anthoceros structure and types of reproduction
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The class Anthocerotopsida, comprising about 6 genera and 300 species, is characterized by a flat, thalloid gametophytic body with unicellular rhizoids and the absence of air chambers. It includes the families Anthocerotaceae and Notothylaceae, with notable genera such as Anthoceros and Notothylas, primarily found in moist habitats. Reproduction occurs both vegetatively and sexually, with unique features like symbiotic associations with cyanobacteria and the presence of gemmae in some species.
anthoceros structure and types of reproduction
- 2. General Characters: 1. This class is represented by about 6 genera and 300 species. 2. Plant body is flat, dorsiventral, thalloid, gametophytic and variously lobed. 3. Smooth walled rhizoids are present. 4. Internally the thallus is not differentiated into zones. 5. All cells are alike. 6. Air chambers or air pores are absent. 7. Each cell has a single chloroplast and each chloroplast contains one to many pyrenoids. 8. Mucilage cavities open on the ventral surface by slime pores. 9. Sex organs are embedded in the thallus.
- 3. 10. Antheridia develop either singly or in groups in closed cavities called antheridial chambers. 11. The sporophyte is differentiated into foot, an intermediate zone or meristematic zone and capsule. 12. Due to the presence of the meristematic zone, the sporophyte shows indeterminate growth i.e., it continues to grow indefinitely. 13. Archesporium is amphithecial in origin. 14. Sporogenous tissue forms the fertile spores and sterile elaters. 15. Elaters do not have spiral thickenings and are known as pseudo elaters. 15. Capsule wall is four to six layered thick and epidermis has the stomata. 16. Capsule matures from apex to base and usually dehisce by two valves.
- 4. • The class Anthocerotopsida has only a single order Anthocerotales. • Muller (1940), Proskauer and Reimers (1954) divided the order Anthocerotales in two families: • Family 1.Anthocerotaceae (e.g., Anthoceros) • Family 2.Notothylaceae (e.g., Notothylas). • Family-Anthocerotaceae (4 or 5 genera; important genus Anthoceros) 1. The capsule is linear and vertical. 2. The stomata are present on the wall of capsule. 3. The archesporium develops from amphithecium. 4. The elaters are four-celled, smooth or thick-walled; 5. Thickening band may or may not be present.
- 5. Family-Notothylaceae (single genus-Notothylas) 1. The capsule is cylindrical and horizontal. 2. The stomata are not found on the wall of capsule. 3. Archesporium arises from endothecium and amphithecium. 4. Elaters are short and stumpy. 5. They have irregular thickening bands. • However, according to top bryologists, there is only one family, i.e., Anthocerotaceae. • About five or six genera are included in this family. • These genera are—Anthoceros, Phaeoceros, Aspiromitus, Notothylas, Dendroceros and Megaceros. • Four genera are universally recognised, they are—Anthoceros, Megaceros, Dendroceros and Notothylas. • This group is placed intermediate between Hepaticopsida (Hepaticae) and Bryopsida (Musci).
- 6. • All species are terrestrial and cosmopolitan in distribution. • The species grow in very moist and shady places like slopes, rocks or sides of the ditches. • Some species are found growing on decaying wood. • Unlike other bryophytes Anthoceros is usually not well adapted to resist dry conditions. • In India Anthoceros is represented by about 25 species. • Out of these three species of Anthoceros viz., A. himalayensis, A. erectus and A. chambensis are commonly found growing in the Western Himalayan region at an altitude of 5000-8000 feet (Kashyap, 1915). • These species are also found growing in Mussoorie, Kulu, Manali, Kumaon, Chamba valley, Punjab, Madras and in plains of South India.
- 7. Gametophytic Phase of Anthoceros • 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. • Anthoceros form small rosette like plant. • Unicellular rhizoids are attached to the underside of the thallus. • Small mucilaginous cavities are present on the ventral side. • These cavities contain colonies of a blue green alga like Nostoc. • Stomata like small slits are present on the dorsal side of the thallus. • Mucilage oozes out through these slits.
