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Jan's Guide to Mosses and Liverworts

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Mosses and liverworts, in addition to hornworts collectively make up a group of land plants known as bryophytes. While mosses consist of a leafy stem attached to substrate (e.g. soil, rocks or tree's) via branched rhizoids. Liverworts are formed of either stems with leaves, or a thallus (a rounded, strap-shaped or branched plate of cells) and have unicellular rhizoids. The main difference between leaves and thalli is that leaves are always one cell thick, while thalli are multiple cells thick.

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One of the main features distinguishing bryophytes from other land plants is a lack of vascular tissues (phloem and xylem), specialised cells involved in the transport of nutrients and water around plants. Instead they can absorb water across their whole surface area due to having absent or very thin cuticles. Other features include spore containing capsules, stem like seta, and strengthening cells such as the midrib in thalli or the nerve found on the leaves of certain moss species.

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Moss Plant Diagram
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Liverwort Thallus Diagram
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Life Cycle

The lifecycles of mosses and liverworts are practically identical with the main differences being the capsule structure and associated spore release mechanisms. Liverwort capsules are generally colourless and much simpler than moss capsules e.g. they do not have calyptra or lids. The main method of spore release in liverworts capsule splitting. On the other hand moss capsules are green for at least some stages of development and can be much more complex in strucure, often with 4, 8, 16 or 32 'teeth' forming a structure known as the peristome. This structure can open and close in response to humidity (open in low humidity and closed in high) and the number and structure of teeth can be used for identification. Additionally moss capsules also have stomata, pores which allow gas exchange to occur, in their walls. Mosses also posses a larger variety of spore release mechanisms with methods including, capsule rotting, breaking around the circumference or loss of the lid.

The sex organs are found either at the plant apex or the leaf axils (point where the leaf stem joins the main plant stem) and consist of an antheridia and an archegonia. The archegonia is flask-shaped and as it is the female sex organ each contains an egg cell (a.k.a an ovum) stored in the base. The antheridia is the male sex organ and is densly packed with motile cells known as antherozoids which swim through a film of water in order to reach the egg and fertilise it. The fertilised egg then develops into a capsule which eventuallys release the spores so they can be transported far away from the parent plant by the wind, animals, or other methods. Spores then germinate into a structure known as a protonema which is very filamentous and bears resemblance to green algae. Next protonema cells form buds from which the more recognisable moss/liverwort plant will grow and from here the cyle repeats.

Moss Capsule Diagrams
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Liverwort Capsule Diagrams
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Bryophyte Lifecycle Diagram
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While the above lifecyle is an example of sexual reproduction like many plants bryophytes are also able to undergo asexual reproduction via vegetative propogation. In bryophytes this can occur from thalli, leaves, stems and rhizoids of plants. Examples of propogation structures include...

  1. Gemmae : either 1-2 cells or a ball or plate of cells formed on leaves or rhizoids.

  2. Bulbils : small detachable buds with finger-like underdeveloped leaves which form on leaf axils.

  3. Paraphyllia: clusters of tiny detachable stems with leaves found on leaf axils.

Bryophyte plants can have large amounts of these propogative structures and they are often distinct enough to act as identification aids.

Collecting and Preparing Samples

While using a hand lens can be useful for starting the identification process many important features can only be properly observed using a microscope and therefore it is often neccessary to take samples back for later examination if accurate identification is desired. While initially any container is fine for storage samples should not be left in plastic or away from the light for more than a day. If you want to keep the samples for an extended period of time then the best way to store them is to first let them air dry to prevent fungal growth and then store them in paper packets (see below diagram).

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Important information to note down when collecting samples includes habitat information e.g. acid or calcaneous ground, species of tree found on, whether near water etc, as well as the date and location from which the sample was taken.

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Whenever you are collecting samples it is crucial that you only take the minimum amount required and to check that this is not the only example of that specimen in the area. This is especially important if you suspect you may be collecting a very rare species, e.g. one that is only found in certain highly specialised environments, as some species are so rare that they should not be collected at all.

Moss Identification

Mosses generally fall into one of two main groups, either Acrocarpous or ​Pleurocarpous mosses. Which group a moss falls into can be determined by looking at overall shape, position of the capsule and mid-leaf cell shape. Acrocarpous mosses tend to grow in tufts or cushions with short upright minimally or completely unbranched stems with capsules forming at the apex and compact leaf cell shapes such as round, square, hexagonal or short rectangular. On the other hand Pleurocarpous mosses grow in mats or felts, with highly branched and often horizontal stems with capsules growing laterally and below the stem apex and elongated leaf cells which can be rhomboid, rectangular of spindle shaped.

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Other features used to help identify species are leaf shape, arrangement and colour, presence and length of a nerve, apex shape, leaf margin cell shape and presence of teeth, as well as presence and position of propogation structures.

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Liverwort Identification

The first step in identifiying liverworts is to determine whether the specimen is thalloid or leafy. Liverwort leaves have a number of distinguishing features which include ...

  1. Presence or absence of a third row of small leaves along the underside of the stem known as underleaves (may be hidden by the rhizoids).

  2. Whether the two main row of leaves are arranged in a succubous (upper leaves are on top of lower leaves) or a lucubous (lower leaves on top of upper leaves) formation when viewed from the upper side.

  3. Presence of conduplicate leaves, leaves which are folded into an upper and lower lobe.

  4. Whether leaves are plane, bi-lobed or tri-lobed.

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