Bryophyta is defined as, plants with distinguishing characters as follows: “Vascular system absent, gametophyte dominant, sporophyte attached to gametophyte and homosporous. Bryophyta shows following characters.
- These are the first land plants.
- They were evolved from green algae.
- They are poorly adapted for life on land. So they are present only in damp shady places.
- Conducting and strengthening tissues (xylem and phloem) are absent in bryophytes.
- The transport of water, minerals, prepared food and substances take place by diffusion.
- The plant body is with or without cuticle. Some bryophytes have very thin layer of cuticle.
- Water is absorbed by general surface of plant.
- The bryophytes are also called amphibious plants as they cannot live without water. They need water for their reproduction
- Bryophyta is non-vascular flowerless plant.
Alternation of generation or Life Cycle or Reproduction of Bryophyta
Bryophytes plants show regular alternation of generations. These generations are heteromorphic.
Gamete producing generation is called gametophyte. It is haploid, dominant, independent and free living generation in bryophytes. There are two body forms of gametophytes in bryophytes.
- It may be thallus as in many liverworts.
- It is differentiated into structures, resembling to stem, leaves, absorbing and anchoring organs rhizoids, as in mosses and some liverworts.
Spore producing generation is called sporophyte. It is a less conspicuous generation in bryophytes. It partially or completely depends on gametophyte for its nutrition. The sporophyte generally consists of foot, seta and capsule.
The sporophyte is diploid (2n). It produced haploid spores by meiosis in sporangia. These spores are of one kind. So sporophyte of bryophyte is homosporous. These spores germinate to produce gametophyte. The entire development of sporophyte takes place within the gametophyte generation. Even after full development, the sporophyte remains attached to gametophyte. It does not contain chloroplasts and cannot perform photosynthesis. So it depends on gametophyte for nourishment.
The alternation of generation is an important phenomenon. It produces variation and selects the best genetic makeup among the organisms for survival and adaptation in the changing environments.
The gametophyte produces male and female reproductive organs either on same plant or different plants. These reproductive organs are protected by a covering of sterile cells (protective cell). These sex organs produce gametes by meiosis.
- Antheridia (sing. Antheridium): These are male reproductive organs. Antheridia produce male gametes called antherozoid. These antherozoids are motile. They are produced in large number.
- Archegonia (sing. Archegonium): These are female reproductive organ. A single egg is formed in each archegonium.
Fertilization takes place ¡n water. Antherozoids (1n) move towards archegonia chemtactically (chemical attractions). A single antherozoid (1n) fuses with an egg (1n) to form a diploid (2n) zygote.
The zygote is retained within the female sex organ (archegonia). After resting period, the zygote divides by mitosis to form diploid embryo. This embryo develops to from sporophyte. This sporophyte is also diploid.
Adaptation in Bryophyta to Land Habitat
Bryophytes generally develop following adaptive characters for terrestrial environments:
- Bryophytes have compact multicellular plant body. It reduces the surface area exposed to dry land. So it helps ¡n the conservation of water. They also have a layer of cuticle. It further reduces the loss of water by evaporation.
- Their photosynthetic tissues develop special chambers for the absorption of CO2, without losing much water and exposure to light.
- They develop rhizoids for absorption of water and anchorage.
- They show heterogamy (different types of gametes). Their egg is non-motile and it contains stored food. Their sperm is motile.
- They develop special multicellular reproductive organs called antheridia and archegonia. These reproductive organs produce and protect gametes.
- They develop multicellular embryo. Embryo is retained and protected inside the female reproductive body during its development.
- They show alternation of generation. They develop gametophyte and sporophyte generations. Gametophyte produces gametes and sporophyte produces spore. The alternation of generation produces best genetic combinations which help them for adapting the terrestrial conditions.
Classification of Bryophyta
Bryophytes are divided into three classes: Hepaticeae, Musci and Anthocerotae.
- Hepaticeae (liverworts)
- Anthocerotae (Horn worts)
- Liverworts are the simplest bryophytes.