- 9. Hornwort phylogeny and the morphological features of Anthoceros punctatus. (a) Hornwort phylogeny (Villarreal & Renner, 2012). Genera whose species could be successfully transformed using our improved protocol are marked with red arrowheads. (b–j) Morphology of A. punctatus. (b) Germinating spore that develops into a thallus. Scale bar: 4 μm. (c) The thallus is irregularly shaped, lacks specialized internal differentiation and is composed of mucilage chambers and parenchyma cells. Scale bar: 2 mm. (d) Scanning electron microscopy of thallus. Scale bar: 100 μm. The male (antheridia are shown with a yellow arrow) (e) and female (archegonia) reproductive organs are embedded in the thallus. Scale bar: 0.5 mm. (f) Thallus with sporophytes (see white arrow). (g) Longitudinal section of the foot (see yellow arrow) surrounded by the involucre showing the basal meristem and differentiating sporogenous tissue. Scale bar: 100 μm. (h) Transverse section of the sporophyte showing its morphology. From the centre to outside: columella, spores, pseudoelaters, assimilative tissue, epidermis, and stomata with substomatal cavities. Scale bar: 100 μm. (i) Transmission electron microscopy of the chloroplast (yellow arrow: pyrenoid). Scale bar: 1 μm. (j) Hand section of A. punctatus thallus showing ellipsoidal cavity colonized by cyanobacteria. Scale bar: 100 μm.
- 10. Hornwort phylogeny and the morphological features of Anthoceros punctatus. (a) Hornwort phylogeny (Villarreal & Renner, 2012). Genera whose species could be successfully transformed using our improved protocol are marked with red arrowheads. (b–j) Morphology of A. punctatus. (b) Germinating spore that develops into a thallus. Scale bar: 4 μm. (c) The thallus is irregularly shaped, lacks specialized internal differentiation and is composed of mucilage chambers and parenchyma cells. Scale bar: 2 mm. (d) Scanning electron microscopy of thallus. Scale bar: 100 μm. The male (antheridia are shown with a yellow arrow) (e) and female (archegonia) reproductive organs are embedded in the thallus. Scale bar: 0.5 mm. (f) Thallus with sporophytes (see white arrow). (g) Longitudinal section of the foot (see yellow arrow) surrounded by the involucre showing the basal meristem and differentiating sporogenous tissue. Scale bar: 100 μm. (h) Transverse section of the sporophyte showing its morphology. From the centre to outside: columella, spores, pseudoelaters, assimilative tissue, epidermis, and stomata with substomatal cavities. Scale bar: 100 μm. (i) Transmission electron microscopy of the chloroplast (yellow arrow: pyrenoid). Scale bar: 1 μm. (j) Hand section of A. punctatus thallus showing ellipsoidal cavity colonized by cyanobacteria. Scale bar: 100 μm.
- 11. Dorsal Surface • The dorsal surface of the thallus may be smooth (A. laevis) or velvety because of the presence of several lobed lamellae (A. crispulus) or rough with spines and ridges (A. fusiformis). • It is shining, thick in the middle and without a distinct mid rib.
- 12. Dorsal Surface • The dorsal surface of the thallus may be smooth (A. laevis) or velvety because of the presence of several lobed lamellae (A. crispulus) or rough with spines and ridges (A. fusiformis). • It is shining, thick in the middle and without a distinct mid rib. Ventral Surface • The ventral surface bears many unicellular, smooth-walled rhizoids. • Their main function is to anchor the thallus on the substratum and to absorb water and mineral nutrients from the soil. • Many small, opaque, rounded, thickened dark bluish green spots can be seen on the ventral surface. • These are the mucilage cavities filled with Nostoc colonies. • In the month of September and October the mature thalli have erect, elongated and cylindrical sporogonia. • These are horn like and arise in clusters. • Each sporogonium is surrounded by a sheath like structure on its base. It is called involucre.
