Discipline: Botany Paper: Archegoniate Lesson: Morphology, Anatomy and Reproduction of Marchantia Lesson Developer: Dr. Anita Sehgal, Dr Somdutta Sinha Roy Department/College: Miranda House Lesson Reviewer: Dr Veena Ganju Department/College: Deshbandhu College Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
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Table of Contents
Systematic Position of MARCHANTIA Habitat and Distribution Habit Gametophyte Phase (The plant body) o o
External features Internal features
o Apical growth and Dichotomy Reproduction
Fertilization Sporophyte Phase
o Vegetative Reproduction o Sexual Reproduction Structure and development of Antheridiophore Structure and development of Archegoniophore
o o o
Development of the sporophyte Structure of the sporophyte Dehiscence and dispersal of the spores
The Young Gametophyte o
Spore morphology and spore germination
Life Cycle
Salient Features
Exercises
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Marchantia(March.) L. SYSTEMATIC POSITION: Division: Bryophyta Class: Hepaticopsida Order: Marchantiales Family: Marchantiaceae Genus: Marchantia (March.)L. HABITAT AND DISTRIBUTION: Marchantia
is
the
most
advanced,
highly
differentiated
member
of
the
family
Marchantiaceae. The genus is named after Nicholas Marchant, who was the director of Orleans Botanical garden, Blois, France. It is found in moist, shady places near water sources and areas of burnt ground. Marchantia is widely distributed and about 65 species are known from all over the world. In India ~ 11 species are represented mostly in the Himalayan region. The three most common species are- M. polymorpha (Fig. 2), M. nepalensis and M.palmata. HABIT: Each spore on germination forms a thallus which is dichotomously branched with a distinct apical notch and prominent mid rib (Fig. 1A, 2). As compared to Riccia, the thallus is larger, shows prominent lobes and is better adapted to terrestrial habitat. It generally forms patches (Fig. 1B, C, 2) and grows extremely well on damp, burnt soil. The green thallus represents the gametophyte phase of the plant. GAMETOPHYTE PHASE: (A)
External Morphology: The adult gametophyte of Marchantia is fleshy,
green, dichotomously branched thallus with a distinct median groove. It grows prostrate on the substratum and branches freely by dichotomy (Fig.2). Occasionally, it also takes up a rosette form (Fig. 3A-C) due to frequent branching. Each branch has a distinct apical notch and the actively dividing apical cells are present just
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Fig.1 Marchantia: A-C Habit. A, C. Gametophytes with antheridiophores B. Gametophytes with archegoniophores Source: A. http://www.gopixpic.com/marchantia-sp/ B. http://www.cpbr.gov.au/bryophyte/photos-captions/marchantia-sp-35. html C. http://waynesword.palomar.edu/bryoph1.htm
Fig.2 Marchantia polymorpha : Gametophytes showing prominent midrib and dichotomy Source: http://wnmu.edu/academic/nspages/gilaflora/marchantia_polymorpha.html behind the notch (Fig. 3D). The thallus is flattened and dorsiventrally differentiated. The dorsal surface shows the presence of distinct rhomboidal or polygonal areas called ‘areolae’ with a tiny hole in the centre (Fig. 3F). The pores carry out the gaseous exchange. Each rhomboidal area represents the corresponding air chamber lying below. The dorsal surface also shows the presence of several, small cup like structures called the gemma cups (Fig. 3E, 7A-D) that are generally present along the mid rib region. The gemma cup has a fringed, membranous margin and contains numerous gemmae (Fig.7C, D).
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Fig. 3 A-F Marchantia. A-C. Habit –Rosette stage. D. Bilobed thalli with a notch (arrow). E. apical part of the thallus showing two gemma cups with membranous margins (arrows). F. Upper surface of the thallus enlarged to show rhomboidall areas with an air pore (arrow) in the centre. A.
http://oregonstate.edu/dept/nurseryweeds/weedspeciespage/liverwort/liverwort_thalli.html
B. http://flickrhivemind.net/Tags/hepatophyta,marchantia/Interesting C. http://www.quazoo.com/q/Marchantiales D. http://www.utas.edu.au/dicotkey/dicotkey/Lworts/MARCHANTIACEAE/ZMarch_bert_ vent.html E. http://waynesword.palomar.edu/bryoph1.html Each gemma is green, flat, multicellular body (Fig. 6F) that arises from the bottom of the gemma cup. On maturity, the gemmae get detached, fall onto the ground and give rise to two new thalli each. On the ventral surface, mid rib is represented by a low ridge. It shows the presence of two kinds of appendages scales and rhizoids (Fig. 4C-I). Scales are of two types i.e. appediculate (median) and spathulate (peripheral) (Fig. 4H, I). Both types of scales are multicellular, one celled thick, and purple due to presence of anthocyanin and are arranged in two-four rows on either side of the midrib (Fig. 4D,E). The rows of appendiculate scales occur near the mid rib, therefore, termed median whereas those of the spathulate are found towards the periphery and thus called peripheral. Each median scale arises obliquely from the ventral surface and is divided in to a large body and an apical sub rotund appendage by a narrow constriction (Fig. 4H). However, the latter is absent from the peripheral scales making them appear like a spoon or spathulate (Fig. 4I). Presumably, both types of scales protect the growing tip from desiccation and retain water through capillary action. Besides
scales,
the
ventral
surface
also
bears
numerous,
elongated,
unicellular,
unbranched rhizoids (Fig. 4C, E). The cells of the lower (ventral) epidermis elongate to form two types of rhizoids - namely smooth-walled and tuberculate (Fig. 4F, G). The smooth-walled rhizoids are thin walled, slightly larger in diameter (Fig. 4F) and mostly found near the mid rib region. They grow directly into the soil and help in anchorage and absorption. On the other hand, the tuberculate rhizoids are thick-walled, narrow and have peg like invaginations towards the interior of the cell (Fig. 4G). They generally originate
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Fig.4 A-I. Marchantia. A. Thallus showing dichotomy- two young lobes (yellow stars) and two old lobes (white stars). B. Thallus showing initiation of dichotomy (yellow star) and two old lobes (white stars). C, D. Lower surface of thallus showing rhizoids (arrow) in the middle and scales (arrow) on either side of the midrib. E. Lower surface of the thallus
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(diagrammatic sketch) showing midrib, rhizoids and scales. F. A smooth- walled rhizoid. G. A tuberculate rhizoid. H. A marginal scale. I. A peripheral scale (diagrammatic). A. https://www.google.co.in/search?newwindow=1&tbm=isch&q=marchantia+life+cycl e+animation&revid= B.
