Mosses have chloroplasts

Evolution and biodiversity of plants


1 The mosses (Bryophyta)

1.1 General overview

The mosses are all small to very small plants (a few mm to a few cm in size). Only a few species have returned to aquatic life (e.g. Fontinalis) as a secondary feature, the vast majority are land-living. Moist habitats show the greatest biodiversity, but mosses also occur in very dry locations at times. Phases of drought are survived with practically no measurable metabolic activity and largely dehydrated in a state that is often called "latent life".

In all mosses, the gametophyte represents the long-lived phase of the generation change that is predominant in terms of biomass. The sporophyte always consists of a single, usually clearly stalked sporangium (spore capsule) and dies after the spores formed therein have matured. The differentiation of the sporophyte is essentially limited to a sometimes complicated histological differentiation of the spore capsule. The capsule usually has a clear stalk, which is always a smooth carrier (sporangiophore) without external differentiation and never covered with leaves or hair. The capsule stalk is almost always formed by the sporophyte (seta), only in exceptional cases is it formed by the gametophyte (pseudopodium in gossips and peat mosses). The short-lived sporophyte remains in connection (1) with the gametophyte throughout its life and, in contrast to the situation in ferns, never becomes a plant independent of the gametophyte.

The mosses are usually isospore. Anisosporia only occurs in a few deciduous mosses (e.g. Macromitrium, Orthotrichum) (2) in front. A sexual differentiation into morphologically identical male and female spores from which male and female moss plants (gametophytes) emerge is more common (e.g. Polytrichum, Marchantia). If different spores are formed, two female and two male spores result from a meiosis, the differentiation occurs through sex chromosomes.

While the systematic classification is particularly based on the sporophytes, which are usually not found all year round, the morphology of the gametophyte is particularly important for recognizing and identifying the mosses. There are both thallose and leafy mosses. Thalli can be single-layered or multi-layered (tissue thallus). With the exception of the leaf base and the occasional midrib, moss leaves are always single-layered. So-called paraphyllia can stand between the leaves on the stem of the moss. Paraphyllia are single-row hairs or multi-row flat structures which, in contrast to rhizoids, contain chloroplasts. Sometimes the leaf clusters of reduced side shoots look similar to paraphyllia, they are then called pseudoparaphyllia. It is anchored to the substrate by epidermal hairs called rhizoids. The rhizoids are multicellular (leaf moss) and then have characteristic, crooked cell walls, or they are unicellular (3) (liverwort). In liverworts, the rhizoids can have cone-like wall reinforcements on the inside. Real roots are never found in mosses. The gametophyte is persistent and can repeatedly develop gametangia and carry sporophytes over many years.

Numerous mosses are able to regenerate from the smallest fragments. Some species form brood buds that break off easily, others form spherical expansion units (brood bodies, gems). In addition to the exogenous formation of brood bodies (only in liverworts), an endogenous formation of such reproductive units occurs. It is similar to the formation of spores in that the cell content is used up during the formation of the 1-4 spreading units and the empty cell wall is left behind during spreading. Such units of spread differ from spores or conidia above all in that they do not represent stages of rest or duration. They often continue to grow directly on the mother plant and are then released on a plant in very different sizes.

There are flowering plants that at first glance vegetatively may be very similar to mosses. These are, for example, small-leaved cushion plants, and one species (Mayaca sellowiana) even bears the name "moss with flowers" in its homeland (southern Brazil). In contrast to moss-like flowering plants, mosses never have roots built from tissue but only thread-like rhizoids There is a differentiation into different cells and tissues, but vascular bundles with water guide elements (trachea, tracheids) are never found. Typical water guide elements occur rarely and as isolated cells only in the sporophyte. Stomata or similar structures are only found on the spore capsule or on the thallus some thallous liver disease, but never on the leaflets of leafy mosses. Except for the (sometimes missing) midrib, these leaflets are single-layered. A gap opening would be pointless here and would not be as in higher plants lead into a respiratory cavity, but simply to the other side of the leaflet.


