Pygmy pocket moss (Fissidens exilis) COSEWIC assessment and status report: chapter 3

Species Information

Name and classification

The moss Fissidens exilis Hedw. belongs to the family Fissidentaceae and order Fissidentales. Within the genus, it belongs to section Aloma Müll. Hal. (Beever 1999). The genus name means “split-tooth,” referring to the teeth surrounding the open end of the spore-bearing capsule, while “exilis” means small, or slender.

Synonyms of F. exilis include F. bloxamii Wilson (Steere 1950), Bryum viridulum Dicks., Dicranum exile (Hedw.) Muhl., Schistophyllum exile (Hedw.) Lindb., Skitophyllum exile (Hedw.) Bach. Pyl., and Hypnum minutum Wilson (Missouri Botanical Garden 2002). The species has also been classified as a variety of F. bryoides, F. viridulus, and Dicranum palmatum.

Description

Steere (1950) provides the most thorough description of Fissidens exilis, but it is also described and illustrated well by Crum and Anderson (1981). These descriptions are summarized below. Illustrations from Steere (1950) are reproduced in Figure 1. Definitions for technical terms used in the description below may be found in Crum and Anderson (1981) or in Magill (1990).

General:Leafy plants of F. exilis are dark green to dark brown in colour. They are minute, growing 1.0 – 2.0 mm high, and are scattered or gregarious (growing together, but not closely enough to form a turf). They may be mixed with other species of Fissidens (Molnar 1975, Steere 1950). Plants are unbranched and erect or prostrate at their bases. Persistent, abundant protonemata (filamentous, chlorophyllose precursors to recognizable leafy moss plants) are found with the plants and independently. The colour and size of F. exilis makes it very difficult to spot in the field, and impossible to determine with certainty without the aid of a microscope. It should be noted, however, that other small species of Fissidens (e.g. F. bryoides and F. taxifolius are 8 mm tall or less) have been collected abundantly in southern Ontario (Ireland & Ley 1992).

Leaves: Leaves are crowded in a single plane at the apex of the stems, arranged in 2-4 pairs of variable size with the largest pair (1 – 2.0 mm long) occurring at the tip. Leaves are oblong-lanceolate and curved with acute or obtuse and apiculate apices.

The distichous (in two rows) leaves of the genus Fissidens display characteristic morphology: the lower portion of the adaxial (inner) side of each leaf is doubled, forming two ‘vaginant laminae’ that generally envelop (hence the name ‘pocket moss’) the stem and the bases of the leaves above (Figure 1). The abaxial (outer) side of each leaf is known as the ‘dorsal

Footnote1Figure 1. Illustrations of Fissidens exilis from Steere (1950)

  • A. Habit of leafy plant bearing sporophyte, x21.
  • B-C. Detail of gametophyte plants, x33.
  • D. Single leaf, x63, showing characteristic Fissidens morphology:
    • a. vaginant lamina
    • b. dorsal lamina
    • c. apical lamina
    • d. costa
  • E. Areolation (cell pattern) of margin of vaginant lamina to show intramarginal border, x408.
  • F. Cell detail of apex of vaginant lamina, at junction with costa, x408.
  • G. Cell detail of apical lamina, from margin towards costa, x 408.
  • H. Cell detail of leaf apex, x 408.
  • I. Detail of capsule, showing peristome, x 63.

Figure 1. Illustrations of Fissidens exilis from Steere (1950)

Lamina,’ while the top (undoubled) part of the adaxial side of the leaf is called the ‘apical lamina.’ In Fissidens exilis, the vaginant laminae, one of which is tapered to the stout costa (leaf midrib) at its tip, measure about one half of the leaf length, and are not joined distally, except at the costa. The dorsal lamina does not reach the leaf base.

The costa reaches to within a few cells of the leaf apex. Leaf margins are subentire or appear shallowly toothed because of projecting cells above and appear particularly dentate in the vaginant laminae.

Perigonial leaves (leaves of the male inflorescence) consist largely of vaginant laminae with the apical lamina represented by a short or long spine. Leaf margins are conspicuously toothed in appearance as a result of projecting cell tips. Perichaetial leaves (leaves of the female inflorescence) are much larger than the vegetative leaves, but are otherwise similar to them in appearance.

Leaf cells: Cells of F. exilis are smooth with thick, colourless walls. Upper leaf cells (apical and dorsal laminae) are isodiametrically polygonal, measuring 8 – 15 μm in diameter. Cells of the vaginant laminae are larger and longer, and less regular in shape. A few marginal rows of cells have thickened cell walls, and the basal cells, which are much elongated, extend upward in most leaves to form an intramarginal (two-three rows of cells in from the leaf margin) border.

Seta: The seta (stalk supporting the capsule) is pale, becoming reddish with age. It varies in length from 2 – 5 mm.

Capsule: The capsule, which contains the spores, is erect and oval or short-cylindric, measuring 0.5 – 0.8 mm in length. Once the operculum (lid of the capsule) has fallen off, the capsule constricts below its mouth. The operculum is conic-rostrate with a straight or curved beak covered early in development by a small calyptra. True to the name Fissidens, meaning ‘split-tooth,’ the (red) peristome teeth (surrounding the capsule mouth) are divided at their tips.

Sexuality: F. exilis is rhizautoicous, meaning that the male perigonia (containing the sperm-producing antheridia) occur on protonemata buried in mud at the base of leafy plants that bear the female perichaetia (containing the egg-producing archegonia). The perigonia are often buried in the soil at the bases of the leafy stems and may appear to be dissociated from the plants, a condition that led Steere (1950) to describe F. exilis as dioicous (perigonia and perichaetia occur on separate plants).

Steere (1950) notes some similarity between Fissidens exilis and F. pauperculus, F. closteri, and F. pellucidus, but suggests that “the peculiarly differentiated margin of incrassate and elongated cells” readily distinguishes F. exilis. Steere also suggests that the examination of herbarium material of F. minutulus, F. viridulus, and F. bryoides may result in the discovery of new populations of F. exilis.

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