http://waynesword.palomar.edu/bryoph1.html
C. http://oregonstate.edu/dept/nurseryweeds/weedspeciespage/liverwort/liverwort_thalli.html D. http://waynesword.palomar.edu/bryoph1.html
E. –I. Authors from under the scales and grow parallel to the soil surface in dense mats. The function of the tuberculate rhizoids is to help in water absorption and distribution to the entire thallus. At maturity, the thalli show several gametophores or gametangiophores which are upright, stalked, umbrella-shaped structures (Fig.5,9). They are of two kinds, antheridiophores (Fig.1A, C, 5) and archegoniophores (Fig.1B, 5). The former bear antheridia and the latter archegonia. The two types of sex organs are borne on different gametophytes hence Marchantia has separate male and female plants. (B)
Internal Structure of the Thallus (Fig. 5A-F): The thallus of Marchantia
shows high degree of internal differentiation and is divided into three distinct zones: (1) Epidermal zone (2) Upper or dorsal assimilatory zone or photosynthetic region (3) Lower or ventral storage zone (1) Epidermal region consists of a single layer of upper and lower epidermis (Fig. 5A-C). The cells of epidermis have thin, inner walls and thick, outer walls and possess a few chloroplasts. The outer wall of the epidermis is resistant to water, checks the rate of transpiration and and thus is protective in function. The upper epidermis also shows presence of air pores or areoles (Fig. 3F,5D). Each pore is surrounded by four to eight tiers of 4 to 5 cells each (Fig. 5E). These tiers are arranged in rings one over the other and form a chimney like structure around the pore. The upper and lower rings of the pore are narrower than the middle and the opening is slightly raised above the upper epidermis. The cells of lower ring sometimes are papillose, project into the
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passage in such a manner so as to give a star shaped appearance (Fig. 5F). Each pore opens into an underlying air chamber (Fig. 5B,E). Unlike stomata, these air pores do not open and close but carry out gaseous exchange during photosynthesis and respiration and are, thus, analogous to the stomata of higher plants. (2) Photosynthetic zone (Fig. 5A-C, E) lies below the upper epidermis. It consists of a row of small, distinct air chambers. Each air chamber is bounded by one cell layer thick, chlorophyllous partitions (two to four cells in height) and communicates with the exterior through a barrel shaped pore. Several short, simple or branched filaments of oval cells rich in chloroplast arise from the floor of the air chamber. Each areole in the upper epidermis corresponds with an air chamber below. Since the cells of air chambers are chlorophyllous, this zone is the principal seat of photosynthesis. (3) Storage region (Fig. 5A-C) lies just below the photosynthetic zone. It is made up of thin walled parenchymatous cells and makes up the major part of the thallus. The cells are polygonal, mostly colourless and are compactly arranged. These cells have the major function of storage and have starch, mucilage and oil-bodies stored in them. The mid rib region is the thickest part of the storage region but becomes progressively thinner (3-5 layers) towards the edges (Fig. 5A). The cells towards the ventral surface have thicker wall and is bound by the lower epidermis. The lower epidermis shows two kinds of appendages i.e. scales and rhizoids. The rhizoids assist in absorption of water and minerals whereas scales help in retaining the moisture that helps Marchantia grow under dry conditions.
APICAL GROWTH AND DICHOTOMY: The thallus of Marchantia grows by division of a group of meristematic cells present in a horizontal row just behind the apical notch. Cell division in this group of meristamatic cells results in the increase in the length of the thallus lobe. Frequently, some of the median cells of the horizontal row of apical initials divide vertically to form a tissue that separates the growing point into two sets of apical initials (Fig. 4A). Subsequently, each set functions as the growing point of a new thallus lobe. This is the initiation of
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dichotomy and becomes marked by the addition of more and more tissue between the two sets of apical initials (Fig. 4B).
Fig. 5 A-F Marchantia. A. V.S. thallus (diagrammatic sketch) showing internal structure. B, Marked out area in Fig. A enlarged to (outline) show upper epidermis, air chambers (photosynthetic) and storage region. Note the rhizoids and scales originating from the lower epidermis. C. V.S. thallus (cellular) showing photosynthetic and storage zone. D. Pore in surface view. E. V.S. thallus showing pore in the sectional view. F. Air pore as seen from
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above (upper) and as seen from below
(lower) surrounded by the papillose cells of the
lowermost tier. A. http://dita2indesign.sourceforge.net/dita_gutenberg_samples/dita_encyclopaedia_br itannica/html/entries/entry-d1e27277.html B. http://dita2indesign.sourceforge.net/dita_gutenberg_samples/dita_encyclopaedia_br itannica/html/entries/entry-d1e27277.html C. http://www.vcbio.science.ru.nl/en/virtuallessons/hepatophyta/ D. http://www.nativeorchid.org/news201112.html E. http://flickrhivemind.net/Tags/hepatophyta,liverwort/Recent F. http://dita2indesign.sourceforge.net/dita_gutenberg_samples/dita_encyclopaedia_br itannica/html/entries/entry-d1e27277.html
REPRODUCTION: The gametophytic thallus of Marchantia reproduces both vegetatively and sexually. Vegetative Reproduction: Vegetative reproduction (Fig. 6A-F) in Marchantia can occur in the following three ways: (1)Fragmentation: The aging thallus generally starts disintegrating from the base and when this reaches the point of dichotomous branching, the two lobes separate and with the help of apical growth form two different thalli (Fig. 6A-C). (2)Adventitious Branches: Generally these adventitious branches arise from the ventral surface of the thallus (Fig. 6D). Though it has been reported by various authors that nearly all the parts of the thallus are capable of giving rise to adventitious branches but in M. palmata stalk and disc of the female gametophores is known to form adventitious branches. These branches detach and develop into a new individual as the tissue connecting them to the thallus decays.
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Fig. 6 A-F.
Marchantia: Methods of vegetative reproduction. A-C. Fragmentation of the
thallus. D. Adventitious branches arising from the lower surface of the lobes of antheridial disc. E, F. By gemmae (F) present in the gemma cup (E). Source:http://biologyworks.com/IMAGE-SALE/GBProject/images/marchantia_life_cycle.jpg (3) By formation of Gemmae (Fig.6E, F, 7 A-D, 8A-D): Gemmae are stalked, disc –like,
multicellular
structures
(Fig.6F,7C,
8C)
produced
inside
specialized
structures called gemma cups or cupules (Fig.6E, 7A-D). Gemma cups are shallow, cup like structures with fringed margins (Fig. 7C, 8A, B). They develop on the dorsal surface of the thallus around the mid rib region (Fig. 7A, B). As discussed earlier, gemmae are biconvex, disc like, multicellular structures with two prominent notches opposite to each other. In each notch is present a growing point (Fig.6F, 8C). The gemmae are borne on a short, single celled stalk attached to the floor of the gemma cup (Fig.6F, 8C). Figures 8A (cellular) and 8B (diagrammatic) show a longitudinal section passing through the thallus bearing a gemma cup. Interestingly the photosynthetic zone present on the dorsal surface of the thallus continues to be present in the fringed margin as well. Several gemmae can be seen in surface as well as sectional view attached to the base of the gemma cup with the help of single celled stalk. In addition to gemmae, the
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cup also bears several, short, glandular hair (Fig. 8A, B) that secrete copious mucilage. This secreted mucilage helps mature gemmae to absorb water and get detached from the base of the gemma cup. Gemmae are carried away from the parent thalli by the action of water and give rise to new thalli under suitable conditions.