1.2 The generation change of the mosses

The spore of a moss first germinates with one (4) thread. This thread can branch and is called the protonema (5). After a while, one or more thread tips form a three-edged apex cell (6) and turn into a leafy moss plant. After some time, the moss plant forms male and female gametangia at the top of the Sprösschen (summit-fruited moss) or at the top of short basal side branches (side-fruited mosses). Male and female gametangia can occur on one plant (monoecious mosses) or be distributed among different individuals (dioecious mosses). In the case of the Monözischen mosses, a further distinction can be made as to how the gametangia are distributed on the plant (Figure: gender distribution).

The female gametangia contain only one single gamete, the egg cell. Such a gametangium is called oogonium in lower plants. In the case of mosses and ferns, it is called Archegonium (7) for historical reasons. The archegonium has a wall made of a layer of cells. It consists of a belly that surrounds the egg cell and a chimney-like neck. The neck surrounds a single row of cells, the so-called neck canal cells. Between the cervical canal cells and the egg cell there is a larger cell, the abdominal canal cell. When the archegonium opens, the abdominal canal and neck cells mucus so that the spermatozoids can swim to the egg cell.

Through fertilization, the egg cell becomes the zygote, which further develops into a sporophyte as the cells divide. First of all, the wall of the archegonium follows this growth. But when the capsule stalk (seta) begins to grow, the archegonia wall is torn off at the base and lifted up onto the capsule as a hood (calyptra). There is a fixed division sequence when developing the capsule. The tip cell of the sporophyte first divides into four quadrants, then each quadrant is divided into an inner and an outer cell. The outer layer forms the later capsule wall as amphithecium, the inner cells as endothecium (8) both the spore-forming layer (archespor) and the sterile central column (columella). The Amphithecium differentiates itself in the tip area of ​​the capsule into the opening structures typical for the individual moss groups.

The antheridia also have a wall made of a layer of cells. The numerous cells within this wall are the spermatogenic cells. Each of them forms two spermatozoids, which are released by dissolving the cell walls in the interior and opening the shell at the tip. The spermatozoids reach the archegonia by chemotactic attraction (so-called gamons). Longer distances can be covered by hurling the spermatozoids away when water drops hit them. This "spray spread" is favored in some species by a cup-like cover ("perianth"; not to be confused with the perianth of flowering plants!) Around the antheridia. So that the spermatozoids can reach the archegonia lying at the tip of the moss shoots, this must be completely covered with water. This water cover is mostly in the form of capillary water between the stems and the bases of the leaflets. This "outer water conduit" makes an inner water conduit largely superfluous. The mosses therefore lack highly developed water conduction structures such as phloem or xylem.


1.3 The main groups of Mosses

1.3.1 Moss (Bryopsida) Bryidae

The best-known and largest group of mosses are the leaf moss s.str. (9) (Bryidae). The morphology of the capsule and, in particular, the opening of the capsule is essential for the systematic classification. This peristome lies under a cover and is made up of numerous teeth. In the relatives of women's hair moss (Polytrichum, Polytrichales), these teeth are made up of cells (see schematic cross section). In the case of the other deciduous mosses, these teeth do not consist of numerous whole cells, but rather around the thickened walls between two adjacent rows of cells. The cells themselves tear (schematic cross-section). Such teeth, which only consist of cell walls, can stand in two rows or in one row. One then speaks of a double or a single peristome.

In the middle of the capsule is a sterile column, the columella. The cells around this column all go through meiosis and all turn into spores. Sterile cells between the spores as in the liverwort do not occur in the deciduous moss. The sporophyte is a typical disposable product and dies after the spores have matured. However, it is relatively tough and durable, which is why the capsules can be found in many species all year round.

When making the determination, the question of the number of peristome teeth often plays a role. For this it is important to realize that the capsule is formed by a regular sequence of divisions. In this sequence, first 2, then 4, 8, 16, 32 or 64 cells and a corresponding number of teeth are formed in the layer that forms the peristome. In exceptional cases, several teeth can arise from each cell. But this means that an additional division of the cells or a further splitting of the teeth means a doubling of the number of teeth. So if an identification key asks "32 persistome teeth or more" with the alternative "fewer peristome teeth", it means "16 or even fewer". That makes the tiresome counting much easier, because it doesn't matter at all whether the result is 30 or 34 is, but only whether it is one or the other number from series 2x is.

