrhenatherum elatius.

At least the lower florets in the spikelet have an awned lemma which emerges from the back (abaxial side) of the lemma from the lower third. This type of awn is very long and consists of a twisted (helical) column and a distal limb. A bend or knee occurs where the limb joins the column (a geniculate awn). When the fruit is dispersed, drying and wetting causes the awn to twist and the angled limb flicks the fruit along the ground as it does so, until the fruit stumbles upon and falls into a suitable crevice where it may germinate.

Above: a floret removed from its glumes

Above: the pair of glumes which enclosed the spikelet

Above:  the ovary with its pair of feathery stigmas.


Alopecurus pratensis (Meadow Foxtail)

Meadow Foxtail

Young 'foxtails'.

Meadow Foxtail

Meadow Foxtail

Meadow Foxtail

Meadow Foxtail

Meadow Foxtail

Meadow Foxtail

Briza media (Quaking Grass)

Quaking Grass

A quaint grass whose spikelets readily vibrate in the breeze. The inflorescence is a loose panicle (with each flower borne on its own distinctive branch) that is more or less pyramidal in outline. This grass is perennial, producing a short rhizome. Its most characteristic habitat is well-grazed calcareous grassland (as here) but it occurs on a wide range of soil types, particularly dry and well-drained soils.It occurs in Europe, parts of Asia (northern and eastern), South Africa and has been introduced into North America, Australia and New Zealand. This species reaches 80 cm in height, but is often much shorter. Briza maxima (Greater Quaking Grass) is a Mediterranean species with much larger spikes and is a popular ornamental in gardens.

Quaking Grass

Above: a spikelet of Briza media.

Quaking Grass

Quaking Grass


References

Cope, T. & Gray, A, 2009. Grasses of the British Isles (BSBI Handbook No. 13). Botanical society of the British Isles. Avilable at: https://www.nhbs.com/grasses-of-the-british-isles-book

Matzk, F., Prodanovic, S., Ba ̈umlein, H., and Schubert, I. 2005. The Inheritance of Apomixis in Poa pratensis Confirms a Five Locus Model with Differences in Gene Expressivity and Penetrance. The Plant Cell, 17: 13–24.  http://www.plantcell.org/content/17/1/13.full


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Article updated: 8 May 2021, 6 July 2021