The spore-producing cells undergo meiosis to form spores, which disperse with the help of elaters , giving rise to new gametophytes. Thus, the life cycle of liverworts follows the pattern of alternation of generations. Liverwort Life Cycle : The life cycle of a typical liverwort follows the pattern of alternation of generations.
Spores are released from sporophytes and form the gametophyte. Male gametes fertilize female gametes to form a zygote, which grows into a sporophyte. This sporophyte disperses spores with the help of elaters; the process begins again. Liverwort plants can also reproduce asexually by the breaking of branches or the spreading of leaf fragments called gemmae.
In this latter type of reproduction, the gemmae small, intact, complete pieces of plant that are produced in a cup on the surface of the thallus are splashed out of the cup by raindrops. The gemmae then land nearby and develop into gametophytes. The hornworts Anthocerotophyta belong to the broad bryophyte group that have colonized a variety of habitats on land, although they are never far from a source of moisture.
The short, blue-green gametophyte is the dominant phase of the lifecycle of a hornwort. The narrow, pipe-like sporophyte is the defining characteristic of the group. The sporophytes emerge from the parent gametophyte and continue to grow throughout the life of the plant. Stomata appear in the hornworts and are abundant on the sporophyte. Photosynthetic cells in the thallus contain a single chloroplast.
Meristem cells at the base of the plant keep dividing and adding to its height. Many hornworts establish symbiotic relationships with cyanobacteria that fix nitrogen from the environment. Hornworts : Unlike liverworts, hornworts grow a tall and slender sporophyte. The life cycle of hornworts also follows the general pattern of alternation of generations and has a similar life cycle to liverworts.
The gametophytes grow as flat thalli on the soil with embedded gametangia. Flagellated sperm swim to the archegonia and fertilize eggs.
However, unlike liverworts, the zygote develops into a long and slender sporophyte that eventually splits open, releasing spores. Additionally, thin cells called pseudoelaters surround the spores and help propel them further in the environment. Unlike the elaters observed in liverworts, the hornwort pseudoelaters are single-celled structures.
The haploid spores germinate and produce the next generation of gametophytes. Like liverworts, some hornworts may also produce asexually through fragmentation. Life Cycle of Hornworts : The life cycle of hornworts is similar to that of liverworts. Both follow the pattern of alternation of generations. However, liverworts develop a small sporophyte, whereas hornworts develop a long, slender sporophyte. Liverworts also disperse their spores with the help of elaters, while hornworts utilize pseudoelaters to aid in spore dispersal.
Mosses are bryophytes that live in many environments and are characterized by their short flat leaves, root-like rhizoids, and peristomes. More than 10, species of mosses have been cataloged. Their habitats vary from the tundra, where they are the main vegetation, to the understory of tropical forests. Mosses slow down erosion, store moisture and soil nutrients, and provide shelter for small animals as well as food for larger herbivores, such as the musk ox.
Mosses are very sensitive to air pollution and are used to monitor air quality. They are also sensitive to copper salts. Such salts are a common ingredient of compounds marketed to eliminate mosses from lawns. Mosses form diminutive gametophytes, which are the dominant phase of the life cycle.
Green, flat structures resembling true leaves, but lacking vascular tissue are attached in a spiral to a central stalk or seta. The plants absorb water and nutrients directly through these leaf-like structures. The seta plural, setae contains tubular cells that transfer nutrients from the base of the sporophyte the foot to the sporangium.
Some mosses have small branches. Some primitive traits of green algae, such as flagellated sperm, are still present in mosses that are dependent on water for reproduction. Other features of mosses are adaptations to dry land. Thalloid liverworts, on the other hand, do not look anything like mosses. They do not have stems or leaves; instead their main body is flat, like a green pancake.
It is easiest to tell liverworts apart based on the appearance of the leafy part the dominant, gametophytic generation ; most liverwort sporophytes look very similar. The tissue within the thallus can be quite differentiated, as in the Marchantiales, or can have little differentiation, as in the Metzgeriales.
These relatively undifferentiated thalli are only one or two cell layers thick. Marchantiales typically have thalli that are composed of multiple cell layers. Liverworts have unicellular rhizoids that are located on the ventral bottom side of the gametophyte.
Rhizoids look like roots, but do not absorb water or nutrients. Instead, they attach the plants to their substrate and help with external water retention and conduction. Some thalloid liverworts have scales associated with their rhizoids. These scales can be easily distinguished from the rhizoids because they are multicellular and are usually pigmented.
Some liverworts reproduce asexually by producing gemmae in gemmae cups. Gemmae are vegetative diaspores that can germinate to form a new plant that is genetically identical to the parent plant. The gemmae are dispersed when water droplets fall into the splash cups; the shape of the cup makes the water splash out, and it takes some gemmae with it, carrying them far away from the parent plant.
In general, the gametophytes of leafy liverworts are very small, with leaves that are only about 1 mm long. Although leafy liverworts look very much like mosses, they can be distinguished from mosses by their leaf arrangement. Hahn's Die Lebermoose Deutschlands , Within the spore capsules of many liverwort species there are elaters as well as spores.
Elaters are tubular cells with spiral thickenings and often help in spore release. This photo shows two spores and elaters of Fossombronia foveolata.
Both spores are broken and exuding oily droplets. This photo shows part of a broken spore capsule of the thallose liverwort Fossombronia papillata. As well as elaters you can see spores the round dark brown objects and fragments of the spore capsule wall the flat, often brown-speckled sheets. Elaters do not work in the same way in all species and there are some species where elaters play little or no role in spore release.
On this page you'll see how elaters are arranged and how they work in several species. While many liverworts have elaters there are exceptions. For example the thallose genera Sphaerocarpos and Riccia lack them and some populations of Fossombronia microlamellata , another thallose liverwort, lack elaters.
Cephalozia bicuspidata is a leafy liverwort, widespread in the Northern Hemisphere. The left hand figure in the following diagram shows an unopened spore capsule, which is elongated in this species. The red lines are the elaters, each attached at one end to the inner wall of the spore capsule.
Each of the red elaters is coated with black spores. Four lines of weakness run along the wall of the capsule, which opens out into four lobes. Once the capsule has opened the contents begin to dry out. Each elater has two spiral bands and is filled with water. The water in each elater begins to evaporate so that the elater becomes distorted, with the thinner areas between the spiral bands being sucked inward.
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