Fig. 7 A-D. Marchantia. A,B. Thalli with gemma cups. C. Side view of gemma cups showing fringed margins (arrow). D. Top view of the gemma cup with several gemmae inside (arrow). Note the occurrence of pores on the surface of the thallus. A. http://flickrhivemind.net/Tags/marchantia B.
http://waynesword.palomar.edu/bryoph1.html
C. http://www.plantzafrica.com/plantklm/marchantiabert.htm
D. http://waynesword.palomar.edu/bryoph1.html
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Fig. 8 A-D. Marchantia: A. V.S. thallus passing through the gemma cup (cellular). B. V.S. thallus passing through the gemma cup (diagrammatic) with gemmae and mucilaginous hair. C. w.m. gemma. D. T.S. gemma Source: A: http://pixgood.com/marchantia-gametophyte.html, B-D: Authors (i)
Development of Gemmae:
Each gemmae arises by the division of a
superficial cell at the bottom of a gemma cup. Thus, all the superficial cells of the inner lining of the gemma cup are potential gemmae initials. Each gemma initial cell first undergoes a transverse divisions to give rise to a lower cell and an upper cell. The lower cell forms a single celled stalk whereas the upper cell divides twice transversely to give rise to a row of four cells. These four cells then undergo both vertical and transverse divisions to form a thin plate like
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structure with slightly swollen centre and two lateral notches that house the growing points (Fig.6F, 8C, D). (ii)
Structure of Gemma:
A gemma is a multicellular, green, biconvex
structure with two deep notches on the opposite sides (Fig.6F, 8C). Each notch has a growing point that consists of meristematic cells. Besides these, there is a basal indentation representing the point of attachment to the stalk. The green colour of gemma is due to the presence of chloroplasts in the thin walled cells. Some of the cells near the thin edges have oil bodies. There are a few colorless cells on both the surfaces termed rhizoidal cells. These are slightly larger than
the surrounding
cells
and
show
dense,
granular
cytoplasm. In addition, there are isolated cells near the margins that contain oil bodies instead of chloroplasts and are known as oil cells. Figure 8D is a transverse section of a gemma showing biconvex outline, the presence of rhizoidal cells, oil cells and two notches. Importantly, the two flattened surfaces of a gemma are alike. Thus, a gemma is an isobilateral, tetra-polar structure that does not show innate dorsiventral symmetry. It is only after a gemma falls on the ground that the surface facing the substratum becomes dorsiventral. The establishment of symmetry is guided by the environmental gradients such as light and temperature etc. (iii)
Germination of Gemma: Under favourable conditions, the rhizoidal
cells located on the surface in contact with the soil give rise to rhizoids. Once the rhizoids are formed, the gemma attains dorsiventrality. The growing points present in the lateral notches begin to divide simultaneously in opposite directions and eventually form two thalli. The two young plants separate once the gemma degenerates. Gemmae from male plants generally give rise to male thalli and those from female plants give rise to female thalli. Thus, gemmae formation provides a very rapid and successful means of vegetative
multiplication
and
does
not
contribute
to
alternation
of
generations. Sexual Reproduction: Marchantia is oogamous and sexual reproduction is dioecious i.e. male and female plants are separate (Fig. 9). The plants enter the phase of sexual reproduction only under very specific environmental conditions namely high humidity and long days. Sex organs in Marchantia are borne on specialized branches called gametophores or gametangiophores (Figs 9,). The gametophore borne on a male thallus is called an
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antheridiophore and the one borne on a female thallus is termed an archegoniophore. Therefore, the gametophores in Marchantia are unisexual and at maturity a thallus can be distinguished as male or female by the presence of structurally distinct antheridiophores or archegoniophores (Fig. 9). Marchantia is thus sexually dimorphic and heterothallic (dioecious). However, a few reports of thalli with abnormal receptacles bearing both archegonia and antheridia in the same receptacle have also been reported. Each Gametophore has two distinct parts, the stalk and the receptacle. Gametophores are borne terminally on the vegetative thallus and are extensions of the vegetative thallus as evident from the fact that the stalks of the gametophores show the presence of rhizoidal grooves and assimilatory chambers with assimilatory filaments (Fig. 10,16C).
Fig.9 Marchantia-male and female gametophytes showing gametangiophores. Source: modified from :https://www.studyblue.com/notes/note/n/unit-3-review-summer-turley-inprogress/deck/11650460
Fig.10 Marchantia. T.S. stalk of antheridiophore. Source: authors
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(1)Structure and development of Antheridiophore: (A) Structure of Antheridiophore (Figs 9, 11A-F, 12, 13A-C): Antheridia in Marchantia are borne on specialized branches known as antheridiophores which are 1-3 cm long, having a stalk and a receptacle (Fig.9, 11B). To begin with the receptacles are small and sessile, but later the stalks elongate (Fig.11C) and attain full length. The stalk shows a dorsiventral symmetry similar to the vegetative thallus (Fig.10,12). It has two longitudinal furrows having rhizoids and scales on the interior side. On the other hand, posterior side shows air chambers with assimilatory filaments (Fig. 12) and corresponds to the dorsal surface of the thallus. Thus, the internal structure of the stalk (Fig. 10, 12) corresponds to that of the thallus and proves its dorsiventral nature. The terminal male receptacle is a flattened, disc shaped structure generally having 8 lobes (Fig. 11A- F). Each lobe has a growing point at its tip. Interestingly, the complex lobed structure of the receptacle arises as a result of repeated dichotomy in quick succession. On each lobe, 6-9 antheridia are arranged in a row. Each antheridium arises by the division of terminal growing point that eventually produces 6-9 antheridia in an acropetal order i.e. the oldest in the centre and youngest near the periphery (Figs 11D,12, 13A-C). Each antheridium is present in a chamber termed antheridial chamber. The antheridium is an oval shaped structure with a short stalk (Fig. 14H) that arises from the base of the chamber. The wall of antheridial chamber is deep purple. Anatomically (Figs.12, 13A-C), the disc is very similar to the vegetative thallus. It has air pores leading to air chambers full of assimilatory filaments arising from its floor. The air chambers alternate with the flask-shaped antheridial chambers (Figs 12, 13A-C).The antheridial chambers open to the upper surface by a pore called ostiole. The upper surface, therefore, shows slight papillate elevations to mark the position of antheridia below (Fig.11C, D, E). A compact region of parenchymatous cells lies below the antheridial chambers. The lower epidermis bears scales and rhizoids (Fig.12).
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. Figure 11 A-F. Marchantia. A. Top view of antheridial discs. B. antheridiophores and gemma cups (line drawing).
