Wonderful documentary film about mosses made by the "Core Facility Cell Imaging and Ultrastructure Research" of the University of Vienna (link. Source: Pogöschnik G., Zechmeister H., Lichtscheidl I.K., 2011: The Kingdom of Plants: Mosses. DVD-Video, in German, but the images are self-explaining)
Mosses are simple, green little land plants, which are considered as "lower plants". Although like some algae (see: Laminaria) they show a clear differentiation, the architecture of the body, called thallus, is more primitive than in "higher plants", like ferns, Gymnosperms and flowering plants. Both leaf-like and stem-like structures are seen, but distinct water-conducting structures like a vascular system with lignified xylem vessels and phloem lack. The most important adaptation to land life which mosses have in common with other land plants is the presence of a thin cuticula that protects them against loss of water. Single-cell and multicellular rhizoids resembling root hairs can be found. These serve for anchorage to the substrate and water uptake, but they are no true roots. The uptake of water and nutrients can occur over the entire thallus. The exchange of CO2 and O2 happens mainly by diffusion (for example through the large surface of lamellae on the leaflets), also through pores occur in Liverworts and through stomata on the capsule of the sporophyte of Hornworts and True Mosses. But the 'leafy' part of the gametophyte lacks stomata.
The same goes for hornworts, but not for liverworts, which completely lack stomata.Due to the absence of roots and an efficient vascular system mosses can only thrive in humid environments without too much sunshine. They remain small in size, hardly ever exceeding 15 cm. Yet, they can survive periods of drought, even periods of complete desiccation. Moss have no flowers or reproductive cones, nor fruits and seeds. Their dispersal goes through haploid spores. Many mosses can follow asexual propagation through fragmentation of the thallus and formation of specific, multicellular bodies, called gemmae.
Life cycle of mosses
Example of the life cycle in mosses
A Polytrichum, antheridial antheridial cups in cross-section with 'leaflets' at the edge (zoom)
B Detail of 'leaflet" of Polytrichium, with 1 lamellae and 2 costa (zoom)
C Detail of the antheridium in Polytrichium (zoom)
D Mature spore capsule of moss with spores (zoom)
E Archegonium of a moss with egg cell (1) and neck(2) (zoom)
Stereo-projection of a leaflet of Polytrichum made with a confocal laser scanning microscope. Depth is visible with red-green glasses, preferably with red right
Gametophyte and sporophyte in mosses
Atrichum Left view: moss plantlets, middle view: gametophyte, right view: -old- sporophyte consisting of seta and capsule; the sporophyte has developed on top of the female gametophyte; photos Stefan Vriend.
The life cycles of all mosses (Liverworts, Hornworts and Bryophytes) are in principle similar. All mosses are haplo-diplonts: haploid (n) and diploid (2n) life forms alternate and in each phase mitotic divisions occur. Only the haploid, dominant gametophyte can survive for longer periods and is found extensively in the field. The fertilization requires the presence of water drops so that antherozoa, the male motile gametes, can reach the ovuli with help of their flagella, and fertilization can take place. Diploid (2n) cells arising from the zygote continue to divide to form a small sporophyte (2n). The sporophyte consists of a stalk (called seta) and a capsule, remains attached to the gametophyte, living in depency with it. In the sporophyte meioses occur leading to the formation of haploid (n) spores. (The sporophyte is thus the only diploid phase of the entire life cycle in mosses).The spores are dispersed by the wind and germinate to filamentous protonemata. From these structures one or more new thalli can grow, closing the circle.
Classification of the mosses
Mosses are the most primitive land plants. All mosses used to be regrouped under the phylum Bryophyta. The Bryophyta were divided in three groups, one of them being the True mosses, named as the Musci. On account of the differentiation of the thallus and the sporangia in particular, a new classification has been proposed recently, in which the Bryophyta are a su-bgroup of the Mosses. According to this most recent insight, the following groups are distinguished:
Hepatophyta (= Liverworts)
Anthocerotophyta (= Hornworts)
Bryophytasensu stricto (= True mosses)
Spore capsule in mosses
Spore capsule of A a Liverwort and B a True moss, C seta, capsule and peritome in Undulate atrichum moss (Atrichum undulata)
Liverworts. Example: Marchantia The most primitive Mosses are the Liverworts with their flat thallus that lay on the ground and bears rhizoïden that attach to the soil. The thallus has nearly always oil cells. There are umbrella-shaped gametangiophores that are either male (called antheridiophores; phore means to carry) or female (called archegoniophores). The male antheridiae open towards the upper part, whereas the one-egg cell bearing female archegonia open towards the lower surface. The edges of the archegoniophore are slightly curved downward (like the cap of mushrooms). As the seta, the stalk at the base of the sporophyte, grows, the adult sporangium pops slightly out. The spore caps have grooves (see example A); after opening of the caps the spores are released within a few hours.
The Hornworts resemble the Liverworts, but they look more like real plantlets. They never have oil cells and each cells has only one or two large chloroplasts. The gametangia are sunken in the upper part of the thallus. The adult sporangia (sporophyte, 2n) protrude as little horns above the surface, hence the name "Hornworts". These horns are formed by strong outgrowth of the seta, the base of the sporophyte. The dispersion of the spores can last for a few weeks after the opening of the grooves in the sporangium. A little more than 100 species are known. In the Netherlands they are quite rare, but they are not completely determined yet.
True Mosses. Examples: Sphagnum, Polytrichum, Funaria
The true mosses have an architecture with a distinct stem-like and leaf-like appearance (although they do not have true stems, nor leaves, or roots or tissues!). The gametangia develop at the top of the male and female gametophytes. After fertilization the zygote develops into a sporophyte, which however remains attached to the female gametophyte. The sporophyte comprises a seta and a capsule which is covered by a cap, called operculum (see example B, operculum green colorized; High-resolution of the operculum of an Atrichum moss). When the capsule ripens and the operculum falls off, the teeth at the edge of the capsule opening (peristome) become visible. The outgrowth of the seta makes that the sporangium extends above the other parts of the plantlet, being exposed to the wind. Within the capsule, spore-producing cells undergo meiosis to form haploid spores. De dispersion of these spores can take days. True mosses are by far the largest and most common phylum of the Bryophytes. More than 500 species are known in the Netherlands.