capsule [ KAP-suhl ] noun: a dry, dehiscent, simple fruit developed from several carpels
In botany, fruits are the ripened, seed-bearing ovaries of flowers. The most common type of fruit, the capsule, differs widely in its structure. Like other simple fruits, a capsule develops from a single ovary. A capsule reaches maturity when its walls begin to dry. When ripe, it dehisces (splits open) to release its seeds. Dehiscence likely results from the tension generated when cells within the wall shrink at different rates during the drying process. (Day, 2000)
However, unlike a follicle or a legume, each of which has a single locule or loculus (cavity or chamber) and splits open along one or two sutures (seams) respectively, a capsule has several locules and can split apart in different ways. For example, a capsule may split along seams from top to bottom into separate segments called valves, or it may dehisce around its circumference or by teeth or by localized pores, or it may break apart by irregular cracks or by the growing force of developing seeds.
Capsules are categorized by how the fruits split open. However, it is important to understand that botanists do not agree on either the standards or terminology for recognizing fruit types, nor on how the terms are defined, so there may be disagreement over how to categorize the fruit of a particular plant species. Some of those inconsistencies are noted here.
Some commonly recognized capsule types are:
Loculicidal [ lok-yuh-luh–SAHYD-l ] adjective: (of a capsule) splitting longitudinally along the center of the locule. The split occurs along what is sometimes called the midrib or dorsal suture. Pictured below (left to right) are examples of immature and ripe fruits for species in nine of the 86+ plant families with loculicidal capsules: Aesculus pavia (red buckeye), Allium tuberosum (garlic chives), Canna indica (canna lily), Euonymous americanus (strawberry bush), Hemerocallis spp. (daylilies), Hibiscus coccineus (scarlet rose mallow), Impatiens capensis (orange jewelweed), Iris sibirica (Siberian iris), and Lilium ‘Stargazer’ (hybrid Oriental lily). Seed dispersal for most of these plants happens when the capsules disintegrate or topple over, spilling their seeds, or the seeds are dislodged by the wind or carried away or eaten by animals. An exception in these examples is orange jewelweed whose capsules exhibit explosive dehiscence, releasing stored elastic energy to disperse the seeds. When a ripe capsule is touched, the five elongated valves tightly coil inwards, ejecting the seeds up to six feet away, hence another of its common names: touch-me-not.
Septicidal [ sep-tuh–SAHYD-l ] adjective: (of a capsule) splitting longitudinally along or through the septum (the seam or partition separating two locules). Pictured below (left to right) are examples of both immature and ripe fruits for species in four of the 35+ plant families with septicidal capsules: Chelone (turtlehead), Datura inoxia (angel’s trumpet), Gelsemium sempervirens (Carolina jessamine), and Yucca filamentosa (curlyleaf yucca).
Septifragal [ sep-TIF-ruh-guhl ] adjective: (of a capsule) breaking away of the valves from the septa, leaving a central column (also known as a columella, Spjut, 2003–2015). Oenothera and Rhododendron are in two of the 15+ plant families that exhibit septifragal dehiscence. Some may describe Oenothera capsules as loculicidal and Rhododendron capsules as septicidal and that is how their capsules split open before separating from the septa and leaving a central column as pictured below. For example, the capsules of Oenothera can be more precisely described as dorsally-septifragal (Odintsova, 2016).
Pyxis [ PIK-sis ] noun, plural pyxides [ PIK-si-deez ]: a fruit capsule with circumscissile dehiscence in which it splits horizontally along the circumference so that the top or lid pops off. Native Jeffersonia diphylla (twinleaf) and Plantago (plantain) are examples in two of the 17+ plant families with pyxides.
Denticidal [ den-tuh–SAHYD-l ] adjective: (of a capsule) opening at small teeth at the top of the capsule. An example is native Cerastium arvense (field chickweed).
Poricidal [ pawr-uh–SAHYD-l , pohr- ] adjective: (of a capsule) opening at localized pores or holes. Examples are native Triodanis biflora (small Venus’ looking-glass) and Papaver orientale (Oriental poppy). Although Papaver is a commonly cited example of poricidal dehiscence, there are some sources that cite it as an example of a ceratium “with seeds attached to an axile partition, or if seeds parietal then without partition” (Spjut, 2003–2015.) or as an operculate rather than a poricidal capsule. When the wind shakes ripe poppy capsules, they can disperse their seeds through the opened pores like a salt or pepper shaker, with the seeds falling near their parent plant creating the opportunity for a colony to form around it.
Silicle [ SIL-i-kuhl ] noun; Silique [ suh–LEEK, SIL-ik ] noun: dry, dehiscent fruits of the Brassicaceae (also known as the mustards, crucifers, or cabbage family) that split open into two valves leaving a persistent central partition that bears the seeds. A silicle has nearly equal length and width while a silique is usually more than three times as long as it is wide. Native Lepidium virginicum (Virginia pepperweed) and invasive Lunaria annua (money plant) are plants with silicles. Crops harvested before they bolt and produce flowers and siliques include broccoli, cabbage, and cauliflower. Cardamine hirsuta (hairy bittercress) has siliques that split open not as they dry, as most capsules do, but rather by using cellular growth in the outer wall of the valves to contract (Mosca et al., 2023). Pressure builds to a point at which the siliques explode with the valves coiling rapidly outward discharging seeds up to a distance of 16 feet. Note that flora may describe silicles and siliques variously as capsules or simply as dry, dehiscent fruits.
Left to right top row: immature and dehisced septifragal capsules of Oenothera biennis and Rhododendron and immature and dehisced pyxides of Jeffersonia diphylla and Plantago; Left to right bottom row: flowers and dehisced denticidal capsule of Cerastium arvense; immature and dehisced poricidal capsules of Triodanis biflora and Papaver orientale; immature and dehisced silicles of Lepidium virginicum and Lunaria annua; and immature and dehisced siliques of Cardamine hirsuta.
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