Grasses (Poaceae) are one of my favourite botanical illustration subjects; I adore drawing and painting them. I have written a blog on my passion for this family of plants before but wanted to take another look at the way grasses are put together, and introduce beginners to the basic anatomy and terminology that’ll help you start to understand these glorious and diverse plants.
Drawing a plant is one of the best ways to begin to understand it, so I hope this crash course in grass anatomy will help.
Anatomy of Grasses: Overview of the Plant
Grasses have long leaves or blades, straight thin roots, a rounded (often hollow) stem (or culm), and a flowering spike. Lots of people may not realise that the top region of a grass plant happens to be the plant’s flowers and seeds. It becomes obvious when you think about a grass like wheat, but other species might fall under the radar.
Bread wheat Triticum aestivum
The culm of a grass has “knees”, these are known as nodes. These nodes might be at a bend in the culm, or just on a straight run of the stem. The culm tends to be swollen at the nodes. They may be hairy or smooth, depending on species. This bending at the nodes is known as genticulate growth. Some people confuse grasses with sedges and rushes; remember that grasses are the only one of these groups which can “bend at the knees”.
The space between these nodes is called the internode. Its length can help differentiate between species of grass.
Blades (leaves) of grass tend to be flat and linear, arranged alternatively up the culm, and have parallel and unbranching veins. They may be broad, or needle like; in some species they roll in on themselves to make bristle-like leaves. Noting if they are hairy or smooth helps determine the species.
The blades of grass grow up the culm rather like a tube, then grow outward. This encircling or tubular covering is known as a sheath. Sheaths may cling tight to the culm or be loose and inflated; yet another thing to look out for if you’re trying to i.d. a grass plant.
Overview of the anatomy of a grass (Meadow oat grass Avenula pratensis)
Ligules are little flaps of membranous tissue that form at the top of the sheath and the base of the leaf blade. They are very cool as their shape varies a great deal from species to species. In many cases they’re tiny, so a hand lens might be handy if you’re going to take a closer look. Some ligules are pointed, some are rough edged, some very thin, some broad and easy to spot. Some species have no ligule, or have a ligule which is reduced to a ring of hairs.
Sometimes the edges of the leaf blade cling to the culm and surround the ligule (as in the second illustration below); these structures are called auricles.
Ligule variety in different species of grasses
The flowering part of a grass plant is called the panicle, flowering spike, inflorescence or flower-head (many of these terms also apply to other families of plant, and botanists use them somewhat differently at times, which can be confusing). These flowering heads consists of lots of tiny grass flowers which are called spikelets.
Diagram showing flowering spike diversity in the Grasses family: Spreeading panicle, flowering spike, Raceme & Compact panicle
If the flowering spike is unbranched, with each individual spikelet attached to the central stem by a stem (or rachis) it’s known as a raceme (as with Rye grass Lolium perenne and Tor grass Brachypodium pinnatum).
Racemes: Tor grass Brachypodium pinnatum and Italian Rye grass Lolium multiflorum
Panicles often refer to grasses whose spikelets are borne at the end of stalks on a branching flowering head. They show an enormous amount of variety both in individual plants (depending on the age and developmental stage of the plant), within species, and (obviously) between species.
Variety of panicle shape: Yorkshire Fog Holcus lanatus showing one panicle still within the sheath, one fully spread at maturity.
Some other grass species with spreading panicles include Cocksfoot Dactylis glomerata, Common bent Agrostis capillaris, and Wavy hair grass Deschampsia flexuosa.
Spreading panicles in Cocksfoot Dactylis glomerata, Common bent Agrostis capillaris, and Wavy hair grass Deschampsia flexuosa
Panicles can also be very compact, and look like one tight structure. This is particularly true of the Meadow foxtail Alopecurus pratensis.
Tight panicle shown by Meadow foxtail Alopecurus pratensis
Some other grasses with tight panicles include Crested Dog’s tail Cynosurus cristatus, Twitch grass Alopecurus myosuroides, and the Foxtails.
Tight panicles shown by Crested Dog’s tail Cynosurus cristatus, Twitch grass Alopecurus myosuroides
Each individual spikelet, or flower, is made of distinct parts. The stalk of each flower is called the rachis, and flowers are arranged alternately, or in a zig-zag fashion along it.
The base of each spikelet, be it one or several distinct flowers, is held in a pair of glumes. These paired glumes have distinct upper and lower glumes, and these structures are important in determining grass species.
The glumes may have bristles or spikes attached to them. These are called awns, and can be long or short, bent or straight, twisted (as with many Oat Avena species), or absent.
Diagram of an individual grass flower or spikelet
Inside the glumes is the floret, which is the stamens and styles of each flower enclosed by two further scales or bracts, the lemma and the palea. You’re down to hand lens work now, but characteristics to look out for are nerves along the middle (or lack of nerves), awns (or lack of awns), hairiness or not, and colour.
Normally, there are three stamens bearing anthers per spikelet; these often hang out beyond the flower; look closely to find purple ones (Timothy grass and Meadow Foxtail), orange ones (Orange foxtail), white, or cream anthers (many of the Bromes). False oat grass Arrhenatherum elatius has bright yellow stamens.
False oat grass Arrhenatherum elatius with yellow stamens and Meadow Foxtail Alopocerus pratensis with purple ones
Some grasses put out lateral shoots, sometimes at quite a distance from the main plant. These are known as tillers, and grow from horizontal rhizomes, or root-like stems which grow along the ground. Grasses can rapidly colonise new habitats with this vegetative form of growth.
Tiller and rhizomes, shown on the Rough meadow grass Poa trivialis
Identifying grasses species: Features to look out for
Habit and habitat
What area is the grass growing in? Is the ground wet or dry? Calcareous or acidic? Disturbed? What season is it?
What shape and height is the plant? Is it erect, tufted, or droopy? Likewise, are the panicles tight or drooping, compact or loose, many branched or not?
Does it have rhizomes and tillers?
How long and how wide are the leaves? Are they hairy or smooth? Flat or inrolled and bristle-like? What colour are they?
Fold the leaf blade back from the stem and find the ligule. Look for its size, shape, edge, presence…
How are these arranged on the stem? How big are they? What colour? What texture?
Compare the size of the 2 glume scales, the number of nerves, awns or not, hairy or not. Are the palea and lemma awned or not? How many nerves do they have?
Spikelets of False oat grass Arrhenatherum elatius and Common Oat grass Avena fatua
If you’re interested in learning more about British and European grasses, there are some really good reference books out there. The “bible” of grasses is C.E. Hubbard’s Grasses; Colour Identification to the Grasses, Sedges, Rushes and Ferns of the British Isles by Francis Rose, Collins Guide to the Grasses, Sedges, Rushes and Ferns of Britain and Northern Europe by Fitter, Fitter and Farrer. You could also take a look at Collins Flower Guide by Streeter although it’s rather arrogant of me to suggest this as the grasses plates were all completed by me (with a great deal of help from David Streeter!)
I hope you'll give the grasses a chance, and end up loving them as much as I do, their beauty and diversity is mind-boggling.
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