Characteristics

General Characteristics:

Liverworts are the most primative members of the Bryophytes and are found in various environmental conditions throughout the world. Liverworts share many characteristics with the green algae of Kingdom Protista, but are very easy to identify in their natural habitat. The common representative of the liverworts(Phylum Hepatophyta) is Marchantia polymorpha. Marchantia, as they are commonly called, are found in moist, humid environments around the world and appear in the form of a thallus, or a flattened body. Twenty percent of the liverworts display this characteristic(picture of a thallus is to right). Although not all of the liverworts appear in this form, Marchantia is a very good representative of the taxa because it clearly displays the major characteristics of the liverworts.

Marchantia polymorpha

The thallus of Marchantia clearly shows the general characteristics that are present in every liverwort. The following characteristics are present in Marchantia, as well as every other member of the liverworts. The bottom surface of Marchantia is ordained with single celled projections of the epidermis called rhizoids. These "appendages" are used as a means of anchoring the thallus to their substrate and have no vascular tissue, so cannot be considered true roots. These appendages can be seen in the thallus cross section to the right as extensions of the lower epidermis. Cell differentiation has a definite role in the liverworts as well. The thallus of the Marchantia is divided, internally, into separate areas of diffentiated tissue that each do a separate task.(cross section is to the right) There are three main subdivisions within the thallus: the epidermis(skin of the thallus), a photosynthetic area(which is towards the upper surface of the thallus and is responsible for all of the photosynthesis of the thallus), and a storage area(that is used to store photosynthates in the form of starch). Liverworts have multi-cellular sex organs and are dioecious plants, meaning that the male and female sex organs appear on separate plants. The male sex organ, the antheridium, produces flagellated sperm which makes it essential to live in a moist environment because the sperm must swim to the female sex organ, the archegonium, in order to fertilize the ovum. The antheridia, plural for antheridium, appear on the upper surface of a structure called the antheridiophore, which resembles the shape of an open umbrella.(pictured to right ) The archegonia, plural for archegonium, appear on the lower surface of a structure called the archegoniophore, which resembles the shape of a palm tree.(also pictured to the right) During fertilization, the sperm must travel from the antheria of the antheridiophore in water to a "female" thallus, and then up the archegoniophore stalk and into the archegonia where the egg is then fertilized.(for life cycle see Life Cycle and Development)

Female Thallus bearing many Archegoniophores

Male Thallus bearing many Archegoniophores

Liverworts also reproduce asexually by means of specialized structures called Gemma cups. These gemma cups appear on the upper surface of the thallus and each contain many small pieces of tissue that are called gemma. When a drop of water hits the gemma cups, the gemma are splashed out, and each of them is then capable of growing into a new thallus.(a gemma cup is pictured to the right)

Derived Characteristics:

Although the liverworts are very primative land plants, they still have many derived characteristics that set them apart from their oceanic relatives, the green algae.(for evolutionary relationships see Evolutionary History) Generally, true plants are easily distinguished by basic characteristics that they possess. Not all members of Kingdom Plantae display all of the characteristics, but an organism must show a few of them in order to be considered a plant. The liverworts display five of these characteristics and are as follows: 1. Distinct tissues(groups of cells similar in form and function that work together for one function){described above} 2. A waxy cuticle that prevents water loss and makes life on life possible(secreted by the epidermis) 3. Stomata- microscopic perferations in the epidermis of the thallus that allows gas and water exchange with the environment(visible in cross sectional view above) 4. Multi-cellular sex organs(see above for further details) 5. Alternation of generations- a pattern of the appearance of sporophyte and gametophyte generations in the life cycle. (see Life History and Development)

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