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CPIE Notebook Project - Key to the Grasses of Hawai‘i Page 4

Grass Inflorescence Types

Grass flowers (called florets) are very small, too small for their detailed structure to be seen by the unaided eye. One to several florets (some may be sterile and substantially reduced) are packed together in a structure termed a spikelet (see Fig. 4A), which is the basic unit of the flower or seed head (termed an inflorescence; see also Page iv). Both the spikelet and the arrangement of spikelets on the inflorescence provide useful diagnostic characters for identifying grasses. So important is the inflorescence and details of its structure, that a grass plant not in flower could be very difficult to identify. Grass keys that encompass a limited number of species may rely on the structure of the leaf, particularly at the blade/sheath boundary called the collar (see Fig. 1A on page 1). However, this approach also requires magnification. Where our key presents a structural detail of the grass requiring magnification (using a dissecting microscope, magnifying lens, or lupe), the first lead of a couplet has a magnifying lens symbol: Magnifying Lens

The grass inflorescence is usually borne at the upper end of a stem called the culm or axis, above the leaves in one of three arrangement types: panicle, spike, or raceme. Spikelets attached not to an axis (termed a rachis in the inflorescence) but to side branches (rays or peduncles), form an inflorescence type termed a panicle. Most grass species have an inflorescence that is technically a panicle. Species described as having spikes or racemes, typically have a culm (the axis) with spicate or racemose branches, so the term panicle is used in our key to indicate a flowering structure that is clearly branched as in Fig. 4B (below, far left). You can read more about open panicle inflorescences on page 14. Excellent drawings are provided by this website: UC IPM Online.

Spikelets

Figure. 4A. Three spikelets of
perennial ryegrass (Lolium perenne), a species with prominent glumes.

 

types of panicles
drawing: panicle of racemes Figure 4B.
  Panicles (branched inflorescences)—
  (left) a much branched, open panicle;
  (center) a simple panicle with sessile
  spikelets (essentially a panicle of spikes);
  (right) a panicle of racemes.

A raceme is an unbranched inflorescence with spikelets attached by short stalks (pedicels) to the rachis. Spikelets that are sessile—attached directly to the rachis without pedicels or branches—form a spike (Fig. 4A). Because pedicels can be very short, the distinction between spikes and racemes is not always an easy one to determine in the field; on the other hand, this distinction is a technicality compensated for in our key by not requiring a decision where the pedicels are extremely short.

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20a (12) Job's tears inflorescences Grass plant with numerous separate inflorescences, some terminal (from end of culm) and others axillary (from a lower node, arising inside the leaf sheath); each inflorescence consisting of an ovoid (egg-shaped), 1/4 inch (5 to 15 mm) green, purple, or (at maturity) lusterous white or bluish, hard cupule on a stout peduncle and supporting two racemes: one male (with anthers) and one female. { A large grass with wide leaves, found typically in or very near streams. Job's tears, pū‘ohe‘ohe. (Figs. 4C, 4G, 4H) [NAT]
Coix lacryma-jobi L.

Figure 4C. Numerous inflorescences of Job's tears (Coix lacryma-jobi) showing cupules with attached staminate racemes. Only basal scar or rachis remain of the pistillate racemes. See also, Figs. 4G & 4H (at bottom of page).

20b

Inflorescence not having a fruit-like, hard cupule supporting two spikelets; not as above

[21]
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[NOTE: THREE CHOICES HERE]
21a (20) Poa anua On a single culm, the inflorescence is extensively branched (is a panicle, Fig. 4B, above, far left) the branches themselves branched, the spikelets attached to clearly visible stalks called pedicels, branches and spikelets typically spread open (that is, an open panicle); OR, if inflorescence not extensively branched, spikelets attached by pedicels, at least some exceeding the spikelets in length (Fig. 4D).

Figure 4D. Part of inflorescence of Poa anua showing 7 spikelets, each on a thin pedicel branching from the culm and each subtended by two glumes (as labeled on one spikelet).

