Description[ edit ] The plant body of a hornwort is a haploid gametophyte stage. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter. Each cell of the thallus usually contains just one chloroplast. In half of the roughly hornwort species , this chloroplast is fused with other organelles to form a large pyrenoid that both enables more efficient photosynthesis and stores food. The pyrenoid is comprised predominantly of RuBisCO , the key enzyme in carbon fixation.
|Published (Last):||27 July 2013|
|PDF File Size:||3.99 Mb|
|ePub File Size:||20.26 Mb|
|Price:||Free* [*Free Regsitration Required]|
Description[ edit ] The plant body of a hornwort is a haploid gametophyte stage. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter.
Each cell of the thallus usually contains just one chloroplast. In half of the roughly hornwort species , this chloroplast is fused with other organelles to form a large pyrenoid that both enables more efficient photosynthesis and stores food.
The pyrenoid is comprised predominantly of RuBisCO , the key enzyme in carbon fixation. By using inorganic carbon transporters and carbonic anhydrases, up to a fold increase in CO 2 levels can be achieved. Many hornworts develop internal mucilage -filled cavities or canals when groups of cells break down. They will secrete hormogonium-inducing factors HIF that stimulate nearby, free-living photosynthetic cyanobacteria , especially species of Nostoc , to invade and colonize these cavities.
There may also be small slime pores on the underside of the thallus. These pores superficially resemble the stomata of other plants. The horn-shaped sporophyte grows from an archegonium embedded deep in the gametophyte.
The sporophyte of a hornwort is unusual in that it grows from a meristem near its base, instead of from its tip the way other plants do. Unlike liverworts , most hornworts have true stomata on their sporophyte as mosses do. The exceptions are the genera Notothylas and Megaceros , which do not have stomata.
The sporophyte of most hornworts are also photosynthetic, which is not the case with liverworts. The pseudo-elaters are multi-cellular, unlike the elaters of liverworts. They have helical thickenings that change shape in response to drying out; they twist and thereby help to disperse the spores. The spores are polar, usually with a distinctive Y-shaped tri-radiate ridge on the proximal surface, and with a distal surface ornamented with bumps or spines Life cycle[ edit ] The life of a hornwort starts from a haploid spore.
In most species, there is a single cell inside the spore, and a slender extension of this cell called the germ tube germinates from the proximal side of the spore. By contrast, species of the family Dendrocerotaceae may begin dividing within the spore, becoming multicellular and even photosynthetic before the spore germinates. Life cycle of a typical hornwort Phaeoceros.
Click on the image to enlarge. From the protonema grows the adult gametophyte , which is the persistent and independent stage in the life cycle. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter, and several layers of cells in thickness. It is green or yellow-green from the chlorophyll in its cells, or bluish-green when colonies of cyanobacteria grow inside the plant.
When the gametophyte has grown to its adult size, it produces the sex organs of the hornwort. Most plants are monoecious , with both sex organs on the same plant, but some plants even within the same species are dioecious , with separate male and female gametophytes.
The female organs are known as archegonia singular archegonium and the male organs are known as antheridia singular antheridium. Both kinds of organs develop just below the surface of the plant and are only later exposed by disintegration of the overlying cells. The biflagellate sperm must swim from the antheridia, or else be splashed to the archegonia.
When this happens, the sperm and egg cell fuse to form a zygote , the cell from which the sporophyte stage of the life cycle will develop. Unlike all other bryophytes, the first cell division of the zygote is longitudinal. Further divisions produce three basic regions of the sporophyte. At the bottom of the sporophyte closest to the interior of the gametophyte , is a foot. This is a globular group of cells that receives nutrients from the parent gametophyte, on which the sporophyte will spend its entire existence.
In the middle of the sporophyte just above the foot , is a meristem that will continue to divide and produce new cells for the third region. This third region is the capsule. Both the central and surface cells of the capsule are sterile, but between them is a layer of cells that will divide to produce pseudo-elaters and spores. These are released from the capsule when it splits lengthwise from the tip.
The tip grows continuously giving rise to separate plants. Tubers escape from desiccation with the formation of a resistant layer of cork cells. Tubers store food and function as perennating organ that germinate into new gametophytes on the return of favourable environmental conditions. They are stalked and often develop mucilage pores. On separation from the parent plant, the detached gemmae grow into a new gametophyte.
Type Anthoceros – Structure, Reproduction and Development
They are present in shady places or in the crevices of rocks. Anthoceros erecrus, A. General structure Anthoceros has thallus body. It is lobed and it has irregular or dichotomous branches.
Life Cycle of Anthoceros (With Diagram) | Anthocerotopsida
Full size table The total assembly length varied between and Mb, which is consistent with the size estimates based on k-mer analysis Table 1 but slightly larger than those from flow cytometry 17 , The genome size variation among the three strains can be largely attributed to the differences in repeat content Supplementary Fig. A combination of ab initio, evidence-based and comparative gene prediction approaches resulted in 24,—25, predicted protein-coding genes Supplementary Table 5. For A. The three hornwort genomes show a high gene density compared to other land plants Supplementary Table 6.
Gametophytic Phase of Anthoceros: i External Features: The gametophytic plant body is thalloid, dorsiventral, prostrate, dark green in colour with a tendency towards dichotomous branching. Such branching results into an orbicular or semi orbicular rosette like appearance of the thallus. The thallus is bilobed A. Dorsal Surface: The dorsal surface of the thallus may be smooth A. It is shining, thick in the middle and without a distinct mid rib Fig. Ventral Surface: The ventral surface bears many unicellular, smooth-walled rhizoids Fig. Their main function is to anchor the thallus on the substratum and to absorb water and mineral nutrients from the soil.