| GRASS STRUCTURE
There are four main parts to a grass plant, the roots, the stem (culm), leaves and flowerhead or inflorescence. Each part has diagnostic features that help in correct identification. It is often the combination of several characters that eventually leads you to the correct name. The grass descriptions in this guide are designed to point out important features of each of these parts. It is very important to get as much information as possible on each part of the plant to help identify the grasses. In other words it is best to have the entire plant, roots and all, to identify a grass.
Grasses grow from fibrous root masses, but sometimes there is a below-ground root-stem structure called a rhizome (Figure 1) that is often mistaken for a root. It is important that you observe whether the specimen has a rhizome or not. Often the root mass may appear to have no rhizome, but if you squeeze the mass you can detect a hard, branchlike projection that is the end of a rhizome that got broken when the plant was removed from the soil. Annual grasses do not have a rhizome, they only have a mass of fibrous roots (Figure 2).
Figure 2Stems or culms
Stems in grasses are actually called culms in grass keys, but we have retained the term "stem" to make it easier for laypeople. Stems sometimes assume varying forms that may at first seem confusing. These forms are very important to grass identification. The single upright stem with a few leaves at the base is the most familiar form, but stems can also originate from clusters (cespitose) or tufts of leaves at the base. Stems can be found below ground (rhizomes) or above ground (stolons), and trail on the ground. In some species, these trailing culms root at the nodes.
Stems arise from the crown, which is the base of the leafy part of the grass plant. It is from this crown that leaves and stems regenerate after grazing or fire. Some grasses, such as Onion Grass (Melica bulbosa), have swollen stem bases that look like bulbs at the crown. At the end of the first season, the dead stem often remains standing and dried leaves from the previous year also remain around the base. It is important to note the presence of old stems and leaves at the crown. This feature provides a clue to whether the grass is annual (living for one growing season) or a perennial (living for more than one season). Some grasses are biennials (they grow vegetatively the first year, and produce a flowerhead during the second year).
Leaves arise either from the crown at the base of the stem or from swellings along the stem called nodes (Figure 3). Nodes are often a dark colour, and in some cases they may be barely noticeable. Some nodes have small hairs on them. The spaces between nodes are called internodes.
Each leaf consists of a sheathing portion called a sheath, which wraps around the stem, and a free portion called a blade. The sheath may be open or closed, an important diagnostic feature with some grasses. If it is closed, it appears like the V in a V-necked shirt. If it is open, the edges of the sheath appear to meet or overlap.
Figure 4The leaf blade often diverts or bends away at right angles from the stem and the sheath. Often a distinct ridge of tissue develops at the bend. From this ridge of tissue or collar there arises the ligule (derived from the word tongue because in many species it looks like a little tongue sticking out) (Figure 4). The ligule sticks up from the ridge and is thought to protect the stem from insects. The sheath is an enclosed surface and insects could easily move from the flat blade down into the sheath; but the ligule forms a small barrier to this. Ligules have two common forms: they either consist of a membrane or a ring of hairs. At the point where the ligule meets the leaf blade there is usually a narrowing or thickening of the leaf known as the collar (Figure 5).
Figure 6Length and width of the leaf blade are often used in keys to help distinguish species. A good metric ruler with a clear millimetre scale is an invaluable tool to grass identification. In addition, the blade may be flat, or "folded lengthwise" (conduplicate) or inrolled (involute). Sometimes the form of the leaf tip is a useful feature. For example, Blue Grasses (Poa spp.) have keeled or prowlike tips, like the prow of a canoe. It is important to check several leaf tips as young leaves are the favoured food of many animals, and that strange tip may have been nibbled into shape.
At the point where the inflorescence or flowerhead begins, the stem officially ends and the stem axis above this point is called a rachis (Figure 7). Sometimes there is a noticeable node at this point or branches begin to point either upward or outward. In this treatment the measurements for the length of the flowerhead are taken from this node to the top of the flowerhead. The branching pattern and the general form of the flowerhead is a useful feature for field recognition. The rachis of the flowerhead may be obviously branched or unbranched. The branched (Figure 8) inflorescence in this treatment has spikelets that are attached to the rachis with a branch that can be clearly observed. Otherwise it is classified as having no branches or a spike. It is important to note whether you are looking at a fully extended flowerhead: the same species can look like a spike when immature. At full maturity, the spikelet is actually branched. An example of this can be seen in Figure 7 and Figure 9. They are both the same species Yorkshire Fog (Holcus lanatus).
