Bryophyta

Diversity of mosses has been classified in approximately 10,000 species, 700 genera, and about 110-120 families. This places the mosses as the third most diverse group of land plants, only after the angiosperms and ferns. Mosses are small plants requiring stereoscopes and compound microscopes for routine examination. The conspicuous green leafy shoots are the gametophytes, haploid organisms, on which the diploid embryo develops into a mature sporophyte (Figure 1). The sporophyte is chlorophyllose and photosynthetic only in early stages of development, and it is mostly dependent on the gametophyte. Moss colonies are a very important element in many ecosystems, from the tundra to the tropical rain forest, reducing soil erosion, capturing water and nutrients, providing shelter for microfauna, and nurseries for seedlings in succession or regeneration processes.

 As a lineage, mosses are a historically crucial group in the understanding of the transition to life on land. The green leafy shoots (gametophytes) retain some features of the green algal ancestors (chlorophylls a and b, starch, sperm with two forward undulipodia), but the needle-like shoots that produce the spores (sporophytes) display key innovations for the life outside water, such as stomates, a simple strand of conductive cells [in an unbranched sporophyte], and airborne spores produced in a single apical capsule (sporangium). This is the simplest structural level among all land plants. The next organizational level is found in two fossil groups: Horneophythopsida and Aglaophyton (Rhynia) major, where the sporophyte is branched and produces several sporangia. The sporophyte shows the most complex structural organization in the tracheophytes.

There is no controversy that the mosses are monophyletic. Synapomorphies for the mosses are: i) leaves in the gametophyte; these green laminar organs are attached densely along the shoot. ii) multicellular rhizoids; these branched filaments composed of a series of multiple cells develop from the surface of the gametophyte axis at the point of contact with the substrate. iii) columnella; this is a cylinder of sterile cells located in the center of the capsule. (Mishler & Churchill, 1984). Other possible synapomorphies are features of male gamete [spermatozoid] ultrastructure (Mishler, Lewis & et al 1994).

Figure 1. Life cycle of a dioicous moss.
Modified from original drawings by Ivy Livingstone. © 1997 BIODIDAC Modified from original drawings by Ivy Livingstone.

The life cycle of a moss alternates a conspicuous gametophyte generation and a dependent sporophyte generation. Some of the salient features of the different life cycle stages are outlined below.

Protonema. Spores germinate and produce a protonema. This is usually filamentous and branched, but in some groups it is tallose or massive. At several places in the protonema, apical cells differentiate and produce the foliose shoots.

Gametophore. An apical cell produces a stem and leaves spirally arranged. The stems produce branches in several combinations of monopodial and sympodial architectures. Leaves are sessile, unlobed, and often with a thickened midrib. Other appendages to the stem are multicellular rhizoids, axillary hairs, paraphyllia, pseudoparaphyllia, and various types of asexual propagules. The patterns of the leaf cell network and leaf cell papillae provide numerous characters for the systematic arrangement of genera and species.

Gametangia. Archegonia and antheridia are produced in groups, with paraphyses among them, and surrounded by perichaetial or perigonial leaves. Autoicous, synoicous, paroicous, heteroicous, and dioicous sexual conditions are found in many taxa.

Seta. The sporophyte consists of the foot, a seta and an apical sporangium. The foot is embedded within the apex of a stem or branch. Ultrastructural details of the transfer zone provide important similarities with other land plants. The seta is short or elongate. It has an internal conductive system that connects the foot and the capsule at both extremes.

Sporangium. A single sporangium or capsule develops distally from an unbranched sporophyte. The sporangium opens by an apical pore, longitudinal splits or most commonly by an operculum. The external cell layer (exothecium) often has stomata, especially in the neck. Underlaying the exothecium there are parenchymatose cells. Both concentric layers constitute the amphithecium (Figure 2A). Internally, the endothecium consists of a cylinder of sporogenous tissue, surrounding a columnella of sterile cells. Moss spores are unicellular, sometimes retaining tetrad marks.

Figure 2. The two basic peristome types of mosses.
A. Cross section of an embryonic sporophyte capsule.
B. Nematodontous peristome (e.g., Polytrichum).
C. Arthrodontous peristome (e.g., Funaria or Bryum).
Drawings copyright © 2000 Efrain De Luna

Peristome. In the majority of mosses, the apex of the capsule (operculum) falls off at maturity and reveals a structure called the peristome (Figure 1). This is a ring of narrow triangular segments surrounding the mouth of the capsule. Changes in moisture conditions cause movements of the peristome and facilitate the dispersion of spores in favorable dry conditions. Two basic types of peristomes are found in mosses: arthrodontous and nematodontous. In arthrodontous peristomes, at the level of the capsule mouth and above, three innermost rings of cells of the amphithecium are involved in the formation of teeth in most taxa. These three concentric rows are known as the "outer", "primary" and "inner" peristomial layers (OPL, PPL, IPL). Most peristomate mosses have the arthrodontous type, in which each tooth is composed of periclinal (tangential) cell wall remnants between two of the three concentric peristomial cell layers (Figure 2C). If the teeth are formed by the tangential walls between the OPL and PPL, the row of teeth is collectively known as the exostome. In the second case, the cell wall remnants are located between the cell rings of the PPL and IPL, therefore the row of segments is known as endostome. A second fundamental type of peristome is nematodontous, structured by narrow columns of entire cell wall remnants. Each tooth consists of agglomerated cylinders formed by the periclinal and anticlinal thickened cell walls (Figure 2B). As in arthrodontous peristomes, peristomial cells derive from the innermost amphithecium, but multiple concentric peristomial layers (four to seven) contribute to the formation of a nematodontous tooth.

The World of Mosses - Ontario, Canada
British Mosses and Liverworts - A Field Guide - UK
Identifying Australian Rainforest Plants,Trees and Fungi: Moss - Australia
The Hidden Forest: Mosses - New Zealand

In Great Britain and/or Ireland:
Plant / associate
fruitbody of Dictyonema interruptum is associated with gametophyte of Bryophyta

Plant / associate
fruitbody of Loreleia postii is associated with Bryophyta