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Liverworts (Hepaticae)

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Liverworts

The dominant part of the liverwort plant is referred to as the thallus, which is the the gametophyte stage. The thallus is a flattened collection of undifferentiated cells. Principally, there is no transport tissue. These flattened structures are anchored to the ground by cell outgrowths called rhizoids (they have no roots). The green thallus has cells in the upper region which carry out photosynthesis.

Liverworts are found on land - just! They are very dependent on water to complete the life cycle and so are usually very close to water. Hence they are in regions with high rainfall (uplands, moorlands and mountains) in the north of Europe. They grow along riverbanks where they become splashed by the fast flowing water. However, they can be found in most habitats as long as the water element is present.

ECOLOGY: Liverworts are very primitive plants. With a lack of any specialised cells they cannot grow very large. For example, they do not have any fibres or supporting tissue. Transport tissue like xylem and phloem would be needed to carry food and supporting tissue would be needed to lift it off the ground. The thallus is a haploid structure (a single set of chromosomes) and develops the reproductive structures, antheridia (male) and archaegonia (female). Hence the thallus is referred to as gametophyte stage. Fertilisation is only possible when water flows across these organs. Only then can the male gamete swim across to the flask shaped archaegonium and fertilise the egg cell. The result is a sporangium which begins to grow out of the thallus. This is a diploid structure (paired chromosomes) and is called the sporophyte stage. Spores are produced in the dark capsule at the end of the sporophyte and when sufficiently extended to clear the ground the structure explodes. This is achieved by small brown elaters, structures which change shape on drying out. The spore will develop into a new thallus if it lands on damp soil. The sporangium shrivels and dies. Thus, there is an "Alternation of Generations" during the life cycle between the gametophyte and sporophyte stage. Ferns and higher plants have a dominant sporophyte stage whilst mosses and liverworts have the gametophyte dominant.

Sporangium stage
Sporangium stage

 


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