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CPIE Notebook Project - Hawaiian Grasses 88888 Page 12

Digitate Inflorescences

Digitate means "finger-like" and a digitate inflorescence is one in which several racemes or spikes come from, or at least appear to come from, a common point at the top of the culm, somewhat like fingers (digits) on a hand. The number of spikes or racemes is given in the key as (for example): (1-)3-9, where the number in parentheses represents the uncommon extreme(s), and the count represents the typical range for the species. In some of these grasses, there may be one or more racemes or spikes located below the more crowded grouping forming the fingers of the flower head. In our digitate inflorescences the spikelets are tightly arranged in two rows along only one side of the rachis, an arrangement termed secund or unilateral.
A digitate inflorescence

Our most common grasses with spikes or racemes arranged digitately are:

  • Swollen fingergrass [46a] —extremely common medium size grass of roadsides and waste areas;
  • Bermuda grass [45a]—extremely common small grass in coastal locations and in lawns, especially in dry areas;
  • Wiregrass [44a]—small, coarse weed grass of lawns and disturbed places.

     

    Figure 12A. Digitate flowering head
    of Rhodes grass (Chloris gayana).

    sticker

  • ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    40a (26 37) Inflorescence of beach wiregrass Inflorescence an umbrella like arrangement of (1-)3-9 spikes. Each spike with two rows of downward-pointing bristled spikelets appearing like teeth on a comb. Rachis usually extending as a conspicuous point beyond last spikelets. Ligule a fimbriate membrane. { Widespread although typically not abundant where found; small running grass. Beach wiregrass. (Fig. 12B) [NAT]

    Dactyloctenium aegyptium (L.) Willd.

    Figure 12B. Beach wiregrass (Dactyloctenium aegyptium) inflorescence of four spikes.

    40b

    Inflorescence of spikes or racemes, with or without bristles, but not comb-like in appearance, the spikelets tending to lay flat against the rachis, the tip of which is hidden by uppermost spikelets

    [41]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    41a (40) Spikelets with awns (bristle-like narrow appendage arising near the upper end of one or more bracts of the spikelet) and at least as long as the supporting bract. (Note: true bristles are stiff hairs arising from the base of the spikelet, not from within the spikelet). Chloris spp. [45]
    41b

    Spikelets without awns or awns very short (less than length of the bracts surrounding the floret) and not conspicuous even with low-power magnification

    [42]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    42a (41)

    Inflorescence of 3 to 20 coarse spikes or racemes. Small to medium spreading or clumping grass

    [43]
    42b Inflorescence of 3 to 9 delicate, narrow spikes or racemes. Small grass spreading by stolons or rhizomes or both. [45]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    43a (42) Spikelets 3+ mm long. Small to medium clumping grass. [44]
    43b

    Spikelets 3 mm or less in length. Course, medium, spreading grass.

    [47]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    44a (43) Inflorescence digitate, but often with one raceme located on culm an inch or two below the 2-6 clustered racemes. Spikelets nearly 1/4 inch (5-7 mm) long, appearing coarsely toothed, but not awned and greatly overhanging the rachis. Culms expanded at base, conspicuously flattened. { Small, coarse, annual clumping grass, typically as a weed, prostrate in mowed lawns, but upright where not so disturbed. Wiregrass. [NAT]

    Eleusine indica (L.) Gaertn.
    44b

    Inflorescence of (5-)7-20 spikes. Spikelet with several short awns, the longest about as long again as a spikelet, the shortest very short and on the upper glume. { Medium perennial grass of open, disturbed areas, verges, and pastures. Rhodes grass. (Fig. 12A) [NAT]

    Chloris gayana Kunth
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    45a (42) Spikelets narrowly ovate with one or more awns. Leaf arrangement distinctly alternate. Some Chloris spp. [50]
    45b

    Spikelets ovate, broadly or not, but without awns. Leaf arrangement alternate or distichous (two-ranked; alternating blades coming off culm in two vertical rows on opposite sides of the axis)

    [46]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    46a (45) Bermuda grass leaves

    Inflorescence of usually 3-6(-9), stiff, thin spikes. Spikelets sessile, broadly ovate, compressed, without awns, falling without glumes and under 1/16 in (3.5-4.5 mm) long. Leaf arrangement distinctly distichous (Fig. 12C, left), especially on short, vegetative stalks. { Small (but quite variable in size), perennial grass spreading by stolons and rhizomes, widely used as a lawn grass. Bermuda grass, mānienie (Fig. 12D) [NAT]

    Cynodon dactylon (L.) Pers.
    46b

    Spikelets narrowly ovate, pediceled, pedicels unequal, spikelets falling at maturity with glumes. Leaf arrangement distinctly alternate. Some Digitaria spp.

    [36]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    47a (43) Culms soft, not woody. Inflorescence of 3-13 spikes in one whorl (sometimes two). African Bermuda grass. [NAT]

    Cynodon nlemfuensis Vanderyst
    47b

    Culms stiff, woody. Inflorescence of 5-17 spikes in one or (usually) two to five whorls. Giant star grass. [NAT]

    Cynodon aethiopicus Clayton & J.R. Harlan
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      SUPPLEMENTAL PHOTOS
    [CLICK ON THUMBNAIL TO OPEN AN ENLARGED IMAGE]

    thumbnail smudge
    Figure 12D. Scanned parts of Cynodon dactylon culm in flower (2 MB)


    ToC TABLE of CONTENTS  Grass inflorescence types INFORESCENCE TYPES  Grass Key Introduction INTRODUCTION  AECOS AECOS, Inc.


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