Introduction
Bryophytes are plants of amphibious zone. During the dry period they become almost brittle in texture. With the onset of rainy season the apparently dried, brittle thalli turn green and become active to carry out the normal life functions.
They are called as “amphibians of plant kingdom”, because they are terestial plants, but require water to complete their life cycle at the time of sexual reproduction.
Bryophytes occupy a position just intermediate between the green thallophytes (Algae) and the vascular cryptogams (Pteridophytes).
They are a group of land plants and are non vascular.
The group Bryophyta (Greek word; Byon = moss, Phyton = plant) and are 2nd largest taxonomic group in plant kingdom.
It include the simplest and most primitive land plants. About 960 genera & 24,000 species have been reported in Bryophyta. Most of the Bryophytes are land dwellers which inhabit damp, shaded and humid localities.
Classification system of Bryophytes :
Braun (1864) was the first person who introduce the term Bryophyta and called it acotyledonae. He included Algae, Fungi, Lichen and Mosses in it.
Schimper (1879) gave Bryophyta as the rank of a division.
Eichler (1883) divide Bryophyta into two groups Hepaticae and Musci.
Engler (1892) divided the division Bryophyta into two classes Hepaticae and Musci, and further divided each of the two classes into 3 orders as follows:
uSchuster (1953,1958) distinguished three classes in Bryophyta : Hepaticae, Anthocerotae and Musci.
uFinally, Rothmaler (1961) changed the nomenclature of the three classes of Bryophyta to : Hepaticopsida (= Hepaticae) Anthoceropsida (= Anthocerotae) Bryopsida (= Musci)
uProskauer(1967) has changed the name Anthoceropsida to Anthocerotopsida.
The modern bryologists, Schofield (1985) classify Bryophyta into 3 classes:
Class 1. Hepaticopsida (Liverwort)
Class 2. Anthocerotopsida (Hornwort)
Class 3. Bryopsida (Mosses)
A. Hepaticopsida
The name hepaticopsida comes from the word “hepatic” meaning liver. Liverworts come under this class.
Hepaticopsida is further divided into 4 orders: Marchantiales (e.g. Riccia), Sphaerocarpales (e.g. Sphaerocarpos), Calobryales (e.g. Calobryum) & Jungermanniales (e.g. Pellia)
Gametophyte plant is either thalloid or foliose.
uIn foliose forms, leaves are without midrib and dorsiventral.
Thalloid is dorsiventral, lobed and dichotomously branched.
Each cell of thallus contains many chloroplasts without pyrenoids.
The leaves of liverworts are lobate green structures similar to the lobes of the liver.
Rhizoids are unicellular, branched and aseptate.
Sex organs are borne dorsally embedded in gametophytic tissues.
The sporophyte is made up of only capsule (in Riccia) or foot, seta and capsule (in Marchantia).
uThe columella is absent in the capsule.
Sporogenous tissues develop from endothecium.
Reproduction: Liverworts can be monoicous or dioicous. Also, they show both sexual and asexual reproduction. Asexual reproduction: It takes place by fragmentation or by the formation of gemmae. Gemmae are produced inside gemma cups. Gemmae are asexual buds which are green and multicellular. The gemma cup develops into a new plant after detaching from the parent plant.(Gemmae: small, intact, complete pieces of plant that are produced in a cup on the surface of the thallus and develop into gametophytes through asexual reproduction.) Sexual reproduction: Antheridium (male organ) and archegonium (female organ) may be present on the same thalli or different thalli. They produce sperm and egg, respectively. After fertilisation, zygote is formed. The zygote develops into a diploid sporophyte, and a few cells of the sporophyte undergo meiosis to form haploid spores. These spores develop into haploid gametophytes, which are free-living and photosynthetic.
Examples of Liverworts: Marchantia Riccia
Life cycle of Liverworts:
The life cycle of liverworts follows alternation of generations:
Alternation of generation: the life cycle of plants with a multicellular sporophyte, which is diploid, that alternates with a multicellular gametophyte, which is haploid.
The gametophyte stage is the dominant stage in liverworts; however, liverwort sporophytes do not contain stomata.
Spores germinate into gametophytes, the zygote develops into a sporophyte that releases spores, and then spores produce new gametophytes.
The liverwort’s life cycle begins with the release of haploid spores from the sporangium that developed on the sporophyte.
Spores disseminated by wind or water germinate into flattened thalli gametophytes attached to the substrate by thin, single-celled filaments.
Male and female gametangia develop on separate, individual plants.
Once released, male gametes swim with the aid of their flagella to the female gametangium (the archegonium), and fertilization ensues.
The zygote grows into a small sporophyte still attached to the parent gametophyte and develops spore-producing cells and elaters.
The spore-producing cells undergo meiosis to form spores, which disperse (with the help of elaters), giving rise to new gametophytes.
B. Anthocerotopsida :
There are around 300 species present in this class. They are commonly known as hornworts. It has only one order i.e. Anthocerotales.
The gametophytic body is flat, dorsiventral, simple thalloid without internal differentiation
Rhizoids are smooth-walled
Each cell has one chloroplast with a pyrenoid
Sex organs are present dorsally embedded in the thallus
The sporophyte is differentiated into foot, meristematic zone and capsule
Sporogenous tissues develop from amphithecium
Pseudoelaters are present in the capsule
The columella is present in the capsule, which originates from endothecium
Reproduction: Asexual reproduction: Vegetative propagation is by fragmentation of thallus and by tubers, which are formed under unfavourable conditions
uSexual reproduction: They reproduce sexually by waterborne sperm, which travel from antheridium to archegonium. A fertilised egg develops into a sporophyte. Sporophyte splits lengthwise to release spores which develop into a gametophyte.
Examples of Hornworts:
Anthoceros
Notothylas
Megaceros
Life cycle of Hornworts:
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.
Pseudoelater: single-celled structure that aids in spore dispersal.
The haploid spores germinate and produce the next generation of gametophytes.
C. Bryopsida
uIt is the largest class of Bryophyta, with around 1400 species. They are commonly called mosses.
Bryopsida is further divided into 5 orders:
Bryales
Andreales
Sphagnales
Polytrichales and Buxbaumiales
The gametophyte is differentiated into protonema and foliose gametophore
Foliose is made up of stem as an axis and leaves without midrib
Rhizoids are multicellular with oblique septa
Sex organs are borne apically on stem
Elaters are absent
The sporophyte is differentiated into foot, seta and capsule
Sporogenous tissues develop from endothecium.
Life cycle of Mosses:
The life cycle of mosses alternates between the haploid gametophyte and the diploid sporophyte, called alternation of generation. The female and male gametophyte produce haploid gametes, which fuse to form a zygote and give rise to the diploid sporophyte. The diploid sporophyte then produces haploid spores, germinating to the haploid gametophyte.
The life cycle of most mosses begins with the release of spores from a capsule, which opens when a small, lidlike structure, called the operculum, degenerates. A single spore germinates to form a branched, filamentous protonema, from which a leafy gametophyte develops. The gametophyte bears organs for sexual reproduction. Sperm, which are released by the mature antheridium (the male reproductive organ), are attracted into the neck of an archegonium (the female reproductive organ). Here, one sperm fuses with the egg to produce the zygote. After cell division, the zygote becomes the sporophyte, and, at the same time, the archegonium divides to form the protective calyptra. The sporophyte usually consists of a capsule and a seta. Asexual reproduction occurs within the capsule and the whole process may begin again.
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