Alteration of generations in bryophytes

Alternation of generation in Bryophytes in brief

1. Alternation of generations in Bryophytes Dr Kavita Rambal H.O.D Botany M.D.College Parel,Mumbai-12

2. Alternation of Generation Alternation of generations is a life-cycle involving two phases of life, which regularly alternate with each other. Alternation of generations describes a plant's life cycle as  it alternates between a sexual phase, or generation and an asexual phase . The sexual generation produces gametes, or sex cells and is called the gametophyte generation. The asexual phase produces spores and is called the sporophyte generation.

3. Alternation of Generation In Bryophytes, the first phase is the gametophytic phase, in which gametes are produced, that contain half the number of chromosomes(Haploid). This is the dominant phase in the life of Bryophytes and reproduces sexually by egg and antherozoid. Egg and antherozoid fuse to produce a zygote, starts the second phase. The zygote germinates to produce the sporophyte(Diploid). Source: Google Images

4. This second phase, the sporophytic phase, is the spore producing phase. The sporophyte cannot exist independently. It is either simple or composed of a capsule, a stalk, and a foot that attaches the sporophyte body to the gametophyte. The sporophyte reproduces asexually by means of spores, which are produced by meiosis and are haploid. Each spore germinates to produce a gametophyte, which is the independent phase. This way, the life cycle is completed. This type of life cycle is called haplodiplontic type of life cycle.

5. Alternation of Generation in HEPATICEAE Gametophytic generation is prominent and independent. Sex organs Antheridia and archegonia develop on it and produce gametes.(Haploid) Fertilization of gametes results in formation of zygote.(Diploid). Zygote develops into sporophyte often called sporogonium(Diploid). Sporophyte is completely dependent on gametophyte.

6. Alternation of Generation in HEPATICEAE Sporophyte very simple in Riccia, some liverworts may show slightly developed sporogonium.(foot, seta and capsule) In primitive capsule entire endothecium forms spores .(Haploid) In advanced type some of the endothecium cells form elaters and elatophores which help in spore dispersal. Thus in Hepaticeae there is definite alternation of generations. The thalloidal, independent gametophyte alternates with parasitic sporogonium (Sporophyte).

7. Source :Google Images

8. Alternation of Generation in ANTHOCEROTAE Prominent generation in Anthocerotae is a gametophytic generation as in other Bryophytes. Like primitive Hepaticeae, thallus or gametophyte is green ,dorsiventrally flattened and lobed. The upper surface is smooth and lower side develops number of unicellular rhizoids. Internally thallus is homogenous and consists of Chlorenchymatous cells. Chloroplast with pyrenoid present.

9. Alternation of Generation in ANTHOCEROTAE Gametophyte gives rise to sex organs (antheridia and archegonia) bearing male and female gametes(haploid) respectively. Gametes fuse to form zygote(diploid) first cell of sporophytic generation. Zygote mitotically divides to give rise to sporophyte. Sporophyte better developed than Hepaticeae, partially parasitic on female gametophyte. Sporophyte differentiated into foot, meristematic region an capsule. Capsule wall inner to epidermis Chlorenchymatous (photosynthetic).

10. Alternation of Generation in ANTHOCEROTAE Some of the sporocytes form pseudoelators. Continuous supply of spores is ensured due to meristematic region. Spores germinate to form gametophytes. Thus primitive gametophyte of Anthocerotae alternates with slightly better developed sporophyte.

11. Source :Google Images

12. Alternation of Generation in MUSCI Comparatively better developed gametophyte of Musci alternates with better developed sporophyte. The gametophyte is erect ,differentiated into rhizoid, stem like(caulloid) and leaf like structures (phylloids). The gametophyte is independent ,autotrophic and free living. Gametophyte gives rise to sex organs and gametes (haploid). Gametes fuse to form zygote (diploid) in presence of water. Sporophyte better developed is semiparasitic, can manufacture its own food material. Thus there is distinct alternation of generations.

13. Source:Google Images

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