ALTERNATION OF GENERATIONS IN BRYOPHYTES

ALTERNATION OF GENERATIONS IN BRYOPHYTES.

The phenomenon in which gametophyte and sporophyte generations regularly alternate with each other is called alternation of generation. The bryophytes show two distinct generations. These generations are gametophyte and sporophyte.

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. The spore mother cells divide by meiosis to form spores. The spore germinates to form gametophyte. The sporophyte generation begins with oospore and ends at spore mother cells.

Significance of Alternation of Generation

Alternation of generation has great importance for platts. 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

The spores produce gametophyte. The gametophyte with better genetic make up will have better chance for survival. On the other hand, the gametophyte with less advantageous characte, istics will be eliminated. These gametes are produced by mitosis. So there is no reshuffling of genes during gametes formation and hence 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 sporophyte. This oospore grows to form new sporophyte. So these genetic variations pass to new sporophyte. The new mature sporophyte further produces genetic recombination. These variations are transferred to new gametophyte. In this way нау, the sporophyte produces a large genetic variability and nature selects the best genetic combinations. So the populations of plants become better adapted to their environment.

DIVISION TRACHEOPHYTA

The vascular plants with xylem and phloem tissues are called tracheophytes. They are most successful group of land plants. They are adapted successfully to rough land habitats. Flowering plants are also tracheophytes. They have dominated the land habitat. The vascular plants show evolution of following complex vegetative and reproductive characteristics. These characteristics have made them the predominant plants of land:

1. They have root, stem and leaves.

2. They have vascular systems in stems, roots and leaves.

3. They protect sporangia. It leads to evolution of seed.

4. They develop pollen tube. So they do not require water for the transmission of male gametes to female gametes.

5. They develop flower and fruits.

6. The show heteromorphic alternation of generation.

7. The tracheophytes are further sub-divided into four sub-divisions:

(i) Psilopsida,

(ii) Lycopsida,

(iii) Sphenopsida

(iv) Pteropsida.