- 14. Key morphological features of Anthoceros agrestis. (a) Light micrograph (LM) of germinating spores. Upper three images, successive stages in globose sporeling production. Lowermost: under low light conditions spore germination involves a germ tube, a long single-celled filament that develops a terminate globose sporeling. Bars, 50 lm. (b) Surface view of the irregularly shaped thallus. Blue arrow: rhizoids. Orange arrow: wavy thallus edge. Bar, 2.0 mm. (c) Top: LM of single-celled rhizoids on the ventral thallus (red arrow). Bar, 150 lm. Bottom: LM of single-celled rhizoid tips (yellow arrowhead) Bar, 100 lm. (d) Sporophytes (blue arrow) growing on the gametophyte (yellow arrow). Bar, 3.0 mm. (e) LM of longitudinal section of thallus with mucilage canals (red arrows). Mucilage clefts on the ventral side are indicated with red arrowheads. Bar, 50 lm. (h) LM of antheridia (red arrow) in an antheridial chamber in longitudinal section. Bar, 50 lm. (f) Surface view of antheridial chamber with yellow antheridia embedded in the dorsal thallus. Bar, 10 lm. (g) Antheridium removed from chamber in (f) showing antheridial body with sperm cells inside and stalk to the lower right. (h) LM of antheridia (red arrow) in an antheridial chamber in longitudinal section. Bar, 50 µm. (i) LM of biflagellate sperm. Coiled cell body is on the left and the flagella are on the right (yellow arrow). Bar, 5.0 µm. (j) LM of two archegonia embedded in the dorsal thallus in longitudinal section showing from the base up: egg cell, ventral canal cell, neck cells and cover cells. Bar, 10 µm. (k) Sporophyte with stomata. Bar, 20 µm. (l) Confocal fluorescence microscopy image of transgenic gametophyte showing single plastids in each cell. Green: green fluorescent protein localized in the plasma membrane expressed under the CaMV 35S promoter. Red, chlorophyll autofluorescence. Bar, 50 µm. (m) LM longitudinal section of thallus with an open archegonium (yellow arrowhead) containing only the egg cell near the apical notch. Dorsal side (red asterisk) and ventral side (orange asterisk). Bar, 25 µm. (n) Higher magnification LM of a stoma with two guard cells surrounding a pore. Bar, 10 µm. (o) Scanning electron micrograph of distal side of a spinose spore. Bar, 10 µm.
- 15. Internal Structure • The vertical transverse section (V. T. S.) of the thallus shows a very simple structure. • It lacks any zonation. • It is uniformly composed of thin walled parenchymatous cells. • The thallus is thickest in the middle. • It gradually becomes thinner towards the margins. • The thickness of the middle region varies in different species. • The outer most layer is upper epidermis. • The epidermal cells are regularly arranged, smaller in size and have large lens shaped chloroplasts. • Each cell of the thallus contains a single large discoid or oval shaped chloroplast.
- 17. • Each chloroplast encloses a single, large, conspicuous body called pyrenoid, a characteristic feature of class Anthocerotopsida. • 25-300 disc to spindle shaped bodies aggregate to form pyrenoid. • The air chambers and air pores are absent in Anthoceros. • However, in a few species intercellular cavities are present on the lower surface of the thallus. • These cavities are formed due to break down of the cells (schizogenous). • The cavities are filled with mucilage and are called mucilage cavities. • These cavities open on the ventral surface through stoma like slits or pores called slime pores. • Each slime pore has two guard cells with thin walls. • The guard cells are non-functional and do not control the size of the pore.
- 18. • With the maturity of the thallus the mucilage in the cavities dries out. • It results in the formation of air filled cavities. • The blue green algae Nostoc invades these air cavities through slime pores and forms a colony in these cavities. • The presence of Nostoc colonies in the thallus of Anthoceros is beneficial for the growth of gametophyte is not definitely known. • However, according to Rodgers and Stewart (1977) it is a symbiotic association. • The thallus supplies carbohydrates to the Nostoc and the latter, in turn, adds to nitrate nutrients by fixing atmospheric nitrogen. • The lowermost cell layer is lower epidermis. • Some cells of the lower epidermis extend to form the smooth-walled rhizoids.
- 19. Reproduction in Anthoceros Anthoceros reproduces by vegetative and sexual methods. (A)Vegetative Reproduction: (B)It takes place by the following methods: 1. By death and decay of the older portion of the thallus or fragmentation: • The older portion of the thallus starts rotting or disintegrates due to ageing or drought. • As it reaches up to the place of dichotomy, the lobes of the thallus get separated. • Thus, detached lobes develop into independent plants by apical growth. • This method is not so common in Anthoceros as in liverworts.