Thallus bearing
C. Side view of an antheridiophore
showing a short stalk and an antheridial disc with several antheridial openings or ostioles. E. A model of C showing the internal position of antheridia in the antheridial chambers with
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their openings on the surface. E, F. Top view of young (E) and mature (F) antheridial discs showing 8 lobes. Note the dehisced antheridia in F. A. http://pixgood.com/marchantia-archegoniophore.html B. http://dita2indesign.sourceforge.net/dita_gutenberg_samples/dita_encyclopaedia_br itannica/html/entries/entry-d1e27277.html C. http://flickrhivemind.net/Tags/hepatophyta,marchantia/Interesting D. http://mcgregor.sbs.auckland.ac.nz/2011/02/01/marchantia-polymorphaantheridia/141u-marchantia-3/ E. http://wnmu.edu/academic/nspages/gilaflora/marchantia_polymorpha.html
F. http://www.plantzafrica.com/plantklm/marchantiabert.html
Fig. 12 Marchantia. L.S.antheridiophore Source: http://imgarcade.com/1/antheridium-marchantia/
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Development of the Antheridium: The development of the antheridium in Marchantia is very similar to that of Riccia. A single superficial cell on the dorsal surface, near the growing apex of a lobe of the receptacle enlarges and becomes the antheridial initial (Fig. 14A). The initial undergoes a transverse division to form a lower cell and an upper cell (Fig. 14B). The lower cell does not divide further but the upper cell divides transversely once again to produce an upper primary antheridial cell and a lower primary stalk cell (Fig. 14C). The primary stalk cell undergoes irregular divisions to form a short stalk. The primary antheridial cell undergoes a few transverse divisions to form a short 2-4 celled filament (Fig. 14D). These cells initially undergo two vertical divisions at right angle to each other (Fig. 14E) and then subsequently divide periclinally to form a layer of outer jacket initials and inner primary androgonial cells (Fig. 14F). Jacket initials undergo several anticlinal divisions to form a single layered jacket around the antheridium. Primary androgonial cells also divide several times to give rise to a mass of androgonal cells (Fig. 14G). The cells formed after the last division are called the androcyte mother cells (Fig. 14H). Each androcyte mother cell is cuboidal in shape and has two dot-like, dark staining structures in the cytoplasm termed centrosomes (blepheroplasts). Androcyte mother cells then divide diagonally and give rise to two daughter protoplasts with one centrosome each; these are called androcytes or sperm cells. Centrosome then moves to one corner of the sperm cell and elongates and gives rise to two flagella. Eventually the nucleus of the sperm cell also elongates and it metamorphoses into a sperm. Each sperm is coiled, biflagellate with elongated nucleus and very little cytoplasm occupying posterior end of the nucleus in the form of a vesicle. The latter shows plastid filled with starch and mitochondria. Thus, the sperm can be divided into three parts namely, (a) the head piece (b) the nuclear portion and (c) the cytoplasmic portion. (B) Dehiscence of the Antheridium:
Mature antheridium lies deep in the
antheridial cavity; it is oval in shape, attached to the base of the cavity with the help of a short stalk and has a single layered jacket (Fig. 14H). Dehiscence of the antheridia (Fig. 11F) takes place in presence of water which is either collected in the male receptacle or rises from the rhizoidal
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Fig. 13 A-C Marchantia: One of the lobes of the antheridial disc enlarged to show air chambers (black arrow), antheridia (AN) inside the antheridial chambers (AC), air chambers and rhizoids (R ). Source:
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A: http://imgarcade.com/1/antheridium-marchantia/ B: http://imgarcade.com/1/antheridium-marchantia/ C:
http://pixgood.com/marchantia-archegoniophore.html
Fig. 14 A-H Marchantia: A-G- early stages in the development of antheridium. H. Mature antheridium. Source: authors groove of the antheridiophore. Water then reaches the antheridial cavity through ostioles and when it comes in contact with the antheridium, some sterile cells in the upper portion of the jacket disintegrate. Figure 11F shows the ruptured ostioles as they appear on the surface view.
Mature
androcytes are then released in the form of a milky droplet or slimy mass
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from the upper surface of the receptacle which then spreads out on the surface of the male receptacle as a thin film. Wall of the androcytes then dissolves releasing the flagellated, motile sperms or antherozoids.
(2) Structure and development of Archegoniophore (Figs 15-19): Structure
of
Archegoniophore:
The
female
gametophore
or
the
archegoniophore (also called carpocephallum) is a sexual branch that originates from the apex of the thallus (Fig. 15B-E). It consists of a stalk and a receptacle (Fig. 15A-F, 16A) and shows internal structure similar to that of the thallus (Figs 16C, 18A, B). The part of the stalk that shows the presence of air chambers with assimilatory filaments corresponds to the dorsal surface of the thallus and the one that shows two longitudinal grooves with rhizoids and scales corresponds to the ventral surface. The stalk of the archegoniophore is slightly longer than that of antheridiophore (2 to 3 inches). The female receptacle is also an 8-lobed structure like the male receptacle, arising due to repeated dichotomy of the growing apex and figure 15 A shows several such receptacles. The archegonia are produced in an acropetal succession by the repeated divisions of the apical growing point located at the tip of each lobe. Therefore, 8 distinct rows of archegonia, with 12 to 14 archegonia in each row are present on the receptacle (Fig. 16C). (A) Position of the Archegonium: When the female receptacle is young and its stalk is short (Fig. 17A), the archegonia stand upright and their necks point upwards. As the first formed archegonia reach maturity and fertilization occurs, a few important developmental changes occur in the archegoniophore. Post fertilization, the stalk elongates very rapidly (Fig. 15B-F, 16A) and tissue in the central, sterile region of the upper surface of the receptacle starts dividing very rapidly. This rapid cell division in the upper tissue as compared to lower tissue results in growing points being pushed downwards and inwards towards the stalk and results in the archegonia getting transferred to the lower surface (Fig. 17B, C). Thus, in the mature archegoniophore, the archegonia hang upside down with the
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Fig.15 A-F. Marchantia A.Top view of several archegoniophores. B-E. Stages in the development of archegoniophores. Note increase in the length of stalks. A. http://flickrhivemind.net/Tags/marchantia
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B. https://www.google.co.in/search?newwindow=1&tbm=isch&q=marchantia+life+cycl e+animation&revid= C. https://www.google.co.in/search?newwindow=1&tbm=isch&q=marchantia+life+cycl e+animation&revid= D. http://blogs.ubc.ca/biology321/?page_id=3363 E. http://flickrhivemind.net/Tags/marchantia F. http://dita2indesign.sourceforge.net/dita_gutenberg_samples/dita_encyclopaedia_br itannica/html/entries/entry-d1e27277.html
youngest archegonia near the stalk and the oldest ones near the periphery (Figs 16C, 17D, 18 A,B).The margin between the two successive lobes grows outwards as long, stout, green cylindrical processes called the rays. These are usually nine in number, extend out and hang downwards like the ribs of a small umbrella in a young receptacle (Figs 15C, E, F, 16A, 18A-D). However, at maturity they spread widely and give the female receptacle a stellate or a star-like appearance (Fig. 15D, 24A). After inversion, a plate like tissue also develops due to downward growth of rays. This is called perichaetium or involucre (Fig.16C). It surrounds a single row of 12 to 14 archegonia.The perichaetium is a two-lipped, curtain-like structure. It is one-celled in thickness and has a fringed or lacerated margin. Each row of archegonia is thus separated from its neighbouring rows by the perichaetium. In addition to the perichaetia, each archegonia is surrounded by a cup-like perigynium or the pseudoperianth (Fig. 16B, C). The dorsal region of the receptacle as well as the rays show the presence of air chambers, photosynthetic filaments and air pores similar to that of the thallus. (B)
Structure of Archegonium: The mature archegonium of Marchantia is a flask like structure (Figs 16B) with a short, multicellular stalk that attaches the archegonium to the under surface of the receptacle (Fig. 16C, 18A-D). The archegonium can be differentiated into two parts namely basal, swollen venter and a long neck. Venter has a single, sterile layer of wall enclosing a venter cavity with two distinct cells, namely, lower large egg cell (Fig. 18C) and an upper, smaller ventral canal cell. The neck has characteristic six vertical rows of cells called neck cells enclosing 4 to 8 neck canal cells (Figs 16B, 18C). The mouth of the neck is closed by 4
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Fig. 16 A-C. Marchantia A. Thalli bearing archegoniophores. Note the long stalks (arrow). B. Single archegonium. C. L.S. Archegoniophore showing the internal structure. Source: A.
http://facstaff.cbu.edu/~seisen/SeedlessPlants.html, B,C. authors
Fig. 17 Marchantia. A-D. L.S. through the archegoniophores at various stages of inversion. Source: authors
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Fig. 18 A-D Marchantia A. L.S. through the archegoniophore. Note the bent rays with inverted archegonia and a groove with rhizoids.
