Understanding grass morphology and identification is crucial for students of botany, agriculture, and ecology. Grasses, belonging to the family Poaceae, exhibit a diverse range of physical characteristics that allow for their precise identification. This comprehensive guide will break down the key morphological features of various grass species, aiding in accurate recognition and study.
Grass Morphology and Identification: A Foundational Overview
Accurate identification of grasses relies on observing specific traits across different plant parts. These include the habit (overall growth form), ligule, leaf blades, inflorescence (flower arrangement), spikelets, glumes, and florets. We will explore these features using examples from common genera like Brachiaria, Cynodon, Bromus, Lolium, and Paspalum.
Habit and General Growth Patterns
The habit describes the plant's growth form and root structure, which is a primary identification tool. Different species display unique habits:
- Perennial: Many grasses are perennial, meaning they live for more than two years. Examples include Brachiaria ruziziensis, Cynodon nlemfuensis, Bromus carinatus, and Paspalum notatum.
- Annual: Lolium multiflorum is an example of an annual grass, completing its life cycle within one growing season.
- Caespitose: Growing in dense clumps or tufts, seen in Brachiaria dictyoneura, Bromus catharticus, and Lolium multiflorum.
- Mat-forming: Spreading over the ground to create a dense cover, characteristic of Cynodon nlemfuensis, C. plectostachyus, C. dactylon, and Paspalum notatum.
Rhizomes (underground stems) and stolons (above-ground creeping stems) are critical for identification:
- Brachiaria ruziziensis has short rhizomes, while Cynodon nlemfuensis and C. plectostachyus lack rhizomes. Cynodon dactylon and Paspalum atratum have elongated rhizomes.
- Brachiaria humidicola, Cynodon nlemfuensis, C. plectostachyus, C. dactylon, and Brachiaria decumbens are characterized by the presence of stolons. Brachiaria dictyoneura lacks stolons.
Culms (stems) also vary in length, diameter, and growth direction:
- Brachiaria ruziziensis: decumbent, 50–100 cm long, 3–4 mm diam., rooting from lower nodes.
- Cynodon dactylon: geniculately ascending, 8–40 cm long, 0.5–1 mm diam.
- Bromus carinatus: 30–100 cm long, with culm-internodes that can be glabrous or pubescent distally.
Leaf Characteristics for Grass Identification
Leaves provide numerous diagnostic features, including the ligule and leaf blades.
Ligule: This is a small appendage at the junction of the leaf sheath and blade. It can be a fringe of hairs or a membrane.
- Fringe of hairs: Found in Brachiaria ruziziensis, B. dictyoneura, B. humidicola, B. brizantha, B. decumbens, and Paspalum notatum.
- Ciliolate membrane: Present in Cynodon nlemfuensis (0.3 mm long) and Cynodon dactylon (0.2–0.3 mm long).
- Eciliate membrane: Found in Cynodon plectostachyus (1 mm long), Bromus carinatus (1–3 mm long), B. lanatipes (1–2 mm long), B. catharticus, Lolium multiflorum, L. perenne, Paspalum dilatatum, P. plicatulum (2–3 mm long), and P. atratum.
Leaf Blades: Important traits include shape, dimensions, and surface texture.
- Shape: Lanceolate (Brachiaria ruziziensis, B. dictyoneura, B. brizantha, B. decumbens), linear (Brachiaria dictyoneura, B. humidicola, B. decumbens), or flat (Cynodon nlemfuensis, C. plectostachyus, Paspalum plicatulum).
- Dimensions: Brachiaria ruziziensis has blades 10–25 cm long, 11–15 mm wide. Cynodon dactylon has smaller blades, 1–12 cm long, 2–4 mm wide.
- Surface: Can be pilose (Brachiaria ruziziensis), scaberulous (Cynodon nlemfuensis, C. dactylon), scabrous (Cynodon plectostachyus, Bromus lanatipes), or glabrous.
- Margins: Cartilaginous in Brachiaria ruziziensis.
- Apex: Acuminate (Paspalum plicatulum) or attenuate (Paspalum atratum).
Inflorescence Structures in Grasses
The inflorescence is the arrangement of flowers, typically racemes or panicles in grasses.
- Racemes: Common in Brachiaria, Cynodon, and Paspalum species. They are often unilateral (flowers on one side) and borne along a central axis.
- Brachiaria ruziziensis: (3–)5–7(–9) racemes, 5–10 cm long, central axis 4–12 cm long, rhachis broadly winged.
