23 February 2022<\/span><\/p>\n
The grass spikelet is often the dispersal unit, disseminule, or seed unit for the grass plant. \u00a0Grass flowers lack the colorful, showy parts that would normally spark human curiosity towards identifying a plant. \u00a0The structures associated with a grass flower are usually bland shades of green, yellow, or brown. \u00a0Many people shy away from studying grasses because of their perceived difficulty to identify. \u00a0In the grass family there is considerable variation in the appearance of the structures surrounding the grass flower that can lead to confusion when attempting to make an identification; however, the ability to recognize and understand the placement of the structures within the spikelet can make identification at, subfamily, tribe, genus, and species level much easier. \u00a0Here the discussion will focus on the basic diagnostic features of the grass spikelet and their locations.<\/p>\n The grass flower is simple in structure, typically consisting of a pistil<\/span>, three stamens<\/span>, and two lodicules <\/span>(Figure 1<\/a>). \u00a0The pistil consists of an ovary<\/span>, containing a single ovule<\/span>, to which two styles<\/span> are attached at the apex, each supporting a feathery stigma<\/span>. \u00a0Grasses are wind pollinated and the feathery stigmas provide ample surface area for pollen grains<\/span> to attach. The long, thread-like filaments<\/span> of the stamens are attached at the base of the flower. \u00a0At the distal end of the filament is attached the pollen-producing anther<\/span>. \u00a0The tiny, hyaline<\/span>, scale-like lodicules are located at the base of the flower and are considered to be representative of the perianth<\/span> found in other flowering plants; grasses lack the showy petals and sepals found in many other flowering plants. The lodicules swell during anthesis<\/span>, forcing open structures surrounding the flower and exposing the stigmas for pollination<\/span>. \u00a0The parts of the grass flower that may be useful to identification of mature dispersal units may include the length and colour of the anthers (if present), the remnants of the style bases that may remain attached to the apex of the mature fruit, and the shape of the lodicules.<\/p>\n <\/a>.<\/p>\n Figure 1.<\/strong> Typical grass flower consisting of one pistil with a superior ovary, three stamens, and two lodicules. From Meyer, 2001.<\/p><\/div>\n The one-seeded indehiscent fruit<\/span> of a grass plant is called a caryopsis (grain)<\/span>\u00a0and it is derived from the superior ovary<\/span>. The ovule <\/span>is attached to the placenta <\/span>of the ovary wall by a short funiculus <\/span>(stalk) along the ventral suture of the ovary or at the ovary base. \u00a0This point of attachment is visible as either a line or a spot on the ventral <\/span>surface of the caryopsis and in grasses this area is called the hilum <\/span>(Figure 2<\/a>).<\/p>\n <\/a>.<\/p>\n Figure 2.<\/strong> Grass caryopses in ventral view showing different shapes of the grass hilum, in which the point of attachment of the mature seed (former ovule) within the fruit is attached by a short funiculus to the pericarp (former ovary wall). From left to right, Eragrostis curvula<\/em> (weeping lovegrass), Cenchrus echinatus<\/em> (southern sandbur), and Deschampsia danthonioides<\/em> (annual hair grass), in which the hilum is a small spot near the base of the caryopsis and Lolium perenne<\/em> (perennial ryegrass) and Elymus repens<\/em> (couchgrass or quackgrass) in which the hilum is long and linear. (H = hilum)<\/p><\/div>\n The outline of the embryo <\/span>can be seen on the dorsal <\/span>side of the caryopsis (Figure 3<\/a>).<\/p>\n <\/a>.<\/p>\n Figure 3.<\/strong> Grass caryopses in dorsal view showing different shapes and sizes of the embryo compared to the length of the caryopsis. The red line indicates the upper end of the embryo. In these examples, the ratio of embryo to caryopsis length ranges from about one-quarter the length of the caryopsis to equal to the length of the caryopsis. From left to right: Ventenata dubia<\/em> (soft ventgrass), Deschampsia cespitosa<\/em> (tufted hairgrass), Diplachne fusca<\/em> subsp. fascicularis<\/em> (bearded sprangletop), Setaria italica<\/em> (foxtail millet), Cenchrus spinifex<\/em> (coast sandbur), Zizania aquatica<\/em> (annual wildrice).