- 20. 2. By tubers: • Under unfavorable conditions or prolonged drought, the marginal tissues of the thallus get thickened and form the perennating tubers. • Their position varies in different species. • They may develop behind the growing points (A. laevis) or along the margins of the thallus (A. hallii, A. pearsoni). • In A. himalayensis the tubers are stalked and develop along the margins on the ventral surface of the thallus. • The tubers have outer two to three layers of corky hyaline cells which enclose the tissue containing oil globules, starch grains and aleurone granules. • They are capable to pass on the unfavorable conditions. • On resumption of favourable conditions tubers produce new thalli.
- 21. 3. By Gemmae: • In some species of Anthoceros like A. glandulosus, A. propaguliferus, A. formosae many multicellular stalked structures develop along the margins of the dorsal surface of the thallus. • These structures are called gemmae. When detached from the parent thallus, each gemma develops into new plant. 4. By persistent growing apices: • Due to prolonged dry summer or towards the end of the growing season, the whole thallus in some species of Anthoceros (A. pearsoni, A. fusiformis) dries and gets destroyed except the growing point. • Later it grows deep into the soil and becomes thick under unfavorable conditions. • It develops into new thallus. • It is more a method of perennation then multiplication.
- 22. 5. By apospory: • In Anthoceros, unspecialized cells of the many parts of the sporogonium (for e.g., intercalary meristematic zone, sub epidermal and sporogenous region of the capsule) form the gametophytic thallus. • This phenomenon is called apospory (Schwarzenbach, 1926, Lang, 1901). • The thalli are diploid but normal in appearance e.g., A. laevis.
- 23. Sexual reproduction: • Sexual reproduction is oogamous. • Male reproductive bodies are known as antheridia and female as archegonia. • Some species of Anthoceros like A. longii, A. gollani, A. fusiformis, A. punctatus, A. crispulusand A. himalayensis are monoecious • Some species like A. erectus, A. chambensis, A. hallii, A. pearsoni and A. laevis are dioecious. • The monoecious species are protandrous i.e., antheridia mature before archegonia.
- 24. Antheridium Structure: • A mature antheridium has a stalk and club or pouch like body. • The stalk attaches the antheridium to the base of the antheridial chamber. • Stalk may be slender and composed of four rows of cells (A. punctatus, A. erectus) or more massive (A. laevis). • A single or a group of two to four or more antheridia are present in the same antheridial chamber. • A single layered sterile jacket encloses the mass of androcytes which metamorphosis into antherozoids. • In some species for e.g., A. punctatus and A. erectus jacket layer is formed of four tiers of cells.
- 25. Antheridium Structure: • Each tier appears to be composed of elongated rectangular cells. • The cells of the upper most tiers are triangular with a narrow end towards the apex. • Each cell of the jacket consists of plastids. • At maturity these plastids change their colour from green to red to bright orange. • Young antheridia are, therefore, green and mature one turn bright orange or reddish. • A mature antherozoid is unicellular, uninucleate, bi-flagellated and has a linear body. • The flagella are of almost the same length as the body.
- 26. Dehiscence of Antheridium • Water helps in the dehiscence of the antheridium. • As the antheridia mature the roof of the antheridial chamber breaks down irregular, exposing the antheridia in a cup like chamber. • The antheridia absorb water and the uppermost tier of triangular cells fall apart releasing a mass of antherozoids. • After dehiscence the antheridium loses turgor and collapses. • It is followed by other antheridia to converge towards the opening in the roof and in this way a continuous stream of antherozoids is possible. • It explains the formation of large number of sporophytes in Hornworts.
- 27. Archegonium • Archegonia develop in the flesh of the thallus on dorsal surface. • The place of an archegonium on a thallus can be identified by the presence of a mucilage mound. • A mature archegonium consists of two to four cover cells, an axial row of four to six neck canal cells, a venter canal cell and an egg. • The development of the archegonium starts on the dorsal surface of the thallus from a single superficial cell which acts as an archegonial initial. • It can be differentiated from other cells by its dense protoplasm. • The archegonial initial may divide by transverse division to form an upper primary archegonial cell and lower primary stalk cell (A. crispulus, A. gemmulosus) or it may directly functions as primary archegonial cell (A. erectus).