- Cynodon nlemfuensis: 4–8(–13) racemes, 4–7(–10) cm long, straight or flexuous, rhachis flattened.
- Paspalum dilatatum: (2–)3–5(–11) racemes, 4–11 cm long, central axis 2–20 cm long.
- Panicle: Found in Bromus species and Lolium. A panicle is a branched inflorescence.
- Bromus carinatus: Open panicle, lanceolate, 10–25 cm long. Primary branches appressed or ascending.
- Bromus catharticus: Open panicle, 10–40 cm long, with flexuous branches.
Spikelet packing also varies:
- Adaxial, 2-rowed: Brachiaria ruziziensis, B. dictyoneura, B. humidicola, B. decumbens, Lolium multiflorum, L. perenne.
- Broadside to rhachis, 2-rowed: Cynodon nlemfuensis, C. plectostachyus, C. dactylon.
- Abaxial, 2-rowed: Paspalum notatum.
Detailed Analysis of Fertile Spikelets
Spikelets are the basic units of grass inflorescences, containing one or more florets.
- Composition: Many species, like Brachiaria and Cynodon, have 1 basal sterile floret and 1 fertile floret. Bromus and Lolium species have multiple fertile florets (e.g., Bromus carinatus has 4–12, Lolium multiflorum has 5–15).
- Shape and Compression:
- Elliptic, dorsally compressed: Brachiaria ruziziensis, B. dictyoneura, B. humidicola, B. brizantha, B. decumbens, Paspalum notatum, P. plicatulum, P. atratum.
- Ovate, laterally compressed: Cynodon nlemfuensis, C. plectostachyus, C. dactylon, Bromus catharticus.
- Oblong, laterally compressed: Bromus lanatipes, Lolium multiflorum, L. perenne.
- Length: Varies significantly, e.g., Cynodon dactylon (2–2.5 mm long) vs. Bromus carinatus (15–45 mm long).
- Disarticulation: How the spikelet breaks apart at maturity.
- Falling entire: Brachiaria and Paspalum species.
- Breaking up at maturity, disarticulating below each fertile floret: Cynodon, Bromus, and Lolium species.
Glumes and Their Identifying Characteristics
Glumes are sterile bracts at the base of the spikelet, offering key identification points.
- Dissimilar glumes: Brachiaria ruziziensis, B. brizantha, Bromus carinatus.
- Similar glumes: Brachiaria dictyoneura, B. humidicola.
- Lower glume absent or obscure: Lolium multiflorum, L. perenne, Paspalum notatum, P. dilatatum, P. plicatulum, P. atratum.
- Length relative to spikelet/floret: Varies greatly. For Brachiaria ruziziensis, the lower glume is 0.33–0.5 length of spikelet, while the upper glume is 1 length of spikelet.
- Texture and Keels: Can be membranous (Brachiaria, Cynodon, Bromus lanatipes), herbaceous (Cynodon, Bromus carinatus), cartilaginous (Paspalum notatum), or chartaceous (Bromus catharticus).
- Many glumes are without keels (Brachiaria, Bromus lanatipes), while others are 1-keeled (Cynodon, Bromus carinatus, B. catharticus).
- Venation: Number of veins varies (e.g., lower glume of Brachiaria ruziziensis is 11–13-veined; upper glume 7-veined).
- Apex: Obtuse (Brachiaria ruziziensis), acute (Cynodon nlemfuensis), or acuminate (Bromus carinatus).
Florets: The Reproductive Units
Florets are individual flowers within a spikelet, consisting of lemma, palea, and reproductive organs.
- Fertile Lemma: The outer bract enclosing the flower.
- Shape: Elliptic (Brachiaria ruziziensis, B. dictyoneura, B. humidicola, B. brizantha, B. decumbens), ovate (Cynodon species, Bromus carinatus, Paspalum species), oblong (Bromus lanatipes, B. catharticus, Lolium species).
- Length: Cynodon dactylon (2–2.5 mm long) vs. Bromus catharticus (12–20 mm long).
- Keel: Presence or absence of a keel (prominent ridge). Brachiaria and Bromus lanatipes have lemmas without keels, while Bromus catharticus has distinctly keeled lemmas.
- Surface: Papillose (Brachiaria ruziziensis), striate (Brachiaria decumbens, Paspalum dilatatum), granulose (Brachiaria brizantha), or scaberulous (Lolium multiflorum, Bromus catharticus).
- Apex: Obtuse (Brachiaria ruziziensis), acute (Cynodon species), apiculate (Brachiaria humidicola), dentate (Bromus carinatus, Lolium multiflorum), or lobed (Bromus catharticus).