<\/p><\/div>\n The embryonic root-shoot axis is attached to the single, shield-shaped, cotyledon <\/span>called the scutellum<\/span>, which is pressed up against the endosperm <\/span>and is modified to absorb nutrients from the endosperm during seed germination and seedling development (Figure 4<\/a>).<\/p>\n <\/a>.<\/p>\n Figure 4.<\/strong> Triticum aestivum<\/em> (common wheat) caryopsis. A – Dorsal view (top) and lateral view (bottom) of caryopsis. B \u2013 Longitudinal section of caryopsis exposing the embryo in lower right area of caryopsis and remaining portion of the caryopsis filled with solid endosperm. C \u2013 Close-up view of embryo showing the root-shoot axis immediately beneath the outer dorsal surface of the pericarp and the scutellum (modified cotyledon) compressed between the root-shoot axis and the endosperm.<\/p><\/div>\n Mature caryopses may be nearly round in cross-section or compressed (Figure 5<\/a>).<\/p>\n <\/a>.<\/p>\n Figure 5.<\/strong> Grass caryopsis compression. A \u2013 Lateral compression of caryopsis in Deschampsia danthonioides<\/em> (annual hair grass). Caryopsis shown from left to right in lateral, dorsal, and ventral views, and the cross-sectional view illustrated below. B \u2013 Dorsal-ventral compression of caryopsis in Diplachne fusca<\/em> subsp. fascicularis<\/em> (sprangletop). Caryopsis shown from left to right in dorsal and ventral views, and the cross-sectional view illustrated below. The green line indicates the point of cross-sectional slice. (E = embryo, EN = endosperm).<\/p><\/div>\n The shape and length of the hilum, length of the embryo relative to length of the caryopsis, and compression of the caryopsis can be useful diagnostic features (Figure 6<\/a>). Typically, the bulk of the mature caryopsis is filled with endosperm (Figure 4<\/a>). \u00a0During early development the endosperm is somewhat fluid in composition and as the caryopsis matures the endosperm may remain fluid or become semi-solid to solid depending on the species. In a study of mature caryopses of 169 grass genera, Terrell (1971) categorized the condition of endosperm as liquid to solid (Table 1<\/a>). \u00a0Liquid endosperm in mature caryopses can remain in the liquid state for up to 50 years (Dore, 1956; Terrell, 1971). The condition of endosperm in the mature caryopsis is a useful diagnostic character.<\/p>\n <\/a>.<\/p>\n Figure 6.<\/strong>\u00a0 Grass caryopses in dorsal view showing embryo, ventral view showing hilum, lateral view, and cross-sectional view (dorsal side up).\u00a0\u00a0 A \u2013 Diskisperma dubium<\/em>, green sprangletop; B \u2013 Eragrostis curvula<\/em>, weeping lovegrass; C \u2013 Digitaria sanguinalis<\/em>, crabgrass; D \u2013 Sporobolus airoides<\/em>, alkali sacaton; E \u2013 Cenchrus ciliaris<\/em>, hairy buffelgrass; F \u2013 Phleum pratense<\/em>, timothy; G \u2013 Poa trivialis<\/em>, rough bluegrass; H \u2013 Alopecurus arundinaceus<\/em>, creeping foxtail; I \u2013 Dactylis glomerata<\/em>, orchardgrass; J \u2013 Phalaris minor<\/em>, littleseed canarygrass; K \u2013 Chloris gayana<\/em>, rhodesgrass; L \u2013 Phalaris canariensis<\/em>, canarygrass; M \u2013 Cenchrus americanus<\/em>, pearl millet; N \u2013 Echinochloa crus-galli<\/em>, barnyardgrass; O \u2013 Cenchrus clandestinus<\/em>, kikuyugrass; P \u2013 Panicum miliaceum<\/em>, proso millet; Q \u2013 Sorghum bicolor<\/em>, sorghum; R \u2013 Panicum virgatum<\/em>, switchgrass; S \u2013 Oryza sativa<\/em>, rice; T \u2013 Secale cereale<\/em>, rye; U \u2013 Avena sativa<\/em>, oat; V \u2013 Triticum aestivum<\/em>, common wheat; W \u2013 Hordeum vulgare<\/em>, barley; X \u2013 Zizania aquatica<\/em>, wild rice; Y \u2013 Bromus diandrus <\/em>var. rigidus<\/em>, ripgut brome; Z \u2013 Nassella cernua<\/em>, nodding stipa; AA \u2013 Bromus inermis<\/em>, smooth brome; BB \u2013 Bouteloua eriopoda<\/em>, black grama; CC \u2013 Glyceria declinata<\/em>, waxy mannagrass; DD \u2013 Melica californica<\/em>, California melic; EE \u2013 Briza maxima<\/em>, big quakinggrass.<\/p><\/div>\n <\/a>Table 1.<\/strong> \u00a0Examples of endosperm condition in mature caryopses. Determination made by pressing on caryopses with sharp-pointed forceps (Terrell, 1971). Table from Meyer (2020).<\/p>\n![\"\"](\"https:\/\/seedidguide.idseed.org\/wp-content\/uploads\/2023\/01\/Grasses-4.jpg\")
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