Figure 10The nitty gritty of grass identification gets down to the structural details of the flowering units called spikelets and the flowers within them (Figure 10). Grasses bear their flowers on highly modified side branches called spikelets. At the spikelet base, closest to where the spikelet meets the main branches of the flowerhead, there are two highly modified leaves called glumes (Figure 11). The glumes hold the flower or flowers, which are arranged alternately along the rest of the branch above them. The form of the glumes, their size (length), whether or not they exceed the inner flowers in length, and surface texture, are all useful features to observe. A good hand lens, or in some cases a binocular dissecting microscope, is very useful to view these characteristics . In the key we have added an illustration of a magnifying glass to indicate where you will need additional magnification.
Figure 12Awns are pointed extensions of either the tip, back or base of the glume (Figure 12). They can be long or short, straight, curved or even bent. In some species, the glume is also modified and is awnlike for its entire length.
Figure 13The reproductive parts are arranged on the branch above the glumes. For the purpose of this guide we are calling florets, flowers. Flowers can be widely separated along the spikelet, or jammed tightly together (Figure 13). The number of flowers in a spikelet is an important feature to note. Some genera and species have only one flower in each spikelet, whereas others bear ten or more within the spikelet. Some flowers in the spikelet are sterile (empty). It is important to note whether these sterile flowers are above or below the fertile flowers. Some species have an empty flower at the end of the spikelet, and others have a small extension of the branch of the spikelet.
Figure 14Non-grasses have petals surrounding the flower to attract the attention of pollinators, but grasses are wind pollinated. They bear small, leaflike bracts or scales to protect the seed until it is mature. The grass seed used to seed a lawn is actually naked, but in nature most seed when it is dispersed retains two, leaflike covers. The outside (located away from the stem) and sometimes larger leaflike part is called the lemma (Figure 14), and the inside (closest to the stem), usually smaller, part is called the palea. The palea may be reduced in size or missing entirely but the lemma is always there. The lemma cuddles or holds the palea and the stamens and pistil inside. Lemma and palea form, size and hairiness are critical features in grass identification. These parts often absorb water and hold it close to the seed for germination.
One of the most useful features of the lemma is the character of the awns. It is important to note first of all whether an awn is present or not. Its length is measured from the tip to the point where the awn joins the lemma or glume. Is the awn straight or bent, or -- as in the case of California Oatgrass (Danthonia californica) -- twisted several times (Figure 15)? Note whether the awn is attached at the end, middle or base of the lemma or glume. If you think awns are not important, recall how after running through a meadow or field you had to pull out the spikey bits of grass from your socks or pants. The spikey bits may well have been florets jabbed into you clothing by the awns. Dog owners will be familiar with the various types of awned grasses, some of these can inflict pain and infection to the dog's soft body parts. Needle-and-thread Grass (Stipa sp) is a nuisance to cattle owners.
Other useful awn features to note are whether it arises at the tip of the lemma, or between a couple of teeth as in Bromes (Bromus). Sometimes the base of the lemma is modified into a structure called a callus. The callus may have no hair, be covered by a short tuft of hairs called a beard, or long wavy hairs that appear weblike. The weblike callus is particularly important in deciding which Poa species you have. The awn and the callus may actually be very important in orienting the seed and helping to work the seed deeper into the soil. Experiments have shown that some grass seeds show reduced germination in various soil types if the awn is removed. In some soils awns are necessary for successful germination.
Finally we get to the reproductive heart of the matter, the anthers (pollen part) and the stigma (ovary) within the palea and the lemma of each flower. Sometimes the grass floret may have already ripened and there will only be a hard fruit called the caryopsis. Perhaps the most important feature to note is whether both are present or not. Some species may have several flowers in a spikelet, but only one may be fertile -- that is, having either anthers or an ovary.
In some species the flowers are unisexual, having separate sexes in separate flowers. Because of this, the ideal time to collect grasses is when they are in full flower. In most grass species the anthers are "just hanging out" between the palea and lemma of the spikelet (Figure 16). This is the most colourful time in flowering of the grass: most anthers are yellow or rust coloured.