MARCHANTIA
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Characteristics
- The common habitat is moist and shady places.
- The plant body is thalloid. The thallus is dorsiventral, flat and dichotomously branched. The gametophyte is the dominant phase of plant life.
- The dorsal surface contains diamond-shaped markings, which has central pore in the middle for gaseous exchange. There are chambers present internally beneath the polygonal markings.
- The ventral surface contains scales and rhizoids. The rhizoids are unicellular and the root-like structure. The main function is to anchor the plant to the substratum and absorb water and minerals.
- The reproductive bodies are present on the dorsal surface.
- They bear a cup-shaped structure called gemmae for asexual reproduction.
- Sexual reproductive organs are born on the stalks called antheridiophore and archegoniophore. They contain male and female reproductive organs called antheridia and archegonia, respectively.
- The upper epidermis consists of air pores, which open in the air chamber present in the photosynthetic zone. The upper epidermis also contains few chloroplasts.
- Beneath the air chamber and photosynthetic zone lies the storage zone. It lacks chloroplasts and is made up of parenchymatous cells. They store protein, starch, oil and mucilage.
- The rhizoids and scales are extended from the lower epidermis.
Reproduction
Marchantia reproduces asexually as well as sexually.
Asexual Reproduction
Asexual reproduction in Marchantia is by fragmentation or by forming specialised structures known as gemmae.
Gemmae
They reproduce asexually by gemmae, which are asexual buds. They are formed in the receptacles known as gemma cups. Gemma cups are present on the dorsal surface, near the midrib of the gametophytic thalli. Gemmae are multicellular and green.
Gemmae detach from the parent plant and germinate to give rise to the new plant.
Sexual Reproduction
Marchantia is dioecious. Male and female sex organs develop on different thalli. Male reproductive organ is antheridia and the female reproductive organ is archegonia. They are born on the mature gametophyte and are erect and modified stalks known as antheridiophore and archegoniophore, respectively.
Antherozoids are produced in the antheridium. They are biflagellated and produced from the androcytes.
The archegonium is a flask-shaped structure. It consists of several neck canal cells, a ventral canal cell and an egg.
Fertilization
They need water for fertilization like other bryophytes. The neck canal cells and the ventral canal cell disintegrate and form a mucilaginous mass, which oozes out as the archegonia swells after absorbing water. It consists of chemical substances, which triggers the chemotactic response. The antherozoids get attracted and swim towards archegonia. One of the antherozoids fuses with egg and fertilization takes place. The male and female nuclei fuse together to form a diploid cell called the zygote.
Sporophyte
The diploid zygote does not undergo meiosis (reduction division) immediately. It rather divides mitotically and develops into a multicellular structure called the sporophyte. The sporophyte is differentiated into foot, seta and capsule. It is not the free-living stage, it is dependent on the gametophyte for nourishment.
Some of the cells of sporogenous tissue called spore mother cells (diploid) divide meiotically to produce haploid spores.
These haploid spores are released by the dehiscence of the capsule. Under favourable conditions, they germinate to form the new haploid plant or gametophyte.
Life Cycle
Marchantia show alternation of generation, i.e. the haploid sexual and diploid asexual phase alternates. The life cycle of Marchantia is haplodiplontic. Both the haploid and diploid phases are represented by multicellular structures.
The main free-living plant body is the gametophyte (haploid). The short-lived sporophyte (diploid) stage is dependent on the gametophyte for anchorage and nourishment.
The male and female gametophyte gives rise to antherozoids and an egg respectively, which fuse to form the diploid zygote. The zygote divides by mitotic division to form multicellular sporophyte. The spore mother cells divide by meiosis to form the haploid spore, which germinates to form the haploid gametophyte.
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