- Awns: Bromus and Lolium multiflorum lemmas are awned (have a bristle-like appendage). Bromus carinatus has a subapical awn 3–15 mm long.
- Palea: The inner bract.
- Often involute and without keels (Brachiaria species, Paspalum species).
- Palea keels can be ciliolate (Bromus catharticus), scaberulous (Lolium species), or eciliate (Cynodon dactylon).
Flower Characteristics:
- Anthers: Brachiaria ruziziensis has 3 anthers, 3.5 mm long. Bromus lanatipes has 3 anthers, 2.5–4 mm long.
- Ovary: Often with a fleshy appendage above style insertion and pubescent on the apex (Bromus species).
Fruit and Seed Features
Fruit (caryopsis) characteristics are also useful for identification.
- Adherent pericarp: Present in Cynodon, Bromus, and Brachiaria decumbens.
- Hairy at apex: Bromus carinatus, B. lanatipes.
- Apex fleshy: Bromus carinatus.
- Hilum linear: Bromus carinatus, Lolium multiflorum, L. perenne.
Comparative Identification of Key Grass Genera
To further assist in grass morphology and identification, let's compare some closely related species:
Brachiaria Species Comparison
| Attribute | B. ruziziensis | B. dictyoneura | B. humidicola | B. brizantha | B. decumbens |
|---|---|---|---|---|---|
| Habit | Perennial, decumbent | Perennial, caespitose | Perennial, stoloniferous | Perennial, caespitose | Perennial, stoloniferous |
| Rhizomes | Short | Absent | Absent | Absent | Absent |
| Stolons | Present | Absent | Present | Absent | Present |
| Leaf Blades | Lanceolate | Linear/Lanceolate | Linear/Lanceolate | Linear/Lanceolate | Linear/Lanceolate |
| Infl. Racemes | (3–)5–7(–9) | 3–8(–12) | 2–3(–4) | (1–)2–16 | 2–7 |
| Spikelet Size | 4–5 mm long | 4–6 mm long | 4–6 mm long | 4–6 mm long | 4–5 mm long |
| Lower Glume | Oblate, 0.33–0.5 LS | Oblong, 0.75–1 LS | Oblong, 0.75–1 LS | Ovate, 0.33 LS | Ovate, 0.33–0.5 LS |
| Fertile Lemma | Papillose surface | Papillose surface | Papillose surface | Granulose surface | Smooth surface |
LS = Length of Spikelet
Cynodon Species Comparison (Bermuda, African Star, Santo Domingo Star)
| Attribute | C. dactylon (Bermuda) | C. nlemfuensis (Santo Domingo) | C. plectostachyus (African Star) |
|---|---|---|---|
| Habit | Perennial, mat forming, rhizomes elongated | Perennial, mat forming, no rhizomes | Perennial, mat forming, no rhizomes |
| Culm Length | 8–40 cm | 30–60 cm | 30–90 cm |
| Ligule | Ciliolate membrane, 0.2–0.3 mm | Ciliolate membrane, 0.3 mm | Eciliate membrane, 1 mm |
| Leaf Blades | Flat or conduplicate, 1–12 cm long, 2–4 mm wide | Flat, 5–16 cm long, 2–6 mm wide | Flat, 10–30 cm long, 4–7 mm wide |
| Infl. Racemes | 4–6, 1.5–6(–8) cm long | 4–8(–13), 4–7(–10) cm long | 7–20, 3–7 cm long, in whorls |
| Spikelet Length | 2–2.5 mm | 2–3 mm | 2.5–3 mm |
| Fertile Lemma | Ovate, 2–2.5 mm, midvein pubescent | Ovate, 2–3 mm, midvein pubescent or ciliate | Ovate, 2.5–3 mm, midvein ciliate |
Bromus Species Comparison
| Attribute | B. carinatus | B. lanatipes | B. catharticus |
|---|---|---|---|
| Habit | Perennial, short-lived | Perennial | Perennial, caespitose |
| Culms | 30–100 cm | 40–90 cm | 80–100 cm |
| Butt Sheaths | Not woolly | Woolly | Not woolly (pubescent leaf-sheaths) |
| Ligule | Eciliate membrane, 1–3 mm | Eciliate membrane, 1–2 mm | Eciliate membrane |
| Inflorescence | Open panicle, lanceolate | Open panicle | Open panicle |
| Spikelets | Elliptic, 15–45 mm, 4–12 florets | Oblong, 10–30 mm, 6–8 florets | Ovate, 16–40 mm, 6–12 florets |
| Lower Glume | Lanceolate, 5–9 mm, 3-veined | Lanceolate, 1(–3)-veined | Lanceolate, 10–15 mm, 5–9-veined |
| Fertile Lemma | Ovate, 11–15 mm, 7-veined, dentate, 1-awned (3–15 mm awn) | Oblong, 8–11 mm, 5-veined, awned (2–4 mm awn) | Oblong, 12–20 mm, 9–13-veined, keeled, lobed, 1-awned (1–9 mm awn) |