B. Part of L.S. ray enlarged to show
archegonia (A). C. Part of B further enlarged to show the archegonia (arrows) containing eggs (eg). Note the ventral canal cell in the upper archegonium. D. Part of ray showing archegonia (arrows). The lower archegonium contains 2-celled pro-embryo. A. http://quizlet.com/9738975/taxonomy-1-24-flash-cards/ B. http://blogs.ubc.ca/biology321/?page_id=3363 C. http://pixgood.com/marchantia-gametophyte.html D. http://flickrhivemind.net/Tags/marchantia cover cells (or lid cells). Each archegonium develops a collar like structure called the perigynium or the pseudoperianth around the base of the venter (Fig.
16C).
Post
fertilization,
the
perigynium
that
surmounts
the
archegonial stalk grows and completely covers the developing sporophyte.
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Fig. 19 A-L Marchantia. A-K. Early stages in the development of archegonium. E, K transverse sections. See text for details. L. Mature archegonium. Sources: authors.
(C) Development of the Archegonium: Archegonium develops from a single, superficial cell near the apical growth point of each lobe of the receptacle, known as the archegonial initial (Fig. 19A). This archegonial initial enlarges in size and divides transversely to give rise to lower primary stalk cell and upper primary archegonial cell (Fig. 19B). Primary stalk cell divides to give rise to a short, multicellular stalk. On the other hand, primary archegonial cell undergoes three successive, vertical, intersecting divisions to form three peripheral initials, surrounding the central, fertile, primary axial cell (Fig. 19C, D, E). The primary axial cell divides transversely to form an upper, smaller primary cover cell and a lower, larger primary central cell (Fig. 19F). Three peripheral initials then undergo one vertical division each to form six jacket initials (Fig. 19G). Subsequently, the six jacket initials and the central cell divide transversely to form two tiers of seven cells each (Fig. 19H). The outer six cells of the upper tier functions as the neck initials and the inner central cell as the primary neck canal cell. The outer cells of the lower tier act as venter initials and the inner central cell as primary ventral cell.
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The six neck initials divide transversely to form a tubular neck which is made up of six vertical rows of neck cells. Primary axial cell divides transversely to form upper primary cover cell and lower central cell. Primary cover cell divides vertically two times to give four cover cells. The central cell divides transversely to give upper primary neck canal cell and lower primary venter canal cell (Fig. 19I). Primary neck canal cell divides to form 4 to 8 neck canal cells that are enclosed by six vertical rows of neck (Fig. 19J, K). Six lower primary venter initials divide anticlinally to form a single layered wall or jacket around the venter which encloses the venter cavity. Meanwhile, inside the venter cavity, the primary venter canal cell divides to form upper, smaller ventral canal cell and lower, larger egg cell (oosphere) (Fig. 19L). The mature egg contains plastids, numerous mitochondria, small vacuoles and a large central nucleus with prominent nucleolus. Dehiscence of archegonium is triggered by the presence of moisture. As the egg matures, the ventral canal cell separates from the wall cells and the lowermost neck canal cell. This is followed by the degeneration of all neck canal cells of the axial row except for the egg. The disintegerated cellular material absorbs moisture and exerts pressure on the cover or lid cells and forces them to separate, thus making a thorough passage for the sperms to reach the egg.
Following are the differences between male and the female plants:
Male Plant
Female Plant
1. The stalk of the antheridiophore is 1 to 3cm long; and is comparatively shorter and slender.
1. The stalk of the archegoniophore is 2 to 3 cm long; comparatively longer and stouter than that of antheridiophore.
2. Male receptacle is concave with wavy margins.
2. Female receptacle is convex in the centre and umbrella shaped when young and becomes star shaped at maturity due to formation of rays.
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3. There is no inversion of the receptacle at maturity.
3. There is an inversion in the female receptacle due to more growth in the central region as compared to the lower region.
4. Antheridia are upright and embedded in the receptacle tissue on the upper side. The upper surface therefore is marked by slightly papillate elevations to mark the position of antheridia below.
4. Initially the archegonia are upright but post –fertilization they hang down from the lower surface of the receptacle. No papillate elevation on the upper surface.
5. Mature antheridia are present in 8 rows (one row on each lobe) and arise in an acropetal order.
5. Mature archegonia are also present in 8 rows but in a basipetal order due to inversion of the female receptacle.
6. Perigynium, perichaetium, calyptra and rays are absent
6. Perigynium, perichaetium, calyptra and rays are present.
Fertilization/ Syngamy: Marchantia is a dioecious plant i.e. male and female receptacles are borne on separate thalli (Fig.9). This added to the fact that sex organs of Marchantia are raised on the receptacles makes the sexual reproduction in Marchantia uncertain. Fertilization is completely dependent on the presence of water or moisture for the sperm to swim and reach the mouth of the ripened archegonia. The sperms can reach the female receptacle in the following ways: (i) Slightly concave male receptacle acts as a splash cup for the falling rain drops and the splash droplets containing sperms fall on the neighboring, nearly sessile female receptacle. (ii) Splashed sperms fall on the ground and swim in rain water to reach the female receptacle. (iii) Both male and female plants are submerged in rain water and the sperms are released and swim through the rain water. (iv)Sperms may be carried by very small insects. All these alternatives are possible only if male and female plants are growing together or are situated close to each other. Once the sperm reaches the female receptacle, it moves towards the archegonium due to the presence of certain proteins or potassium salts present in the mucilaginous material. Latter is formed
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by degrading neck canal cells and venter canal cells and finally oozes out of the open neck of the archegonium. As mentioned earlier, stalks of the archegoniophores elongate rapidly after the fertilization (Fig. 24B, C). Rays (Fig.16C) are formed by the rapid division of the tissue between the two apical growing points. A cylindrical sheath called perigynium present around the base of individual archegonium grows to form a large envelope (Fig. 20, 21, 22). The venter cells of an archegonium also divide periclinally to form two to three layered calyptra. Both calyptra and perigynium surround and protect an individual, growing sporophyte whereas the tow-lipped, fringe-edged perichaetium is a covering that grows around a single row of archegonia (Fig.21E, 22). Many archegonia on a female receptacle may be fertilized but all of them do not develop into sporophytes. The diploid zygote formed as the result of fertilization secretes a wall around it and undergoes active cell division (Fig. 18D) to form a spherical embryo (Fig.20E, 21A, B). SPOROPHYTE Sporophyte phase starts with the zygote and terminates with the spore mother cells and elaters. It represents the diploid phase (diplophase) of the life cycle of Marchantia. Development of the Sporogonium: The first division of the oospore or zygote (Fig. 20A) starts within 48 hours of fertilization. The first transverse division of the zygote gives rise to upper epibasal and lower hypobasal cell (Fig. 18D,20B). The next division is at right angle to the first and gives rise to a four celled globular embryo (Fig. 20C). This four celled stage is called the quadrant stage. The next stage is the octant stage typically and after that
the
embryo
grows
vigorously
(Fig.
20D,
E).
