Occurrence of bryophytes
Bryophytes are terrestrial plants. They mostly grow in moist and cold habitats. Some species can grow in dry places. But they depend on moisture for the reproductive stage. Bryophytes have world wide in distribution. They are found in tropical, temperate, subarctic and arctic regions.
Bryophytes show alternation of generation.
The dominant generation in bryophyte is gametophyte. The vegetative plant body is a gametophyte. It has smaller size.
- Plant body: The plants are multicellular. They have simple thallus. Some may be differentiated into simple leaves and stem. Vascular tissues (xylem and phloem) are absent. Roots are absent. Rhizoid performs the function of water absorption and fixing the plant to the soil. Rhizoid may be unicellular or multicellular.
- Reproductive organs: Gametangia are multicellular structures. Male gametangia is antheridia and female is archegonia. Gametangia have a jacket of sterile cells.
- Nutrition: The gametophytic plants are independent. They have chloroplast for photosynthesis. The cells of the gametophyte are haploid.Sporophyte
Sporophyte is totally or partially dependent on the gametophyte for its nourishment. It has simple structure. It is differentiated into foot. seta and capsule. The cells of the sporophyte are diploid.
I. Vegetative reproductive: Vegetative reproduction is common in Bryophytes. It occurs during favourable growing season. Vegetative reproduction occurs only in the gametophytic stage. It takes place by the parts of the vegetative plant or by the production of special vegetative propagules.
2. Sexual reproduction: Sexual reproduction oogamous. Sex organs are always produced on the gametophyte. Male reproductive organ is antheridia and female reproductive organ is archegonia. Male gamete is antherozoid. It is motile. The female gamete is nonmotile oosphere. Oosphere is not liberated from the female gametangia. Both sex organs are multicellular structures. They are borne on short multicellular stalks. They’ develop a jacket of sterile cells.
(e) Antheridia: The male sex organs are called antheridia. They are
borne on short multicellular stalks. Antherozoids or spermatozoids are biflagellate structure with a long coiled body. The antheridium has androgonial cells. These cells give rise to androcytes or antherozoid mother cells. Androcytes give rise to antherozoids.
(f) Archegonia: The female sex organs are archegonia (sing
archegonium). Archegonia are flask-shaped structures. They are borne on short multicellular stalks. The basal swollen portion of the archegonium is called the venter. The upper elongated part is the neck. The venter contains the female gamete or oosphere or egg. Oosphere is not released from the archegonium. A small cell is present upper to the oosphere in the venter. It is called venter canal cell. The neck of the archegonium contains a row of cells called the neck canal cells. When the archegonia are mature the venter canal cells and neck canal cells break. It makes the passage for the entry of antherozoids.
Fertilization: The antherozoid fuse with the oosphere during fertilization. It forms diploid oospore. Oospore is the beginning of the sporophytic stage. The oospore develops into the sporophyte.
3. Asexual reproduction: The sporophyte produces the spores on
maturity by the asexual process. The spore mother cells give rise to spores. Each spore mother cell undergoes meiosis. It produces four haploid spores. Spores are the beginning of the gametophytic stage.
Alternation of Generation
Regular heteromorphic alternation of generation occurs in bryophytes. The gametophyte produces haploid gametes. After fertilization the oospore gives rise to diplopod sporophyte. The sporophyte produces the spores by meiosis. The spores give rise to the gametophyte. Thus both sporophyte and gametophyte generations alternate with each other regularly.
Bryophytes are divided into three divisions:
I. Ilepatieopsida (Liverworts): Liverworts are the simplest bryophytes. They grow on moist rocks and wet soil. Their plant body is gametophyte. There are two forms of gametophytes. Some have thallus body. Thallus is flat or ribbon-like. It is attached to soil by means of rhizoids. Example: Marchantia. Some have plant like body. In this case, the plant body grows upright. It has false leaves. Its stem is differentiated into false stem and leaves. Examples: Porella. The sex organs develop near
the tips of the branches on the upper surface of thallus. Sometimes, sex organ develop on special branches of gametophyte called antheridiophores and archegoniophores as in Marchantia. The sporophyte is dependent on gametophyte. Examples: Porella, Marchantia
- Bryopsida (Mosses): Mosses live in damp places. The adult plan body of moss is gametophyte. Gametophyte has stem and leaves like structures. Their reproductive organs, antheridia and archegonia develop on the tips of different branches. The archegonia and antheridium form clusters. They are mixed with hairs called paraphyses. They show alternation of generation. The spore of mosses produces algae like structure called protonema. Example: Funaria, Polytrichuni
- Anthoceropsida: They are slightly advanced than the Hepaticopsida and Bryopsida. Their gametophyte is highly lobed. It has irregular outline. Antheridia and Archegonia are partially sunken in the gametophyte tissues. Sporophyte does not depend on gametophyte for nourishment anci protection, except in the early stage of development. SporogThyte shows many advanced characters. So it can easily grow on. land as compared to other bryophyte groups. Sporophyte has stomata and chloroplasts in the epidermis. So it can prepare its own food by photosynthesis. It does not obtain food from gametophyte. It also has a band of meristematic tissue at the junction of foot and spore producing region. There is fast growth rate of these meristematic tissues. So the length of the sporophyte continues to increase for an indefinite period of time. In this way, the sporophyte survives even after the death and decay of the gametophyte. Example: Anthoceros.