[80]
21b

Inflorescence otherwise: spikelets attached directly to the axis (without or by very short pedicels); OR spikeletes arranged in spikes or racemes terminating or attached laterally to the culm (Fig. 4B, above, far right); OR a spike-like, closed panicle, spikelets and branches crowded, the branching not seen without magnification and removing spikelets

[22]
21c

Inflorescence absent; spreading growth extensive, but flowering heads cannot be located. (NOTE: a bunching grass without flowering head(s) is either immature (yet to flower) or has flowered and spikeletes have since dispersed; a dry culm/rachis may be evident).

[90]
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[NOTE: THREE CHOICES HERE]
22a (21) Inflorescence a more or less cylindrical head, consisting of a single, elongated cluster of sessile spikelets, that is, a spike; OR a cylindrical raceme or panicle that is spike-like due to dense clustering of spikelets and, if branches are present, these short and of the same general length, or variable in length but pressed close against (appressed to) the axis [23]
22b

Inflorescence consisting of two or more spikes or racemes as branches arranged along an otherwise unbranched culm (see Fig. 4B above, far right); OR the spikes or racemes arising more or less finger-like from a common point at the top of the culm; OR a spike or raceme solitary at top of culm

[24]
22c Kikuyu grass Inflorescence (upon very close examination) is a few-spikelet spike enclosed in the uppermost leaf sheath, so only filaments and/or stigmas are exposed. { forming dense growth from rhizomes and stolens in dry to mesic open areas. Popular pasture forage at higher elevations on Maui and Hawai‘i Islands. Kikuyu grass. [NAT]
Cenchrus clandestinus (Hochst. ex Choiv.) Morrone
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23a (22) & (86) Inflorescence cylindrical or cylindrical tapering upwards to a point (either a spike, raceme, or a closed panicle) and having stiff but soft bristles, giving it a "hairy" foxtail appearance

[44]
23b

Inflorescense more or less cylindrical: a spike or spike-like panicle. Spikelets either unarmed or armed with awns or spines, but lack soft bristles surounding each spikelet (not resembling a foxtail)

[46]
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24a (22) Inflorescence is a single spike or raceme, the spikelets attached along an elongate axis or crowded on the axis at top of culm (plant may have multiple culms)

[25]
24b Inflorescence of two or more spikes or racemes, either crowded finger-like near top (Fig. 4F, below) or spaced out along the culm (Fig. 4B above, two right-side drawings) [26]
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25a (24) Spikelets lacking awns (as in Fig. 4A above);OR awns are extended tips of lemmas or glumes and no longer than the supporting bract

[48]
25a Most spikelets on the single spike or raceme have awns that clearly exceed the spikelet in length

[27]
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26a (24) Inflorescence is two or more racemes or spikes spaced out along the culm (uppermost pair may attach close together)

[30]
26b example of a digitate flower head

Racemes or spikes more than two and mostly arising from a common point or very close together at the top of the culm (arrangement is finger-like or digitate)

Figure 4F. Feathery, purple, digitate spikes of Chloris barbata make this very common grass easy to spot.

[40]
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27a (25) Each spikelet with a (typically 2 to 3+ in) long, dark, twisted awn, these becoming tangled (pili) at maturity. { Medium, perennial clumping grass found in dry, undisturbed areas. Pili, twisted beardgrass. [IND or POL]
Heteropogon contortus (L.) P. Beauv. ex Roem. & Schult.
27b

Not as above

[46]

  SUPPLEMENTAL PHOTOS
[CLICK ON THUMBNAIL TO OPEN AN ENLARGED IMAGE]

thumbnail thumbnail Job's tears plate from Hitchcock, 1922 smudge
Figure 4E. Enlarge to see stigmas of Kikuyu grass (Cenchrus clandestinus) just protruding from the leaf sheath.

Figure 4G. The unusual inflorescences of Job's tears (Coix lacryma jobi)—various floral parts labeled.

Figure 4H. Plate from Hitchcock (1922) for Job's tears (Coix lacryma jobi).


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