Bryophytes are a group of plants having primitive characteristics. From 1836 until the late 20th century several scientists tried different systems for the classification of bryophytes with different divisions.
But the most accepted one is the division of bryophytes into three classes Hepaticopsida, Anthocerotopsida, and Bryopsida. They are further divided into sub-class, order, sub-order, family, etc, based on their specific characteristics.
Current Classification of Bryophytes
In 1957, Proskauer divided the division of Bryophyta into three classes – Hepaticopsida, Anthocerotopsida, and Bryopsida.
Class Hepaticopsida
Hepaticopsida is derived from the word hepatica which means liver. Plants under this class are known as ‘liverworts’ because they look similar to the liver.
The main plant of the class Hepaticopsida is the gametophyte. It is thalloid or leafy in appearance with dorsiventral structures.
- The leaves usually lack midrib.
- Leaves are usually arranged on the main axis in 2-3 rows.
- The cells of leaves contain chlorophyll and oil bodies but there are no pyrenoids.
- There are unicellular rhizoids that help the plant with absorption.
- Sex organs develop from cells on the dorsiventral side or tip of the thallus.
- Archesporium is developed from the endothecium.
- The capsule lacks columella and meristem. Sometimes they do not develop any mechanism for dehiscence.
Orders
Hepaticopsida is divided into four orders each further divided into different suborders. The four orders of the class Hepaticopsida are
- Sphaerocarpales
- Marchantiales
- Jungermanniales
- Calobryales
Order Sphaerocarpales
The plants under this order are thalloid without any pores and air chambers. A flask-shaped involucre encloses the sex organs. Archegonial neck has 6vertical rows of cells. Sporophyte is differentiated into foot, seta, and capsule.
Family: Sphaerocarpaceae
Genus: Sphaerocarpus
Order Marchantiales
Members of this order are thalloid plants having flat habits with dorsiventral sides. The thalloid is branched dichotomously. The thallus shows differentiation internally into upper and lower regions.
The upper region consists of air pores, air chambers, and photosynthetic tissues while the lower region has cells compactly arranged for storage. Thallus has scales and two types of rhizoids- pegged-walled and smooth-walled.
Archegonial neck has 6 rows of cells. The single-layered capsule has thick walls and may or may not have valves for dehiscence.
Campbell divided the order Marchantiales into 5 families- Corsiniaceae, Marchantiaceae, Monocleaceae, Ricciaceae, and Targionaceae.
Order Jungermanniales
Plants under this order have simple thallus or one with a leafy axis. Their rhizoids are usually smooth-walled. The archegonial neck has 5 rows of cells while the antheridium is stalked.
Sporophyte differentiates into foot, seta, and capsule. The capsule wall is multilayered. The spore mother cells and sterile elaters arise from the sporogenous cells. Valves present in the capsule aids in dehiscence.
Order Jungermanniales is subdivided into 2 suborders,
- Metzgerineae (Anacrogynae)- Pelliaceae
- Jungermannineae (Acrogynae)- Porellaceae
Order Calobryales
Plants in the order Calobryales have an axis with 3 rows of leaves arranged around it. Their leaves are simple, dorsiventral, and have margins. The archegonial neck comprises 4 rows of cells and exists in groups at the apex. Their sporophyte is differentiated into a foot, seta, and capsule.
Family: Calobryaceae
Genus: Calobryum, Haplomitrium
Class Anthocerotopsida
Plants of this class have dorsiventral thalli and no internal differentiation. Air chambers have mucilage cavities. Their rhizoids are smooth-walled and there are no scales.
Cells have a large chloroplast and a pyrenoid. The endogenous antheridia arise from the dorsal side and exist as solitary or in groups. Archegonia is also on the dorsal side. Sporophyte has a foot, a meristematic zone, and a terminal capsule, but no seta.
The capsule possesses a sterile central columella. Spore mother cells and pseudoelaters are produced from archegonium from amphithecium. The capsule has valves that can twist hygroscopically to disperse the spores.
This class has only one order- Anthocerotales.
Order Anthocerotales
There are only two families in this order- Anthocerotaceae and Notothylaceae
Family: Anthocerotacea- the genus Anthoceros.
Plants here have long, cylindrical capsules with chloroplasts and stomata. Columella arises from endothecium while amphithecium gives rise to spores.
Family: Notothylaceae- the genus Notothylus
Plants have a short capsule. They are horizontal, compact, and seen at the margins. Capsules lack chloroplast, stomata and columella.
Class Bryopsida
Plants under the class Bryopsida show a protonema-like prostate thallus and an erect axis with leaves. Leaves are spirally arranged. The thallus has multiple rhizoids and oblique seta.