Figure
21A
shows
an
archegoniophore depicting young, developing sporophytes. The four epibasal cells of the octant gives rise to the capsule and the four hypobasal cells of the octant forms the foot and the seta (Fig. 20F, 21B). Periclinal divisions in the capsule part give rise to the inner endothecium and outer amphithecium. Amphithecium divides anticlinally to form a single layered capsule wall. Cells of the capsule wall develops thickening in their inner wall at maturity. The endothecium gives rise to the archesporium which undergoes repeated divisions to form massive sporogenous tissue (Fig. 21B). Cells of the sporogenous tissue are elongated and similar to begin with. Later, they divide
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diagonally and are differentiated into two kinds of cells namely elater mother cells and spore mother cells (Fig. 21G) in equal proportion. The elater mother cells are long and narrow (Fig. 20G, H). They further elongate to form sterile elaters (Fig. 21C, 23C). The latter are diploid, long fusiform cells with two spiral thickenings on their inner walls at maturity. Elaters lose their protoplasmic content at maturity (Fig. 23C). They are hygroscopic and help in spore dispersal by squiggling movement which helps in breaking up the spore mass. Spore mother cells (Fig. 20G, H) divide transversely to form vertical rows of cubical spore mother cells. Each spore mother cell subsequently undergoes meiosis to form tetrahedral tetrad. Later, the spores separate from the tetrads and get intermingled with elaters (Fig. 21C, 23A, D). The elater mother cells do not divide further and directly differentiate into sterile elaters whereas the spore mother cells undergo 3-5 mitotic divisions before undergoing meiosis to form haploid spores. Thus, there is a difference of 3-5 successive cell divisions between the sterile elaters (diploid) and fertile spores (haploid, n=8) and the ratio of an elater to a spore is - 1:!28. The spores are cubical to begin with but become rounded later. They contain dense cytoplasm and a prominent nucleus (Fig. 23B). Development of the capsule is accompanied by the development of foot and seta (Fig. 21D-F, 22, 23A). Foot is completely embedded in the lower surface of the female receptacle and is formed early. Seta elongates little only after the spores mature (Fig. 21D). Besides these changes in the developing sporophyte, the surrounding gametophytic tissue also undergoes following five, significant changes (Fig. 21D-F, 21, 23A): I.
There is a considerable elongation of the stalk of the archegoniophore (Fig. 24B, C).
II.
Calyptra- a 2/3 layered thick covering (Fig. 21B, D-F, 23A) is formed by the periclinal divisions of the venter cells.
III.
Perigynium- a collar like structure at the base of an archegonium eventually forms a single celled thick, cylindrical sheath that encloses a developing sporophyte (Fig. 20G, F, 21B, D, F, 22, 23A).
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Fig. 20 A-G
Marchantia: stages in the development of sporophyte. See text for details.
Source: authors
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Fig. 21 A-F. Marchantia.sporophytes. A. Part of archegoniophore showing stalk (s) and archegonial
disc (AD). Note the developing sporophytes
(sp)
and curtain like
perichaetium (P). B. V.S. through the young sporophyte showing foot (F), seta (SE), capsule (CS); and covered by an inner covering called calyptra (CA) and an outer covering termed (PR). The remains of neck (n) can be seen at the tip of the capsule. C. V.S. through young sporophyte. The capsule contains spores and elators. D. V.S. through young sporophyte showing inner calyptra (ca) and outer perigynium (pr). E. Part of archegoniophore showing stalk, archegonial disc (AD) and three mature sporophytes. Note perigynium (PR) surrounding individual
archegonium and a curtain
like perichaetium (P) covering a row of archegonia in a lobe. F. V.S. through the mature sporophyte showing foot (F), seta (SE), capsule (CS); and covered by calyptra (CA) and perigynium (PR). The mature capsule contains spores and elators. A.
http://flickrhivemind.net/Tags/marchantia
B. http://synbio.org.uk/marchantia/exchange/culture/entry/spore-farm-formarchantia.html C. http://t2.junbi-tracker.com/meta.py?cam=143069&cat=1089413&subid=5c60 D. http://galleryhip.com/marchantia-slide.html E. http://pinstake.com/iv-the-sporophyte-of-a-moss/ F. http://faculty.baruch.cuny.edu/jwahlert/bio1003/bryophyta.html
Fig.
22
Marchantia..
V..S
archegoniophore
depicting
mature
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sporophytes.
Source:
http://botit.botany.wisc.edu/Resources/Botany/Bryophytes/Hepatophyta/Marchantia/Sporop hyte/Sporophytes.jpg.html
Fig. 23 A-D. Marchantia A. L.S. mature sporogonium. B. Spores. C. Part of the elater with a double spiral band of thickenings. D. Part of the capsule showing annular thickenings on the capsule wall, spores and an elator. Source: authors.
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Fig. 24 A-C Marchantia. A. Top view of archegoniophores with sporophytes. B. Side view of an
archegoniophore
showing
a
dehisced
sporophyte
(arrow).
C.
Side
archegoniophores with sporophytes. Note the upturned rays on the right side. Source: A. https://www.anbg.gov.au/bryophyte/what-is-liverwort.html B. http://pixgood.com/marchantia-gametophyte.html C. http://ibis.geog.ubc.ca/biodiversity/eflora/bryophytes.html
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view
of
Fig. 25 A-F. Marchantia:
A. Archegonial head seen from below with the several, mature
yellow sporophytes projecting out (arrow). Part of the stalk (s) of archegoniophore is also
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visible. B. A long stalked (s) mature sporophyte showing ruptured perigynium (pe) at the top, ruptured lobes of calyptra (c) and capsule (arrow) below. Note that the sporophyte is inverted. C . Archegonial head seen from below with a ruptured capsule coming out of white perichaetium (p). The yellow mass of spores (arrow) and green rays (ry) are clearly visible. D. Archegonial head seen from above. Note the nine rays (arrow) and lobes with perichaetium (p) located between the rays. E. Archegonial head seen from below with a ruptured capsule coming out of white perichaetium (p). The yellow mass of spores (arrow) can be seen. F. A stage later than E. showing dehisced capsules (arrow) and dried papery perichaetia (p). A. http://etc.usf.edu/clipart/72800/72800/72800_liverwort.htm B. http://mcgregor.sbs.auckland.ac.nz/2011/02/02/marchantia-polymorphaarchegonium-and-spore-capsule C. ww.flickr.com/photos/blueridgekitties/4912478263 D. https://www.flickr.com/photos/gjshepherd/2883135020/in/photostream/ E. http://nyagyaa.blogspot.in/2012/11/marchantia.html F. http://mcgregor.sbs.auckland.ac.nz/2011/02/02/marchantia-polymorphaarchegonium-and-spore-capsule/ IV.
Perichaetium- a protective, two-lipped, curtain with a fringed margin develops around the each row of archegonia (Figs 21A, F, 22, 25 E, F).
V.
Rays- stout outgrowths, nine in number (Figs 24, 25C-F); and are formed from the margin of the female receptacle between the two lobes. The developing sporogonium is efficiently protected by three sheaths originating from tissue of the female disc. These are the perigynium, the calyptra and the perichaetium or the involucre. The first two sheaths cover an individual sporophyte (Figs 20G,H, 21B,D,F,22,23A) whereas the third sheath covers an entire row of developing sporophytes (Figs 21A, E, 22). As the sporophyte develops, the absorptive foot gets embedded in the tissue of the lower surface of the female receptacle. The region between the foot and the capsule i.e. the seta elongates slightly. The elongation of the seta is an anatomical necessity as the capsule containing the ripe spores and elaters hangs downwards and needs to be cleared of the calyptra and the enveloping sheaths. Figure 22 is a complete vertical section of an archegoniophore depicting two lobes of a receptacle. Each lobe carries 3-5 mature sporophytes cut in vertical sections. Some of them are not in the median
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plane but all of them show differentiation into foot, seta and capsule and are covered by various protective sheaths. A small part of the stalk can also be seen in the section. Structure of Sporogonium: The mature sporogonium of Marchantia is differentiated into three distinct regionsfoot, seta and capsule (Fig. 21E, F, 22, 23A). (i)
Foot. In Marchantia, the foot of the sporogonium is a bulbous structure which is embedded in the tissue of the female receptacle. Main function of the foot is anchorage and absorption of nutrients from the gametophyte for the growth of the developing sporogonium.
