Alteration of generations in bryophytes
- 1. ALTERNATION OF GENERATIONS IN BRYOPHYTES D R K AV I TA R A M B A L H . O. D B OTA N Y M . D. C O L L E G E PA R E L , M U M B A I - 1 2
- 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).
- 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.
- 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.
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