- 28. • The primary archegonial cell divides form the three peripheral or jacket initials and a fourth median cell called the primary axial cell. • The primary axial cells divide by a transverse division to form an outer cell and inner (central) cell. • The outer cell divides by a transverse division to form terminal cover initial and inner primary neck canal cell. • The inner cell directly functions as primary venter cell and divide only once to form upper small venter canal cell and a lower large egg. • Primary neck canal cell divides by series of transverse divisions to form four to six neck canal cells. • Cover initial divided by one two vertical division to form two to four rosette like cover cell at the tip of the neck.
- 30. Fertilization • Water is essential for fertilization. • In the mature archegonium, the venter canal cell, neck canal cells disintegrate and form a mucilaginous mass. • It absorbs water, swells up and becomes out of the archegonial neck by pushing the cover cells apart. • The mucilaginous mass consists of chemical substances. • Many antherozoids caught in the mucilage enter in the archegonial neck because of the chemotactic response, reach up to the egg, and fertilization is affected. • Prior to fertilization, egg enlarges and fills the cavity of the venter. • Fusion of both male and female nuclei results in the formation of diploid zygote or oospore. • Fertilization ends the gametophytic phase.
- 31. • Sporophytic Phase • After fertilization the diploid zygote or oospore still enlarges in size and fills the cavity of the venter of the archegonium. • It secretes an outer cellulose wall. • The first division of the zygote is vertical. • In other bryophytes the first division of the zygote is transverse. • This is the important difference in the development of sporophyte of hornworts and rest of the bryophytes. • The second division is transverse and is so oriented that the upper two cells are usually longer than the lower two (quadrant stage). • All the four cells divide by vertical walls to produce eight cells (octant stage). • The eight cells are arranged in two tiers of four cells each.
- 32. • The lowermost tier forms the foot, the middle tier forms the meristematic zone or intermediate zone and uppermost tier develops into the capsule. • In young sporophyte it is made of four cells but in mature sporophyte it is made of sixteen vertical rows of cells (4 x 4). • The amphithecium divides by a periclinal division to differentiate into an outer sterile layer of jacket initials and inner fertile layer. • The cells of the jacket initials divide by anticlinal and periclinal divisions to form four to six layered capsule walls. • The outermost layer of the capsule wall is called epidermis. • It is characterized by the presence of stomata. • The cells of the inner layers of capsule wall have chloroplast.
- 33. • On maturity the archesporium gives rise to two types of cells. • Spore mother cells and elater mother cells. • Spore mother cells are spherical or oval with dense cytoplasm and large nuclei. • These cells divide by meiotic divisions to form spore tetrads. • Elater mother cells are elliptical with small nuclei. • These cells divide mitotically to form four celled elaters. • The four cells of the elaters may remain attached to each other or may break into 1-celled, 2- celled or 3-celled units. • The broken units are called pseudo elaters. • The elaters are without thickening bands and therefore, called pseudo elaters.
- 34. • By the activity of the meristematic zone various tissues of the capsule are continuously produced so that it becomes elongated. • The young sporophyte of the Anthoceros is surrounded by a fleshy covering or sheath. It is called involucre. • It is developed partly from the tissue of the archegonium and partly from the tissue of the gametophytic thallus. • In young stages the sporophyte is completely surrounded by involucre.
- 35. Structure of Mature Sporogonium • The mature sporophyte consist a bulbous foot and a smooth, slender, erect, cylindrical, structure called capsule. • Capsule varies in length from two to fifteen centimeter in different species. • The sporogonium appears like a ‘bristle’ or ‘horn’, hence the species are called ‘hornworts’. Internal structure • A mature sporogonium can be differentiated into three parts viz., the foot, seta and the capsule. Foot: • It is bulbous, multicellular and made up of a mass of parenchymatous cells. • It acts as a haustorium and absorbs food and water from the adjoining gametophytic cells for the developing sporophyte.
- 36. Meristematic Zone or Seta: • Seta is represented by meristematic zone. • This is present at the base of the capsule and consists of meristematic cells. • These cells constantly add new cells to the capsule at its base. • The presence of meristem at the base enables the capsule to grow for a long period and form spores. • It is a unique feature of Anthoceros and is not found in any other bryophyte.