Affinities of bryophytes
Affinities with algae
It is believed that bryophytes originated from Algae. But present day Algae do not have a structure comparable to the Bryophytes. Bryophytes originated from green Algae. Their ancestors are related to Ulotrichales (Ulothrix-like). Such ancestors formed compact masses of cells by the union of filaments. It adopted them in the terrestrial mode of life. But their vegetative structure remained simple. Bryophytes have following affinities with algae:
1. Some algae and all bryophytes show alternation of generation.
Gametophyte is dominant in both groups.
- Both produces flagellated male antherozoid.
- Both depend on water for fertilization.
- Starch is reserve food material in both groups.
- Cell wall is made up of cellulose in both groups
- Vascular tissues are absent in both.
- Protonema stage of some bryophyte resembles some adult algae.
- Rh izoids are present in both groups. Differences between algae and bryophytes I. Algae are aquatic but bryophytes are terrestrial plants.
- Plant of algae is unicellular or simple multicellular in algae. But the plant body of bryophytes is multicellular and it forms tissues.
- Stomata are absent in algae but present in bryophytes.
- Rhizoids are present only in some algae. But rhizoids are present in almost all bryophytes.
- Asexual reproduction takes place by zoospore formation in algae. But it is absent in bryophytes.
- Reproductive organs are unicellular in algae. But reproductive organs are multicellular in bryophytes. They have sterile cells.
- Oospore undergoes meiosis and form haploid spores in algae. The oospore of bryophytes does not undergo meiosis. Rather it forms diploid sporophyte.
- Sporophyte generation is independent in algae showing alternation of generation. But sporophyte of bryophytes is dependent on gametophyte.
- Algae show isomorphic alternation of generation. But bryophytes show heteromorphic alternation of generation. Affinities with Pteridophytes
Some botanists believe that bryophytes were developed from Ptericlophytes by retrogression. They give following resemblance between these two groups:
- The rootless sporophyte of Psilopsida resembles sporangium of bryophytes.
- Both have terminal sporangia.
- There are great similarities between sporogonium of Horneophyton and Anthoceros.
- There are many similarities between the antheridia and archegonia of both groups.
- Both develop embryo.6. Both have heteromorphic alternation of generation.
Most of botanists do not accept this view. There are following difference between two groups:
- The sporophyte of Pteridophytes is independent. It has root, stem and leaves. But the sporophytes of bryophytes are dependent on gametophyte.
- Vascular tissues are present in Pteridophytes but absent in Bryophytes.
Origin of bryophytes
Following changes occur during evolution of bryophytes from algae.
- Adaptations in habitats: They remained confined to the moist habitats. They also have smaller size. So they do not developed system for the absorption and conduction of water.
- Evolution of sex organs: They developed multicellular gametangia (antheridia & archegonia). Gametangia are covered by sterile cells. These sex organs originated from the multicellular gametangia or sporangia of Algae. Sterilization of the surface layer of cells occurs. These cells form the single layered jacket. In the early evolutionary history, archegonia also produced several female motile gametes. But later most cells of archegonia become sterile except a single one (oosphere). Its motility was lost. The antheridium remained simple. It produces a large number of male gametes. The motility of antherozoid was retained due to availability of water. Further evolution does not occur in the sex organs.
- Evolution of sporophyte: There are two theories about the evolution of sporophytes.
(a) Homologous theory: According to this theory, the sporophyte is not a new structure. It is produced as a result of the direct modification of tht_ gametophyte. Thus sporophyte was a part of gan,etophyte.
(b) Antithetic theory: According to this theory. the sporophyte is a
new structure. Several divisions of vegetative cell occur between the oospore. These divisions produced spores. Oospore in Algae produced the spores directly after meiosis. During the evolution of Bryophyta the oospore does not produced spores directly. It formed tissue by vegetative divisions. This tissue gave rise to diploid sporophyte. Certain cells of this sporophytic tissue then produced the spores after meiosis. The remaining cells formed the wall of the sporophyte. The sporophyte of the simplest
Bryophytes consists of a single layered wall. It surrounds a mass of sporogenous or spore mother cells. Later more cells remained sterile. These cells give rise to foot, seta and capsule of the sporophyte.