The superficial layers of cells produce the sex organs. SPorophyte shows clear differentiaiton. Their multilayered capsule wall contains stomata, two chloroplasts, and the central conductive tissue columella inside. Archesporium produces spore mother cells but no elaters are formed.
Bryopsida is divided into three sub-classes.
- Sphagnidae
- Andreaidae
- Bryidae
Sub-class Sphagnidae
This sub-class has only one order- Sphagnales. Genus: Sphagnum
The plant body of the members is a one-cell thick protonema with multicellular rhizoids and oblique cross walls. Its leafy axis arises from protonema. The leaves lack midrib and have narrow living cells and large colorless dead cells.
Its leafy axis is branched and produces antheridium in the branches and archegonium terminally. Sporophyte is differentiated. The endothecium and amphithecium produce columella and spore mother cells respectively.
Seta is short. The operculum is present and the peristome is absent. The archegonial branch gives rise to pseudopodium after elongation.
Andreaidae
The protonemal thalus gives rise to a dark brown or reddish gametophyte. The plant body has no differentiation. The leaves present are convolute and large.
The endothecium produces archepsorium and columella. The seat is short and pseudopodium is present.
Order Andreales
Family- Andreaeceae
Genus: Andreae, Neuroloma, Aeroschisma
Bryidae
The plant body is filamentous protonema with leafy, erect gametophyte. The leaves have a midrib. The endothecium produces both archesporium and columella.
An air space separates the spore sac from the capsule wall. Trabeculae is present transversely in the air space. A long seta is present with no pseudoelaters but a peristome is present.
There are two orders in this class- Funariales and Polytrichales.
Funariales have small terrestrial plants with broad leaves. There is a spathulae present at the apex arranged in rosette form. Both inner and outer peristomes are present.
Family- Funariaceae
Genus: Funaria
Polytrichales have tall, erect, perennial gametophytes with narrow leaves and midribs. The sporophyte is differentiated. The erect capsule has peristomes seen as a ring.
Family: Polytrichaceae
Genus: Polytrichum
Fossil Bryopsida
Fossils are plant or animal remains preserved in sedimentary rocks from different geological conditions. Since bryophytes do not have vascular tissues or other lignified tissues, they were not as well preserved as other members of the plant kingdom.
The earliest discovered members of fossil Bryopsida are the two species of Muscites– M.betrandi and M.polytrichaceus. They were found in the upper Carboniferous era.
These fossil mosses are incomplete and are found as fragments of stems and leaves. There were no sporophytes or sex organs available in fossil form. However, the available stem remains showed an anatomy close to the mosses with single veins as well as rhizoids having cross walls.
Fossil mosses from Permian beds were found in Angaraland in Russia. Neuberg published an elaborate account of these fossil forms in 1951 and 1960 and also erected new genera named after the locality. She categorized 6 genera into the subclass Bryidae and 3 genera into the subclass Sphagnidae.
Bryidae
Genus Intia has a remarkable resemblance to Mnium and Bryum. Four species of Intia were assigned to this subclass- I.angustifolia, I.falciformic, I.variabilis, and I.vermicularis. The other 5 genera in this subclass are Salaria, Uskatia, Polyssalevia, Bajdaievia, and Buchita.
Sphagnidae
The 3 genera under this subclass are Jungala, Vorcutannularia, and Protosphagnum. The leaves of these fossil forms resemble that of Sphagnum with two types of cells. But the fossils have a midrib and traces of lateral veins as well.
Neuberg included these fossil forms in the order Protosphagnales. But in 1962, Gam regarded this categorization as premature solely based on the vegetative characters.
Muscites is another fossil moss from the Triassic of Natal. Townrow describes that it does not resemble any living moss but in some ways, it comes closer to the living mosses of Leucodontaceae.
Leaf fragments of Sphagnum were found in the upper cretaceous in Greenland. Polytrichites, belonging to Polytrichales are found from the Miocene, along with Peurocarpous and Plagiopodopsis mosses.
Conclusion
The geological records of fossil Bryopsida show that they belong to an ancient group. The meager discovery of the fossil forms does not give enough information regarding their origin or evolution.
However, the fossil evidence indicates that the differentiation of hepatics and mosses had already happened in the Paleozoic era. The evidence dates Bryopsida to the upper Carboniferous era.
References
- Classification of Bryophyta
- Abraham P C. Bryophytes, Pteridophytes, Gymnosperms & Paleobotany. 2000. St. Mary’s Books & Publ.
Additional Reading
- General Characteristics of Bryophytes
- Ecological and Economic Importance of Bryophytes