(ii)
Seta.
In the young sporogonium, seta is a short, thick stalk like structure
which connects the foot with the capsule (Fig. 21B-D). As the spores enter into spore tetrad stage, the seta elongates rapidly resulting in rupturing of calyptra (Fig. 21E) and pushing the mature capsule through the membranous perigynium (Fig. 21F, 23A) and perichaetium (Fig. 21E, 25A). As the sporogonium of Marchantia grows upside down (Fig. 25A-F), seta does not grow too long; as it will then be too near to the substratum and will hinder the spore dispersal over long distances (Fig. 26). (iii)
Capsule. In Marchantia, the capsule is spherical to oval in shape and yellow in colour (Fig 25A-F). Mostly they have one layered, sterile capsule wall (Fig.23A) except on the apical tip due to formation apical cap. Apical cap is formed from the sporogenous cells and projects into the cavity of the capsule. The cells of the capsule wall develop ring-like thickenings on their inner surfaces (Fig. 23D). Spores and elaters (Fig. 23B, C) are present in the cavity of the capsule (Fig. 23A). Spores are also called meiospores as they are product of meiosis and are haploid. Elaters are spindle shaped, elongated cells with two spiral wall thickenings on their wall (Fig. 23C). They lack cytoplasm and are hygroscopic. Main function of elaters is to bring about spore dispersal. As regards nutrition, the immature sporogonium has chloroplasts in their foot, seta and capsule wall as well as in the elaters. Thus, it is dull green and is able to manufacture some of its own food material. However, it is completely
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Fig. 26 Marchantia. Sporophyte dehiscence. Source: http://www.gopixpic.com/450/marchantia-antheridia-sp dependent on the gametophyte for absorption of water and mineral nutrients. As the sporophyte reaches maturity; and dehiscence approaches, the chloroplasts disintegrate. Therefore, mature sporophytes are yellow (Fig. 24B, 25A-F) and completely dependent on the gametophytes for their nutrition. Dehiscence of Capsule: Figure 24A is a top view of several, ripened and star-shaped archegoniophores showing pinkish white perichaetia.
Slight
elongation of the seta after spore formation pushes the capsule out from the protective coverings (Fig. 21A, B, D, 25A). First, the capsule (Fig.25B) breaks through the calyptra (that may still carry the remains of the dried neck at its tip) (Fig. 21A, B); and then through the membranous perigynium and perichaetium (Fig. 21E, F).
Figure 25A shows the undersurface of an
archegoniophore laden with yellow, ripe capsules protruding out of white, membranous perichaetium. The elongation of the seta exposes the capsule to outside atmosphere and eventually the capsule wall splits open along 4-6 lines (Fig. 25B). The split begins from the apex and proceeds to about the
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middle of the capsule. The valves subsequently roll back to expose a mass of yellow spores (Fig. 25C, E, F) and elaters. Figure 25D depicts the top view of archegoniophore shown in fig. 25C. Note the nine, green rays and intervening lobes showing white perichaetia.
Elaters being hygroscopic absorb moisture.
Under dry conditions, elaters twist and upon absorbing moisture uncoil, this movement of the elaters helps in breaking up of the spore mass and eventual spore dispersal (Fig. 26). The perichaetia dry up after the dehiscence of capsules (Fig.25F) and the rays bend upwards (Fig. 24C).
YOUNG GAMETOPHYTE
Spore Morphology and Germination: The meiospores of Marchantia are tiny, rounded, thick walled structures. The wall is differentiated into outer thicker exine or exospore and inner thin intine or endospore. Perinium is absent in many species. Spore has a little mass of granular cytoplasm, single nucleus and reserve food material. Morphologically, spores produced in the tetrad are all alike in Marchantia. However, genetically they are distinct; two spores produces male thalli whereas the other two give rise to female thalli. Thus, Marchantia exhibits functional or physiological heterospory. The spores germinate immediately on coming in contact with the suitable substratum, though the spores can remain viable for about a year. The mature spore (Fig. 27A) imbibes water and increases in size considerably (Fig. 27B). It later develops chloroplasts and cuts off a primary rhizoid on one side (Fig. 27C). The spore then undergoes 2-4 transverse divisions to form a short filament of green cells (Fig. 27D,E). Several forms of this early stage can be seen (Fig.
27F-H) before the second rhizoid gets initiated (Fig.
Subsequently, a row of about 12 cells appear in the apical part of the young plant
27I). (Fig.
27J-L) and persists throught the life span of the adult gametophyte (Fig. 27M,N). Soon a characteristic apical notch appears by the differntial growth of the embryonic cells at the margin of the 30-40 celled thallus (Fig. 27O,P).
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Fig 27. A-P. Marchantia. Stages in spore germination. Source: authors.
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Life Cycle of Marchantia: Life cycle of Marchantia is diplohaplontic and is characterized by sporogenic meiosis and alternation of generations. There are two distinct vegetative individuals in the life cycle – (a) dorsiventrally flattened, chlorophyllous gametophyte (Fig. 28.6) and (b) radially constructed sporogonium (Fig. 28.3). The gametophyte bears gametes and is anchored to the substratum with the help of smooth-walled and tuberculate rhizoids that help in absorption of water and solutes. Associated with the rhizoids are two types of scales that are arranged in two rows on either side of the midrib. Several gemma cups are present along the midrib on the dorsal surface. termed gemmae.
Gemma cups contain numerous tiny, bilobed green structures
Upon germination each gemmae is capable of producing two thalli.
Anatomically, thallus is divided into upper photosynthetic zone and lower storage zone. The photosynthetic zone has an upper epidermis and a layer of air chambers that are separated by vertical partitions of cells. Each air chamber communicates with the outside through a well defined barrel shaped pore situated in the upper epidermis.
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Fig. 28 Marchantia. 1-5 life cycle. Source: https://www.google.co.in/search?newwindow=1&tbm=isch&q=marchantia+life+cycle+anim ation&revid=
From the floor of the air chamber arise simple or branched, short filaments of green cells. Beneath the air chambers is a several layered thick storage region of compactly arranged colorless parenchyma cells. A few mucilage and oil cells are also present in the storage region. The gametophyte grows with the help of a group of meristematic cells. The gametophyte bears antheridia and archegonia in localized areas which are disc shaped receptacles. The male (antheridiophores) and female (archegoniophores) receptacles occur on separate gametophytes (Fig. 28.1). The receptacles are stalked and are terminal in position on the thallus. The terms antheridiophore and archegoniophore encompass both receptacle and stalk. Each archegoniophore has eight lobes and nine rays. The archegonia arise in a row from the tissue of the lobes. Each lobe has a growing point at its tip. Initially, the younger archegonia are closer to the periphery of the receptacle but post-fertilization shift towards the center of the receptacle resulting in archegonial inversion. The fertilized archegonia are pendulous with their necks pointing downwards. Each archegonium is covered by an inner envelope called calyptra and an outer envelope termed perigynium whereas each row of archegonia is protected by two-lipped involucre called perichaetium. The male receptacle has eight lobes. Each lobe has a row of antheridia sunken in the antheridial cavity that opens at the surface by an ostiole.
