Bryophyta is defined as, plants with distinguishing characters as follows: “Vascular system absent, gametophyte dominant, sporophyte attached to gametophyte and homosporous. Division Bryophyta shows the following characters.
Bryophytes plants show regular alternation of generations. These generations are heteromorphic.
Gamete producing generation is called gametophyte. It is the haploid, dominant, independent, and free-living generation in bryophytes. There are two body forms of gametophytes in bryophytes.
Spore producing generation is called the sporophyte. It is a less conspicuous generation in bryophytes. It partially or completely depends on the 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 the 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 the same plant or different plants. These reproductive organs are protected by a covering of sterile cells (protective cells). These sex organs produce gametes by meiosis.
Fertilization takes place ¡n water. Antherozoids (1n) move towards archegonia schematically (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 the resting period, the zygote divides by mitosis to form a diploid embryo. This embryo develops from the sporophyte. This sporophyte is also diploid.
Bryophytes generally develop the following adaptive characters for terrestrial environments:
Bryophytes are divided into three classes: Hepaticeae, Musci, and Anthocerotae.
The bryophytes belong to this class are called liverworts. This class contains 900 species.
Example: Marchantia.
Examples: Perella.
They are slightly advanced than the Musci and Hepaticae.
The bryophytes show two distinct generations. The generations are gametophyte and sporophyte. The gametophyte and sporophyte generations are regularly alternate with each other. This phenomenon is called alternation of generation.
The gamete producing generation is called the gametophyte generation. The gametophyte is haploid generation. It is a 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 a sporophyte generation. So the haploid gametophyte stage begins with spores and ends at gametes.
The spore-producing generation is called the 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.
Alternation of generation has great importance for plants. It helps the plant to adapt to different environments. So it increases the survival of the plants.
The sporophyte produces spores. These spores are formed from spore mother cells by meiosis. It causes the reshuffling of genes. As a result, a great variety of spores are produced. These spores have different genetic combinations.
These spores produce gametophyte. The gametophyte with better genetic makeup will have a better chance of survival. One the 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.
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 the sporophyte. The new mature sporophyte further produces genetic recombination. These variations are transferred to the 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.
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