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If the fruit is a caryopsis and is enclosed within two bracts, the lemma and palea, it belongs to the family Poaceae. The grass disseminule, however, may appear in different guises:
A grass disseminule may be any of the following:
- a caryopsis
- a single floret; floret disseminules may posses accessory structures such as a rachilla base
- several attached florets
- a single spikelet; spikelet disseminules may possess accessory structures such as a pedicel or rachis internode, or involucre of bristles
- a cluster of two or more spikelets
- a portion of the inflorescence, including one or more spikelets
The spikelet is the basic unit of the grass inflorescence (Figure 1). The grass flower matures into the fruit, called a caryopsis, which is enclosed by an inner bract, or palea, which is in turn enclosed by the lemma. The caryopsis with its lemma and palea is called a floret*. Florets are borne alternately along the rachilla. A spikelet may contain one to many florets. At the base of the rachilla are two empty bracts: the upper glume and the lower glume. Spikelets are borne along the inflorescence axis (the rachis).
A disseminule may be easily determined to be a grass if the bracts and structure shown above in Figure 1 are present and easily identifiable. In determining the parts of a grass disseminule, it is advisable to dissect it if possible, and examine from the "outside in", i.e., identify the caryopsis, and then determine the identity of the bracts enveloping it in sequence. While perianth parts may persist on some non-grass fruit disseminules, these would not be arranged nor occur in the same order, as in the grass spikelet. However, there is great variation in spikelet morphology, so that the structure of many grass spikelets diverges from the general structure described above.
One or both glumes or the palea may be reduced or absent (Figure 2A).
The glumes, lemma and palea vary individually with respect to their firmness; they may be hard or leathery, to papery, membranous, or translucent.
Some florets in a multi-floret spikelet may be sterile, lacking a caryopsis, and represented by an empty lemma and palea, or just a lemma (Figure 2B).
The rachilla internodes may be greatly reduced, so that the bracts are closely appressed and may be individually discernible only upon dissection (Figure 3).
Spikelets or groups of spikelets may be subtended by various structures such as hairs, bristles, spines, burrs or sterile branches, and are referred to as an involucre (Figure 4).
The caryopsis is commonly a dry, indehiscent, one-seeded fruit in which the seed is fused to the pericarp in the hilar region only. The grass caryopsis is characterized by two prominent surface features, which may help to distinguish it from true seeds (Figure 5). A caryopsis may be terete, or compressed dorsiventrally (as in Figure 5) or laterally.
1. The hilum of a grass caryopsis is visible on its exterior and represents the attachment of the funiculus of the ovule to the inner ovary wall (Figure 5A). The hilum may be round to oblong to linear, and extends from the base, along the midline of the ventral surface, regardless of compression.
2. The grass embryo is peripheral (contiguous with the testa), extending from the base along the midline of the dorsal surface. (Figure 5B). The embryo is usually visible in relief on the outer caryopsis dorsal surface, opposite the hilum, regardless of compression. The embryo may be broad or narrow, and varies in size. Although its appearance varies, the scutellum and embryonic axis of the embryo are generally discernible.
* The term 'floret' also commonly refers to the grass flower with its lemma and palea.
A good reference for information about spikelet modifications and morphology is:
Clark, L.G. and Pohl, R.W. 1996. Agnes Chase's First Book of Grasses; The Structure of Grasses Explained for Beginners, 4th ed. Smithsonian Institution Press, Washington, D.C., 127 pp.