Lolium Species Comparison
| Attribute | L. multiflorum | L. perenne |
|---|---|---|
| Habit | Annual, caespitose | Perennial, caespitose |
| Culms | 30–90 cm | 30–90 cm |
| Ligule | Eciliate membrane, 1–2 mm | Eciliate membrane |
| Leaf-blades | 6–25 cm long, 4–10 mm wide | 3–20 cm long, 2–6 mm wide |
| Spikelets | 7.5–25 mm, 5–15 florets | 7–20 mm, 4–14 florets |
| Fertile Lemma | Oblong, 5–8 mm, 5-veined, dentate, 1-awned (5–10 mm awn) | Oblong, 5–7 mm, 5-veined, obtuse, unawned |
Paspalum Species Comparison
| Attribute | P. notatum | P. dilatatum | P. plicatulum | P. atratum |
|---|---|---|---|---|
| Habit | Perennial, mat forming | Perennial, caespitose | Perennial, caespitose | Perennial |
| Rhizomes | Elongated | Absent | Short | Elongated |
| Culm Length | 15–50 cm | 40–180 cm | 30–120 cm | 100–150 cm |
| Ligule | Fringe of hairs | Eciliate membrane | Eciliate membrane, 2–3 mm | Eciliate membrane |
| Infl. Racemes | 2(–3), unilateral | (2–)3–5(–11), unilateral | 2–7, unilateral | 10–16, unilateral |
| Spikelet Size | 2.5–3.8 mm | 2.8–3.8 mm | 2.3–3.2 mm | 2–3 mm |
| Upper Glume | Cartilaginous | Membranous, pilose | Membranous, glabrous/puberulous | Membranous, glabrous/puberulous |
| Fertile Lemma | Ovate, 2–3 mm, striate | Ovate, 2.5–3 mm, striate | Ovate, 2–3 mm, granulose, dark brown | Elliptic/Obovate, 2–3 mm, granulose |
Frequently Asked Questions (FAQ) about Grass Morphology and Identification
What are the main parts of a grass plant used for identification?
The main parts used for grass identification are the habit (overall growth), ligule (at leaf sheath/blade junction), leaf blades (shape, size, surface), inflorescence (raceme or panicle type), spikelets (structure, compression, size), glumes (sterile bracts), and florets (lemma, palea, reproductive organs). Detailed examination of these features helps distinguish species.
How do rhizomes and stolons help in grass identification?
Rhizomes (underground stems) and stolons (above-ground creeping stems) are crucial for identifying grass species. Their presence, absence, and characteristics (e.g., elongated rhizomes in Cynodon dactylon, stolons in Brachiaria humidicola) indicate the plant's spread and growth strategy, aiding in differentiation, especially in mat-forming grasses.
What is the difference between a raceme and a panicle inflorescence in grasses?
A raceme is a type of inflorescence where individual spikelets are directly attached to a central axis or stalk, often arranged unilaterally. Examples include Brachiaria and Cynodon species. A panicle, on the other hand, is a more complex, branched inflorescence where the main axis bears branches, and these branches then bear the spikelets. Bromus and Lolium species typically exhibit panicles.
What are glumes and why are they important for grass identification?
Glumes are sterile bracts located at the base of a grass spikelet. They are important for grass identification because they vary in size, shape, texture, venation, and whether they are dissimilar or similar, or even absent (like the lower glume in Lolium and Paspalum). Their characteristics provide specific clues for distinguishing between different grass genera and species.
What does 'disarticulating below each fertile floret' mean in grass morphology?
'Disarticulating below each fertile floret' describes how a spikelet breaks apart at maturity. This means that after the seeds ripen, the spikelet breaks above the glumes and between each individual fertile floret. This specific mode of breaking is characteristic of genera like Cynodon, Bromus, and Lolium, differentiating them from species where the entire spikelet falls as a unit, such as Brachiaria and Paspalum.