Liverworts (Marchantiophyta) PDF

Liverworts What are liverworts? Liverworts are a group of ​non-vascular plants similar to ​mosses​. They are far different to most plants we generally think about because they do not produce seeds, ​flowers​, ​fruit or wood, and even lack ​vascular tissue​. Instead of seeds, liverworts produce spo...

Liverworts What are liverworts? Liverworts are a group of ​non-vascular plants similar to ​mosses​. They are far different to most plants we generally think about because they do not produce seeds, ​flowers​, ​fruit or wood, and even lack ​vascular tissue​. Instead of seeds, liverworts produce spores for reproduction. The liverworts are very primitive plants and many species only contain a single layer of cells. They differ from more advanced plants because they do not have any stomata in their tissue which are used by most plant groups for taking CO2 into their leaves for photosynthesis. What's in a name? The English word "​wort​" means "small plant" and it turns up in names such as Pennywort and Bladderwort. The term liverwort originated from the fact that the early herbalists thought that one of the liverworts had some resemblance to a liver - and some use as medicine for liver ailments. Hence the word liverwort for a "liver-like small plant". As well as the term liverwort you may also see the alternative term ​hepatic used and this comes from the Greek word "hepatikos" - meaning liver. Do not confuse the ordinary English word hepatic (meaning liverwort) with the genus name ​Hepatica​. The latter is in fact in the flowering plant family Ranunculaceae! That family also contains the genus ​Ranunculus​, the plants of which are commonly called Buttercups. Structure of Liverworts Reproduction of Liverworts Liverworts, like all ​bryophytes​, have two forms of reproduction. ​Asexual or vegetative reproduction, and ​sexual​ reproduction. Sexual Reproduction The ​sexual reproduction for leafy liverworts is very similar to the ​mosses​. The sexual parts are contained in small and inconspicuous structures known as ​antheridia (male) and ​archegonia (female), which develop on separate plant bodies. In the thallose liverworts, things are a little different. For example in the Marchantia species the antheridia and archegonia are produced on an umbrella like structure. While in others species they are hidden in small pockets on the leafs In leafy liverworts the antheridia produce mobile antherozoids (sperm), which require a film of water in which to move to the archegonia, where fertilisation takes place. After fertilisation, a new plant develops, which remains attached to the parent plant. This is the ​sporophyte​. The thallose liverworts have another trick; they harness the power of raindrops to project the sperm up to a metre away from the male plant. Thus placing the sperm on special cup shaped splash areas, which are either on the leaf surface, or in the case of Marchantia species, have an inverted umbrella shaped Antheridiophores. Once the sperm has reached the archegonia fertilisation takes place and the sporophyte grows. When the sporophyte has matured in the leafy liverworts, the ​seta that are delicate, white, and glassy, elongate, carrying the ​capsule some 25 - 50 mm into the air. The capsule contains ​spores with hygroscopic ​elaters​, which split open, usually into four segments or valves​, and the spores within are released. Under suitable conditions, each spore can ​germinate and give rise to another gametophyte​. With the thallose liverworts the sporophytes are hidden under the Archegoniophores, or in the case of Marchantia species, in a small structure or pocket on the leaf. The spores are released directly from these structures. The gametophyte can propagate itself vegetatively, and also produce the ​gametes, which give rise to the saprophyte. Asexual Reproduction Vegetative reproduction can occur as a result of older parts of a plant dying off so that the newer branches become separated; by specialised whip-like branches; or by leaves that drop off the plant. In the thallose liverworts a more complicated system is used, with propagative structures called gamma cups forming on the leaves. Each gamma cup gives rise to numerous gametes that are released when water droplets splash into the cup thus transported the gametes to favourable sites to grow into new plants. Classification The ​2000 classification divides the liverworts into two classes: ​Marchantiopsida and Jungermanniopsida​. All the complex thallose liverworts are in the former and all the leafy liverworts in the latter. Simple thallose liverworts are found in both classes, though mostly in the latter. The precise definitions of the two classes are based on a number of microscopic morphological features. For example, in the Marchantiopsida the wall of the spore capsule is single layered whereas in the Jungermanniopsida the wall of the spore capsule has two or more layers (except for the genus ​Haplomitrium​). Spores develop from what are called ​spore mother cells​. In the Marchantiopsida the spore mother cells are unlobed whereas in the Jungermanniopsida they are lobed. There are also differences in the initial stages of the development of the sperm-producing antheridia in the two classes. The class Jungermanniopsida is much, much larger than Marchantiopsida. The leafy liverwort species far outnumber the thallose species. Further differentiation into lower taxonomic ranks is based on a wide variety of gametophytic and sporophytic features. The structure of the sporophyte foot, leaf form, internal thallus differentiation, manner of opening of the spore capsule, gemma structure, protonemal development and nature of elaters are just some examples. All of those are also relevant to one or both of moss and hornwort classification. Since this page doesn't aim to give a full description of liverwort classification there'll be no further discussion of any such features except for one, ​oil bodies​, that are peculiar to the liverworts. Oil bodies, containing a variety of terpenoid oils, are found in about 90% of liverwort species and are distinct cell organelles. Other plants (not just other bryophytes) may have oil droplets but the oil bodies of liverworts are bounded by true membranes. The oil bodies vary greatly in size, shape, colour, chemical composition and distribution within cells. Oil bodies typically disappear in dried, herbarium specimens and are best studied in fresh material. There have been various hypotheses about the functions of oil bodies (e.g. UV protection, cold resistance, herbivore deterrence) but there is insufficient evidence for definite conclusions