Grass Disseminules Explained
Figure 1 is a generalized diagram of a portion of a grass inflorescence with various outer parts “opened” to reveal the inner structure. Most basic to sexual reproductive dispersal in grasses is the caryopsis, which is a combination fruit/seed structure (produced apomictically rather than sexually in a few species). The caryopsis is a dry, indehiscent, one-seeded fruit in which the seed is fused to the pericarp in the hilar region only. It is typically sheathed by two bracts known as the lemma (outermost or lowermost bract) and the palea (innermost or uppermost bract); together, this structure is called a floret. Fertile florets (i.e., those containing a caryopsis) are enclosed individually or in groups within other bracts, the outermost of which are known as the lower glume and, above and to the inside of that bract, the upper glume; together, this structure is called a spikelet. Spikelets occur in various arrangements along the flowering stalk(s) (i.e., rachis) to form the inflorescence. Other grass terminology is defined in greater detail in the text below or in the Glossary.
Using Figure 1 (below) as a guide, let's discuss what can constitute a grass disseminule.
Different disseminule types occur in grasses because species differ with respect to the location(s) and efficacy of their natural disarticulation (i.e., breakage) zones within the inflorescence. In a few taxa, these zones can be missing altogether and the resulting dispersal unit(s) depend upon the breakage of the inflorescence via external forces. When present, the disarticulation zones can occur above the glumes and between florets (or rarely within florets), below the glumes and between spikelets, or even lower on the floral axis, producing clusters of spikelets.
Most grass taxa produce one type of disseminule, this usually being a floret or a single-caryopsis-bearing spikelet (i.e., a spikelet containing one fertile floret and various sterile bracts, often including the lemmas and sometimes the paleas of sterile florets).
However, multiple disseminule types can be produced by species with multiple disarticulation zones (e.g., as in some species of Panicum in which disarticulation can occur below the glumes—producing spikelets as disseminules—or above the glumes and between florets—producing caryopsis-bearing florets as disseminules.) Also, multiple forms of the same disseminule can occur when outer bracts are lost after disarticulation, which is rare but does happen. Most florets tightly envelop their caryopses, but in some species (e.g., some Eragrostis), the caryopsis becomes more or less loose within the floret, especially when the disseminule retains only one of the two floret bracts (i.e., palea or lemma) as a result of the disarticulation process. Thus, the caryopsis alone can constitute one of the disseminule types for these taxa.
The Grass Key is designed to include all probable disseminule types for a given taxon.
In spite of the complexity among grass disseminules, certain groups can be visually recognized. Let's start with the rare case of a taxon that produces vegetatively reproductive bulblets instead of caryopses. For this tool, we are only concerned about Poa bulbosa ssp. vivipara, whose bulblets and clusters of bulblets resemble those shown in Figure 2.
Shown in Figures 3a–e are other grass disseminules in this tool that are distinctive and easy to recognize (at least to genus). They include compound structures such as those found in Hordeum as well as structures subtended and more or less covered by spiny involucres (the two species of Cenchrus in this tool) or clusters of bristles to form fascicles (as in some species of Pennisetum).
Part of the distinctive look of Hordeum disseminules is due to its fan-like arrangement of parts, including awns. An awn is a long, narrow bristle that is often barbed and attached to the dorsal sides of glumes or fertile lemmas in some species. While it is an important character for identifying some species, it is common enough among the grasses of this tool not to be diagnostic alone. However, other attached structures, such as abnormally-shaped sterile spikelets, pedicels or rachis segments, can be more or less diagnostic for some genera (e.g., Sorghum).
The naked caryopsis—without enclosing bracts—is recognized and distinguished from non-grass seeds and fruits by having a hilar scar (often appearing as a distinct dot or line) at the base on one side (the ventral side) of the fruit and the embryo (often wrinkled) at the base on the dorsal side (Figures 4, 5). The embryo is frequently visible in the lateral view as an irregular indentation along the caryopsis starting at the fruit's base and covering ca. 20-80% of the fruit's length towards the apex. Note that, depending on the species, a caryopsis may be more or less terete in cross-section, dorsiventrally compressed or laterally compressed (Figure 4).
In this tool, one grass (Eleusine indica) has a loose pericarp, such that the naked seed can be found as disseminule. Fortunately, it still has that characteristic look of a hilar scar near the base on one side and an embryo near the base on the opposite side (Figure 6).
When working with the various types of bracted disseminules, it is easiest to start by evaluating the apparent number of caryopsis-bearing units and the number of bracts per unit.
Single-unit disseminules can be—
• a fertile floret (i.e., caryopsis enclosed by the 2 floret bracts, although in some species—e.g., Eragrostis—only 1 bract remains sheathing the caryopsis after disarticulation)
• a fertile floret with extra bracts (usually sterile lemmas but definitely NOT glumes) that are attached basally (this occurs rarely—e.g., in Oryza and Phalaris—and is not always immediately distinguishable from a single-unit spikelet)
• a single-unit spikelet (i.e., has 4+ bracts including 1–2 glumes surrounding a single caryopsis)
• the basal remnant of a multiple-unit spikelet (i.e., glumes plus lowermost fertile floret, this happens in a few species where disarticulation zones occur both above and below the glumes, e.g., Schismus)
Multiple-unit disseminules can be—
• floret pairs (this occurs in various species where the usual disseminule is the floret and disarticulation has been incomplete, or in species lacking specific disarticulation zones such as Avena sativa)
• a grouping of at least 1 fertile floret and 1+ similar-looking, terminally-attached sterile florets that typically constitute the apex of a multiple-unit spikelet (this can occur in species where the usual disseminule is the fertile floret, especially when disarticulation does not normally occur between the sterile florets)
• the basal remnant of a multiple-unit spikelet (i.e., glumes plus 2+ attached fertile florets, this occurs in a few species where disarticulation zones occur both above and below the glumes, e.g., Schismus, and can occur in species lacking specific disarticulation zones such as Avena sativa)
• a cluster of spikelets (e.g., Hordeum; in some species the cluster is subtended by a leaf sheath and/or a whorl of hairs or bristles, e.g., Pennisetum, or is enclosed in a spiny cupule, e.g., Cenchrus)
As mentioned above, multiple disseminule types may occur for some taxa more or less naturally. In these cases, the multiple types are listed on the fact sheets under Primary Disseminule and are present on the taxon's icon in the key.
However, an even broader approach was taken towards what could constitute a grass disseminule during construction of the Grass Key. For example, even though the typical disseminule type for Polypogon interruptus is the spikelet, these spikelets are fragile enough that they are also included in the sections of the key limited to floret or caryopsis characters. This means that many of the taxa listed as having the caryopsis as a disseminule, in fact, rarely do so without having suffered sufficient damage to the original disseminule (e.g., spikelet or floret) to remove the outer bracts.
So, if your disseminule is a caryopsis and you go through the key and get down to a few possible taxon identifications, check each taxon's icon and the fact sheet to see whether or not the caryopsis is a typical disseminule for that taxon. This information may help you in making the correct identification.