All these structures including the
gametes i.e. sperms and eggs formed by the gametophyte constitute the haplophase or the gametophytic generation. It begins with the spore (haploid) and ends with the gametes (haploid). Marchantia thalli can also multiply asexually through gemmae present in the gemma cups. This offers an accessory means of multiplying the haplophase, but Interestingly, plays no role in the alternation of generations. Post fertilization, Marchantia switches on to the second phase i.e. sporophytic phase or the diplophase. It begins with the zygote (diploid) (Fig. 28.2) that divides to form a short-lived embryo. The embryo then divides and differentiates into sporogonium- the second vegetative individual in the life cycle of Marchantia. Each sporogonium is solid, radially symmetrical structure, divided into a basal foot, intermediate seta and a terminal capsule (Fig. 28.3). The young capsules of a particular row remain covered by two-lipped perichaetium whereas an individual capsule is enveloped by inner calyptra and outer
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perigynium. The capsules contain elaters (diploid) and spores (haploid). Former are associated with the dispersal of spores. Since the sporogonium of Marchantia produces spores, it is called sporophyte. All the structures produced after fertilization such as zygote, embryo and the sporogonium constitute the sporophytic phase or the diplophase. It ends with the elaters (diploid) and spore mother cells (diploid) that form meiospores (haploid) after meiosis (Fig. 28.4). The spores are the first structures of the future gametophytic generation (Fig. 28.5). Thus, the gametophyte and sporophyte phase are not only closely connected but also alternate with each other. The two critical points in the life cycle are fertilization and meiosis. With the former begins the diplophase or the sporophytic phase and the latter marks the beginning of gametophytic phase or the haplophase. The alternation of one phase with the other in a single life cycle of Marchantia is expressed by a biological phrase ‘Alternation of generations’. Further, as the alternating plants in the life cycle are dissimilar, the alternation of generations is of heterologous type.
Important Features: 1) Plant body is dorsiventral, green and bilobed structure with a distinct central midrib that is more pronounced on the ventral side. 2) Dorsal surface shows numerous polyhedral or rhomboidal areas termed areolae. Each areola has an air pore in the centre. 3) Several gemma cups are present along the midrib zone on the dorsal surface. Each gemma cup is shallow and has a membranous margin. It contains numerous, green, lens shaped structures called gemmae which are capable of carrying out asexual reproduction. 4) Two types of unicellular, unbranched rhizoids i.e. smooth-walled and tuberculate arise from ventral surface and secure the gametophyte to the substratum. 5) Two types of membranous scales i.e. appendiculate and spathulate are arranged in two rows on either side of the midrib on the ventral surface. 6) The thallus grows by the meristematic activity of a group of cells situated in the apical notch.
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7) Internally the thallus is divided into upper photosynthetic and a lower storage zone. The photosynthetic zone shows anupper epidermis in which are embedded barrelshaped pores. Below the upper epidermis is a single row of air chambers. Several branched or unbranched photosynthetic filaments arise from the floor of each chamber. The storage zone consists of several layers of compactly arranged parenchymatous cells. Scales and rhizoids arise from the ventral epidermis. 8) The sexual reproduction is oogamous, plants are dioecious and the sex organs i.e. antheridia and archgegonia are produced on disc-shaped receptacles raised on long stalks. These structures are termed gametophores. The stalks are continuation of the thallus and show two furrows with rhizoids and scales all along the length. 9) The male receptacle is usually eight lobed and lacks rays. Antheridia are arranged in a row (with younger antheridia towards the centre and older towards the periphery) on a dorsal surface of each lobe. Each antheridium is deeply sunk in the antheridial chamber and opens at the surface with the help of ostiole. The sperms are biflagellate, curved structures. 10) The female receptacle is eight lobed and nine rayed. The archegonia are upright to begin with and are arranged in a row (with younger archegonia towards the centre and older towards the periphery) on a dorsal surface of each lobe similar to antheridia. Each archegonium is surrounded by a covering called perigynium. 11) Fertilization takes place with the help of rainwater or dew when the archegonia are upright and female receptacles are sessile. 12) The elongation of the stalk; and formation of rays, calyptra and perichaetium are post-fertilization developmental events. Alternating with the lobes grow out finger like rays from the periphery of the disc. The rays hang downwards in a young receptacle giving it a tiny umbrella shape. However, at maturity they spread out resulting in a star shape. The lobes carrying the rows of archegonia also grow downwards in such a manner that younger archegonia now face the center of the disc. This is termed the archegonial inversion. Calyptra grows from the venter and is a protective covering of an individual sporogonium. Perigynium also grows around an individual sporogonium to form a protective covering.
Perichaetium is a two-
lipped, curtain like structure that encloses an entire row of sporogonia hanging from the undersurface of a lobe.
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13) A mature sporogonium is differentiated into foot, seta and capsule. Capsule wall is single layered and contains a mass of spores and elaters. The elaters are simple, long cells that taper towards the ends. They are hygroscopic and help in dispersal of spores. The ripe capsule dehisces irregularly into 4-6 valves. The elongating seta carries the capsule clear of the covering sheaths such as perigynium, calyptra and perichaetium. 14) Upon germination, each spore produces a small gametophyte which grows by a group of apical cells.
Exercises Q1. Differentiate between: (a) Antheridiophore and Archegoniophore. (b) Smooth- walled and Tuberculate rhizoid. (c) Marginal and Peripheral scale. (d) Photosynthetic and Storage zone. (e) Calyptra and Perigynium. Q2 The gemmae of Marchantia are reproductive structures which do not play any role in the phenomenon of alternation of generations. Justify the truth or falsify the statement. Q3. The formation of rays, perichaetium and calyptra are post-fertilization events. Comment. Q4. The young archegonia are upright but later become pendulous. Describe this archegonial inversion. Q5. The air pores of Marchantia are homologous to the stomata of higher plants. Justify the truth or falsify the statement. Q6. Describe the process of spore formation and mechanism of spore discharge. Q7. Draw a well labeled diagram of V.S. antheridiophore/archegoniophore. Q8. Draw a well labeled diagram of V.S. thallus of Marchantia.
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Q9. Draw a well labeled diagram of V.S. thallus passing through the gemma cup and describe how do the gemmae bring about asexual reproduction. Q10. Fill in the blanks: I. II.
Sporophyte of Marchantia is differentiated in to …………, ......... and .............. Fertilization takes place through .................. mechanism.
III.
The ventral surface of the thallus bears ……………. and ............. scales
IV.
The antheridiophore and archegoniophore are ............... on thallus with the help of ............
V. VI.
Internally, the thallus is divided in to ............... and .......... zones. The antheridiophore has eight lobes whereas archegoniophore has ......... lobes and ........... rays.
Q11. Match the following: I.
Thallus
a) gemmae
Perichaetium
b) splash cup mechanism
III.
Vegetative reproduction
c) archegoniophore
IV.
Fertilization
d) scales
Calyptra
e) spore dispersal
Elaters
f) venter
II.
V. VI.
References Chopra R N (1998) Topics in Bryology. Allied Publishers Limited, New Delhi. Parihar N S (1965) An Introduction to Embryophyta. Vol 1, Bryophyta. Chand book depot, Allahabad. Shaw A J, Goffinet B (2000) Bryophyte Biology. Cambridge Press. Vashishta B R (1993) Botany Part III Bryophyta. S. Chand & Company, New Delhi.
Institute of lifelong learning, university of Delhi
Institute of lifelong learning, university of Delhi