Due to the formation of segments of the three-edged apical cell, the leaves of the moss are initially laid out in three rows, but they quickly reach a dispersed position. The mosses can have a true midrib (several layers of cells), but there are also species without a midrib. Occasionally, the midrib can be very wide and take up most of the leaf cross-section (polytrichum (10)). Sphagnidae (peat moss) and Andeaeidae (gossip moss)

The deciduous mosses in the broader sense include the peat mosses (Spahgnidae) and the klaffmosses (Andreaeidae). What the peat mosses and the peat moss have in common is that the stalk of the spore capsule is formed by the gametophyte (diagram). It is called Pseudopodium and thus also conceptually differentiated from the Seta of the Bryidae belonging to the Sporphyte.

The peat moss comprises only the only genus Sphagnum (peat moss) that is found worldwide. The peat mosses are characteristic of peat bogs, but also occur in other moist and nutrient-poor places. In perhumid tropical areas they can even be found as a cushion on bare rock. The shoot of the peat moss shows a characteristic branching into a trunk and numerous horizontally protruding branches, both of which have small leaflets. The single-layer and always neveless leaflets of the peat moss are made up of a network of living, green cells (chlorocytes) and surrounded by dead cells (hyalocytes). The hyalocytes have one or more pores to the outside. The hyalocates are responsible for the fact that peat moss can store enormous amounts of water like a sponge.

The subarctic bogs grow very slowly. However, since there is practically no remineralization in peat bogs, peat bogs are still an important sink for CO2. The peat that emerges from peat moss over the centuries is an important substrate for industrialized horticulture, among other things because peat is practically germ-free and the substrates made from it do not have to be sterilized. Under today's climatic conditions, pitted raised bogs practically do not regenerate in Germany. The use of these biotopes is therefore associated with a permanent loss in our area.

The Klaffmoose get their name from the capsule that opens with four columns. The most important species (Andrea petrophila) grows on silicate rocks. In the gypsy moss, the spore does not develop into a cell thread as in the Bryidae, but a multicellular body. Only from this a thread then grows out, which, however, quickly reverts to a thallous organization. On this branched thallus, in addition to peculiar so-called thallus leaves, typical moss plants also develop.


1.3.2 The liverwort (Marchantiopsida)

Liverworts have a capsule that opens with (mostly) four flaps. No uniform spore mass is formed, but between the cells that go through a meiosis, numerous cells remain vegetative and form elongated so-called elaters. In the microscopic picture, the Elateren stand out due to their spiral wall reinforcements. The number of spirals is variable and species-specific. Elaterers often remain attached to the capsule wall, especially at the tips of the valves. On the opened capsule, the elaters form a cotton-wool-like structure that appears to be powdered with the spores. In this way, all the spores do not fall out immediately, but are exposed in the loose network of the elater to be spread by the wind. In some species, elaters remain in a small tuft at the tip of the four valves, in others a single tuft remains in the middle at the bottom of the capsule. In contrast to the yellow or brown spores of the moss and ferns, the spores of the liverwort are often green. The sporophyte is very short-lived and usually disappears a few days after opening the spore capsule. The sporophytes are therefore found much less often in the deciduous moss. At the base, the sporophyte is surrounded by a second, usually somewhat blistered, envelope, the so-called perichaetium, in addition to the archegonia wall. Anatomically, liverworts are characterized by the appearance of oil bodies in the cells. These oil bodies occur individually or in groups in individual cells of the thallus and are surrounded by a membrane, so that they could more correctly be addressed as oil vacuoles.

The liverworts have both leafy and thallous forms. The leafy liverworts usually have a three-lined foliage. However, there is practically always a noticeable dimorphism of the leaves, which must be seen in connection with the plagiotropic growth habit. The upright shoot of the (hypothetical) parent plant always lies on the substrate in such a way that one row of leaves faces the substrate and two rows are turned away from the substrate.

