Bryophytes: Liverworts, Hornworts, and Mosses
Bryophytes are classified into three groups by the International Code of Botanical Nomenclature (ICBN).
1. Hepaticopsida (Liverworts)
2. Anthocerotopsida (Hornworts)
3. Bryopsida (Mosses)
A. Hepaticopsida (Liverworts)
The name Hepaticopsida has been derived from the Latin word Hepatica which means liver. In the medieval period, these plants were used in treating liver disorders. Hence the members of this class are commonly known as Hepatics or Liverworts. The class is represented by about 280 genera and 9500 species distributed in both tropical and temperate parts of the world.
Characteristics
1. The dominant phase of the plant is haploid, i.e. Gametophyte (n), while the plant body is thalloid or foliose. The plant body of most liverworts is flat and has dorsiventral symmetry. From the lower side i.e. ventral side some structures help in attachment and water absorption. They are rhizoids, root-like structures. These rhizoids are unicellular and unbranched.
2 The scales can be found on the gametophytic body, but only on the ventral surface. These scales are multicellular and are also known as Amphigrotria, they are involved in controlling water loss or retaining moisture. the Rhiziods and scales are haploid because they are associated with the gametophytic phase.
3. the sex organs are present in the gametophytic phase and are embedded in the thallus on the dorsal side or upper side. the sex organs can also be present on the axile of leaf-like structure e,g Porella
4. the sporophyte is simple or differentiated into foot, seta and capsule. In Ricccia, the seta is shorter, but the sporophyte is represented by a capsule. The columella is absent in liverworts.
5. the function of sporophyte is the formation of spores. The spores are dispersed by Elaters in Marchantia.
6. The sporophyte is non-green which means it is dependent on the gametophyte for food, water, minerals, and attachment. thus sporophyte is known as a total parasite.
7. The true vascular system is absent.
8. Photosynthetic cells contain numerous chloroplasts without pyrenoids.
9. Green cells contain simple or compound oil bodies.
10. Asexual reproduction occurs through fragmentation or gemma formation.
11. Only Marchantia is dioecious, while all other bryophytes are monoecious.
12. Marchantia, Riccia, Pellia, Porella, cyathodium, etc are some examples of liverworts
B. Anthocerotopsida (Hornworts)
The class Anthocerotopsida is a small group of Bryophyta and is represented by 6 genera and 300 species, widely distributed in both the temperate and tropical regions of the world. They are also known as Hornworts because they have a Horn-like structure at sporophyte.
Characteristics
1. The plant body which is the dominant phase is Gametophytic. The plant body is flat. The rhizoids which are present are unicellular and unbranched.
2. The sporophyte is differentiated into bulbous foot, small meristematic seta, and long cylindrical capsule.
3. The scales are absent.
4. The sex organs are presently embedded in the thallus on the dorsal side or upper side.
5. The sporophyte is elongated i.e. Horn-like appearance thus called Hornworts. In Hornworts seta is absent while as capsule contains a sterile part this sterile part is known as the columella.
6. The sporophyte is green which means it synthesis its food, the sporophyte is only dependent on Gametophyte for water, minerals, and attachment. Thus it is called a partial parasite
7. The stomata are present and it contains kidney-shaped guard cells.
8. The true vascular system is absent
9. The pseudoelaters are present in the capsule.
10. Asexual reproduction by fragmentation of thallus and by tubers.
11. Sexual reproduction occurs by gametes
12. Some common examples are Anthoceros, Notothylus and Megaceros, etc
C. Bryopsida (Mosses)
The class Bryopsida is the largest group of bryophytes. It includes 660 genera and 14500 species that occur in almost all parts of the world. They are abundant at altitudes ranging from 1250 to 2500 meters. Some species also occur up to an altitude of 6500 meters. They are better adapted to terrestrial habitats than liverworts and grow on moist soil, rocks, damp walls of old buildings, and on the stem of trees in tropical forests. They are commonly known as Mosses. They are also leafy Bryophytes.
Characteristics
1. The Gametophyte has two growth stages Protonema stage and the Leafy stage. The Protonema stage is a prostrate, creeping, green, and branched filamentous structure. An erect cylindrical stem with persistent leaves and sex organs is the leafy stage.
2. The rhizoids are multicellular, branched, and have oblique septa
3. The scales are absent
4. The sex organs can be present at the tip of the leafy shoot they are known as Gametophore.
5. The sporophyte contains three parts Foot, seta, and capsule. Seta has elongated the function of the capsule is the formation of spores. The columella is present in mosses.
6. The stomata are present over the sporophyte and only one guard cell is present it is ring-like i.e. Annular.
7. In this certain cells are used for conducting water and minerals. The cells which help in the conduction of water are known as Hydriods and the cells which help in the conduction of food are known as leptiods.
8. the most elaborated mechanism for spore dispersal is present in mosses. It occurs with the help of peristome teeth.
9. Elaters are absent
10. Asexual reproduction is by budding and fragmentation of secondary protonema, stolons gemmae, and persistent apices.
11. sexual reproduction occurs with the help of gametes.
12. some common examples are Sphagnum, Polytrichum, Funaria, etc