- They grow on moist rocks and wet soil. They live near water. So there they do not become dry.
- Their plant body is gametophyte. There are two forms of gametophytes:
- Thallus: It is flat, or ribbon-Like. It is branched dichotomously. It is attached to soil by means of rhizoids.
- Plant like Body: In this case, the plant body grows upright, It has false leaves. Its stem is differentiated into false stem and leaves.
- The sex organs develop near the tips of the branches on the upper surface of thallus. Sometimes they develop on special branches of gametophyte called antheridiophores and archegoniophores as in Marchantia.
- The sporophyte is dependent on gametophyte.
- Mosses inhibit damp places. They also grow well in dry places. However, water is essential for the reproduction of mosses. So they grow to form cushions or mats (soft mass).
- 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. These branches may be on the same plant as Funaria or on the different plants as in Polytrichura. The archegonia and antheridium form dusters. They are mixed with hairs called paraphyses.
- They show alternation of generation. The gametophyte and sporophyte generations alternate with each other.
- Gametophyte: Gametophyte produces haploid gametes. These gametes fuse to form zygote. The zygote germinates to form sporophyte.
- Sporophyte: The sporophyte produces spores. The spore of mosses produces algae like structure called protonema. Protonema is absent in liverworts. A bud is formed on protonema. Each bud grows to form haploid gametophyte and complete the life cycle.
Anthocerotae (Horn worts)
- Their gametophyte is highly lobed. It has irregular outline.
- Antheridia and Archegonia are partially sunken in the gametophyte tissues.
- The sporophyte does not depend on gametophyte for nourishment and protection, except in the early stage of development.
- The sporophyte 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.
- sporophyte also has waxy cuticle layer. It reduces loss of water (desiccation).
- sporophyte also has a band of meristematic tissue (The tissues in which tell continuously divides) at the junction of foot and spore producing region. It continues to form new cells toward the spore producing region during the formation, maturation and dispersal of spores from the other end. 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: It is found in the hilly areas.
Alternation of Generation in Bryophyta
The bryophytes show two distinct generations. The generations are gametophyte and sporophyte. The gametophyte and sporophyte generations regularly alternate with each other’s. This phenomenon is called alternation of generation.
1. Gametophyte generation
The gamete producing generation is called gametophyte generation. Gametophyte is haploid generation. It is more conspicuous and dominant generation. The gametes are spermatozoids and eggs. A haploid spermatozoid fuses with a haploid egg to produce Oospore (zygote). The Oospore grows and produces sporophyte generation. So haploid gametophyte stage begins with spores and ends at gametes.
2. Sporophyte Generation
The spore producing generation is called sporophyte. It is diploid (2n) and less conspicuous generation. It is differentiated into foot, seta and capsule (sporangium). The capsule has spore mother cells. These cells divided by meiosis to form spores. The spore germinates to form gametophyte. The sporophyte generation begins with oospore and ends at spore mother cells.
Significance or Importance of Alternation of Generation
Alternation of generation has great importance for plants. It helps the plant to adapt in different environments. So it increases the survival of the plants.
1. Production of variety of spores by sporophyte
Sporophyte produces spores. These spores are formed from spore mother cells by meiosis. It causes reshuffling of genes. As a result a great variety of spores are produced. These spores have different genetic combinations.
2. Formation of gametophyte from spores
These spores produce gametophyte. The gametophyte with better genetic makeup will have better chance for survival. One other hand, the gametophyte with less advantageous characteristics will be eliminated. These gametes are produced by mitosis. So there is no reshuffling of genes during gametes formation and variations are not produced in gametes. The gametes fertilize to form zygote or oospore.
3. Formation of Sporophyte from oospore
The oospore has a new genetic make-up as compared to the parent. This oospore grows to form new sporophyte. So these genetic variation passes to sporophyte. The new mature sporophyte further produces genetic recombination. These variations are transferred to new gametophyte. In this way, the sporophyte produces a large genetic variability and selects the best genetic combinations. So the populations of plants become better adapted to their environment.