The row of leaves facing the substrate (lower leaves (11), ventral leaves, amphigastria) is more or less strongly reduced, in exceptional cases it may even be completely absent. If the lateral leaves turn their upper side down towards the substrate, then the leaf edge pointing towards the tip of the shoot is visible. This type of foliage is called overshot. If the leaflets point the morphological upper side away from the substrate upwards, the edge of the leaf facing the tip dips under the next distal leaf. This foliage is called undershot.

In many liverworts, the side leaves are folded. A distinction is then made between a lower lobe facing the substrate and an upper lobe facing away from the substrate. Upper and lower lobes can have very different morphological designs. There are species in which the upper lobe is larger than the lower lobe. In this case, the lower lobe can be transformed into a kind of water sac (Frullania). But there are also species in which the lower lobe is larger, so that both can be seen from the top and the impression of a doubling of the leaf rows is created. Some liverworts can also develop rhizome-like stolons with mostly reduced foliage.

In the case of thallous liver diseases, there are those with simply built, single-layered thalli and those with highly differentiated, multi-layered thalli. On the underside of the thalli there are rhizoids, the inner wall of which can be either smooth or provided with protruding cones (uvula rhizoids). In addition, so-called ventral scales can be found especially in the midrib area. These ventral scales are flat, transverse, always single-layered and free of chlorophyll. They occur in two longitudinal rows along the middle of the thallus or the midrib.


1The widely used phrase that the sporophyte "parasitizes" on the gametophyte is absurd. Parasitism is understood to mean a relationship between different species, but not the relationship between successive generations or phases of the change of generations of a species.

2Occasionally, the small size difference is considered sufficient to be called heterospore. Because of the way they are formed, both types of spores are present in equal quantities in a single sporangium. In other groups (some fern plants, seed plants) the female macrospores and the male microspores are formed in different sporangia!

3The prothallia of the ferns, which are sometimes very similar to liverworts, have multicellular rhizoids. In the (leafy) liverwort genus Schistochila, multicellular rhizoids occur, but the multicellularity is limited to a thickened to cauliflower-like end section of the otherwise unicellular rhizoid.

4In rare cases, germination with 2-4 filaments occurs. This is actually only conceivable if the spore has already been divided into several cells or at least several nuclei. Whether the thread or the cluster of cells represents the plesiomorphic situation is difficult. In general, the thread is the original form, probably with the characteristic phylogistic concept that the trichal organization is the most original after the unicellular (monadal, capsal or coccal).

5proto- gr. first; nema gr. thread;

6The direction of the diagonally drawn in dividing wall changes by 120 with the successive divisions of the apex cell0that the descendants of the apex cell are arranged in three rows.

7Dillenius (1741) and Linné compared the moss capsule with the anther of the flowering plants. It was later compared by the important moss researcher Hedwig with the fruit of the flowering plants containing the seeds. He called the archegonium, from which the seed capsule emerges, "Pistillidium". Accordingly, he named the male organ "Antheridium" and homologated it with the anther of the flowering plants. Bischoff also believed that the whole archegonium becomes a spore capsule and called it "archegonium" (= "previous fruit" or "fruit becoming"). The terms based on false homologations were preserved when W. Hoffmeister in 1851 the generation change of mosses and ferns had finally cleared up.

8In the seed plants the term "endothecium" denotes the subepidermal layer responsible for the opening of the anthers in the counters of the stamens of angiosperms. If the layer responsible for the opening of the anthers is the epidermis, as in the gymnosperms, this is called "exothecium".

9Without gossip and peat moss

10In Polytrichum the midrib carries lamellar assimilators. Anyone who has only seen Polytrichumg can not understand how one can get the idea to use the description "central rib occupying the entire leaf width" and at the same time to adhere to the single layer of the moss leaf. This is only in comparison to related species such as Atrichum Even if Polytrichum is easy to know and to find, Polytrichum should therefore not be used as the only example for the mosses (eg in school lessons)!

11In the case of flowering plants, the term "lower leaf" denotes the basal part of the leaf bearing the stipules and is delimited from the "upper leaf", which consists of the petiole and leaf blade. The lower leaf of the liverwort is something completely different from the lower leaf of the angiosperms!