Define alternation of generation discuss it with reference to life cycle of Marchantia.
Alternation of generation is a phenomenon in which two generations : haploid gametiphytic generation and diploid saprophytic generation which are morphologically, physiologically and cytologically different, come one after another to complete life cycle of an organism.
Kingdom : Plantae
Division : Bryophyta
Class : Hepaticopsida
Order : Marchantiales
Family : Marchantiaceae
Genus : Marchantia
Habitat : It is found in moist, damp, shady, terrestrial places like wet rocks, trunks of tree, bank of river, damp walls, etc.
Habit : The plant body is gametophytic thallus. The thallus is green, prostate, dorsoventrally flattened and dichotomously branched. The thallus consists of prominent midrib on the dorsal side and many multicellular scales and unicellular rhizoids are present on ventral surface. Scales are of two types : ligulate and appendiculate and two types of rhizoids are smooth walled and tuberculated.
Sexual reproduction : Marchantia is dioecious in nature. Male and female thalli are different. Antheridiophore is born on male receptacle and Archegoniophore is born on female receptacle. Antheridiophores consist of 8 lobes and archegoniophores consist of 8 lobes and 9 rays.
The male gamete is stored inside antheridium which is oval shaped, jacketed and attached to the floor of antheridial cavity. It divides mitotically to produce androcytes which metamorphoses into motile, biflagellate, coiled antherozoids.
The female gamete is stored inside flask shaped structure which is covered by perigynium and perichaetium. Each archegonium consists of neck and venter. The venter consists of egg cell and venter canal cell and in the neck region, there are 4-6 neck canal cells. Dehiscence of antheridium takes place in the presence of water.
When water enters the antheridial chamber, its walls absorb water and disintegrates and the antherozoids come out of the ruptured antheridial walls. During fertilization, the venter canal cell and neck cells degenerate and produce mucilaginous substance. In the presence of water, these mucilaginous substance swells up and give pressure to the cover cells and then antherozoids reach the egg cells showing chemotactic movement and fuses with egg cell forming zygote.
The zygote is diploid which divides and redivides to form embryo and then sporophyte. It consists of 3 structures; foot, seta and capsule. The foot helps for fixation and absorption, seta helps in conduction, capsule helps in formation of spore cell elators.
At maturity, the seta elongates which pushes the capsule and helps in rupturing of covering then the spore liberate out of capsule with the help of elators.
After coming out of capsule, they germinate in the presence of suitable condition. It consists outer exine and inner intine. The exine ruptures and the intine develops into a germ tube which divides and redivides to form new thallus of Marchantia.