- 37. Capsule: Its internal structure can be differentiated into following parts: a. Columella: • It is central sterile pan, extending nearly to its tip. • It is endothecial in origin. • In young sporophyte it consists of four vertical rows of cells but in mature sporophyte it is made up of 16 vertical rows of cells (4 x 4). • In a transverse section these cells appear as a solid square. • It provides mechanical support, functions as water conducting tissue and also helps in dispersal of spores. b. Archesporium: • It is present between the capsule wall and the columella. • It extends from base to the top of the capsule. • It originates from the inner layer of amphithecium. • In upper part of the capsule it is differentiated into sporogenous tissue which produces spores and pseudo elaters.
- 38. • elaters may be unicellular or multicellular, branched or un-branched and may consist of more or less elongated cells. c. Capsule wall: • It consists of four to six layers of cells, of which the outermost layer is epidermis. • The cells of the epidermis are vertically elongated and have deposit of cutin on their walls. • The continuity of epidermis is broken by the presence of stomata. • The stomata are oriented vertically with the axis of the sporogonium and are widely separated from each other. • Each stoma consists of a pore surrounded by two guard cells. • The cells of the inner layers have intercellular spaces and contain chloroplast. • Thus, the sporogonium is partially self-sufficient to synthesize its own organic food but partially it depends on the gametophyte for the supply of water and mineral nutrients.
- 39. • Dehiscence of the capsule Capsule dehisces basipetally i.e., from apex to base. • As the capsule matures, its tip becomes brownish or black. • Vertical lines of dehiscence appear in the jacket layer. • The dehiscence of the capsule is usually by two longitudinal lines, occasionally it is by single line or rarely by four lines. • The capsule wall dries and shrinks at maturity. • The two valves thus separated, diverge and twist hygroscopically. • The pseudo elaters also dry out, twist and help to loosen the spores. • Thus, the twisting of the valves and the movement of the pseudo elaters in the exposed spore mass helps in the shedding of the spores. • Air currents also help in the dispersal of spores.
- 40. Structure of Spore • The spores are haploid, uninucleate, semi-circular with a conspicuous triradiate mark. • Each spore remains surrounded by two wall layers. • The outermost layer is thick ornamented and is known as exospore. • It varies in colour from dark brown to black (e.g., A. punctatus) or yellowish (e.g., A. laevis). • The inner layer is thin and is known as endospore. • Wall layers enclose colourless plastids, oil globules and food material. • Germination of spore and formation of young gametophyte Under favourable conditions the spores germinate immediately in few species. • However, in some the spores undergo a resting period of few weeks or months before germination. • At the time of germination spore absorbs water and swells up.
- 41. • Exospore ruptures at the triradiate mark and endospore comes out in the form of a tube. It is called germ tube. • Contents migrate into the germinal tube where the colorless plastids turn green. • Two successive transverse walls are laid down at the tip of a germinal tube resulting in the formation of three celled filament. • The upper cell divides by a vertical division followed by similar vertical division in the lower cell (quadrant stage). • These four cells again divide by a vertical division at right angle to first to form eight cells (octant stage). • The total tier of four cells functions as apical cells and forms the new gametophyte. • First rhizoid develops as an elongation of any cell of the young thallus. • As the growth proceeds, the mucilage slits appear on the lower surface and these slits are infected by Nostoc.
- 42. Alternation of Generation • The life cycle of Anthoceros show regular alternation of two morphologically distinct phases. • One of these generations is haplophase and the other is diplophase. 1-Haplophase or gametophytic phase • In Anthoceros this phase is dominant and produces the sex organs. • Sex organs produce gametes to form a diploid zygote. 2- Diploid phase of sporophytic phase • Zygote develops into sporophyte. • In Anthoceros sporophyte is represented by foot, meristematic zone and capsule. • The sporophyte produces the spores in the capsule. • The spores on germination produce the gametophyte. • So, in Anthoceros, two morphologically distinct phases (haplophase and diplophase) constitute the life cycle. • The life cycle of this type which is characterised by alternation of generation and sporogenic meiosis is known as heteromorphic and diplohaplontic.