Angiosperms are flowering plant. Their seeds are covered by fruits. Their fertile leaves bear ovules. The fertile leaves are folded and joined to from ovaries. The ovary is changed into a fruit after fertilization. There are 235,000 species of angiosperms out of 360,000 known species of plants. Angiosperms are heterosporous plants. These are most successful plants of the earth. Theses plants produce flower, fruits and seeds.
The plant b.idy angiosperm is sporophyte. It is composed of root, stem leaves and reproductive part i flowers. Root is used for anchoring and absorption of water and salts. Root shows positive geotropism and negative phototropism. In some plants root is also used for vegetative reproduction. Stem give support to the leaves. The main stem of the plant body forms trunk. Trunk gives rise branches. Stein shows negative geotropism and positive phototropism. Leaves are arises from the branches of stern. Leaves are composed of pedicle and lamina Lamina has mesophyll cells for absorption of light and photosynthesis. Leaves also have stomata for exchange of gases and transpiration. Flower is the reproductive part of the body. It is composed of sepal, petals, stamens and carpels.
Life Cycle (Alternation of generation)
Angiosperm shows alternation of generation. Both sporophyte and gametophyte generations alternate with each other. Sporophyte is the dominant generation.
The adult plant of angiosperm is diploid sporophyte. It is heterosporous Sporophyte is a dominant generation. It is differentiated into roots, stem and leaves. It produces flower at 1,.turity. Sporophyte produces microspores and megaspores. A flower is a modified shoot (stem and leaves). A flower consists of a pedicel, thalamus or torus and floral leaves (sepal, petal, stamens and carpals). Thalamus and floral leaves, especially stamens and carpel are much modified. Therefore, they do not look like stems and leaves. Flower has following floral parts:
Sepal and Petals
Sepals and petals are non-essential or non-reproductive parts of the flower. They protect the stamens and carpals. They also attract insects for pollination. After fertilization they may fall off. Sepals and petals are collectively called as perianth.
Stamens and carpals are essential or reproductive parts of the flower. The number of stamens in species is variable. The stamen consists of a filament and anther. The anther lobes are connected by tissues called connective. The anther consists of four pollen sacs (microsporangia). Each stamen develops from a separate priinordium from the floral axis.
Each anther lobe contains two microsporangia. Microsporangium has following parts:
(a) Sporangial wall: Microsporangium is surrounded by sporangial wall. This wall is composed of many layers. The inner most layer of the sporangial wall is called tapetum. It is nutritive in function.
(b) Sporogenous cells:
Sporangial’ wall surrounds sporogenous cells. Sporogenous cells divide to form These cells divide by mitosis to form garnetophytssmicrospore mother cells. undergo meiosis to produce four nucrospores. Microspores germinate to produce male gametophyte.
Microspore is It is uninucleate.lt is covered by two layered wall. The outer layer is called nine and inner is called intine. The exine is thick. It contains thick layer of cuticle, sperm nuclei Sometimes, exine is
Carpel is female reproductive part of a flower. It has central position in the flower. A carpel is composed of three parts: ovary, style and stigma. Ovary is a basal broader part of the carpel. Style forms a
tube like structure. Stigma is the terminal part of carpel. It receives pollen grain. Carpels may be free (apocarps) or fused (syncarpous).
Ovule (Integumented Megasporangium)
The ovary contains one or more ovules. Ovule consists of an outer integument and inner nucellus tissue. Ovules are borne on specialized tissues called placenta. The position of placenta is variable in ovary. Each ovule has a short stalk called funiele. The place of attachment of the funicle with the main body of the ovule is called hilum. Funicle is attached to the basal part end of ovule called chalaza. Pollen tube enters through the micropyle. The body of ovule is filled with Sporogenous tissues called nucellus. Nucellus are surrounded by one or two protective layers called integuments. Ovule has an opening on side in the integument called micropyle. The junction of the nucellus and integument is called chalaza. Nucellus represents megasporangium. Therefore, ovule is known as integumented megasporangium. One of the nucellus enlarges. It acts as megaspore mother cells. This cell undergoes meiosis to form four haploid megaspores. Three megaspore degenerates. Only one remains functional. This megaspore forms embryo sac (female gametophyte).
The gametophyte is an inconspicuous and haploid ( I n) generation. There are separate male and female gametophytes:
The microspore is the first cell of the male gametophyte. Thus the male gametophyte is represented by pollen grain and pollen tube containing two sperms an single tube nucleus. The germination ef m icrospore spore starts with in the microsporangium. Its nucleus divides to form a generative nucleus and vegetative nucleus. The intine grows out of the germ pore to form pollen tube. This semigerminated male gametophyte is called pollm grain. After pollination, the pollen grain is transferred to the stigma. Here it germinates to from complete pollen tube. The generative nucleus divides into to form sperm nuclei. At this stage of development, the pollen grain is called male gametophyte. The pollen tube elongates and carries the sperm nuclei inside the ovule. …dam day
Female Gametophyte (Embryo Sac)
The megaspore is the first –cell of the female gametophyte. Megaspore develops in female gametophyte. The female gametophyte consists of seven cells. Three cells are present towards the chalazal end. These cells are called antipodal cells. A group of three cells called egg apparatus is present towards the micropylar end. The two upper cells of the egg apparatus are called synerdids. One of these cells is called egg or oosphere. A secondary nucleus (2n) is present in the centre of embryo sac.
The fusion of two male gametes with two cells simultaneously is called double fertilization. Double fertilization is a special process found in Angiosperms. The pollen tube grows through the style. It enters the ovule and then reaches the female gametophyte. Here, it discharges the male gametes. One male gamete fuses with the egg to form Zygote nucleus. The zygote nucleus secretes thick wall to form oospore. Oospore is the first cells of the sporophyte generation. The second male gamete fuses with the secondary nucleus and form primary endosperm nucleus. So it is called double fertilization. It is also called as triple fusion. Oospore divides into basal suspensor cells and a terminal embryo cells.
Significance of double fertilization
(i) Double fertilization is an important evolutionary advancement. The storage food is made after fertilization or formation of zygote. It helps the plant to economize on its food resources.
(ii) The fusion of the second male gamete with the secondary nucleus provides a stimulus for the formation of endsperm for the developing embryo. Without this stimulus secondary nucleus remains inactive.
(iii) Double fertilization stimulates the phenomena of xenia and metaxenia. In case of xenia, the pollen stimulates the tissues to surround the embryo. In case of metaxenia, pollen stimulates the formation of seed coat.
The primary endosperm nucleus divides repeatedly. It forms polyploidy nutritive tissue called endosperm. There are two types of seeds for storage of food:
a) Endospermic or albumin* seed: The endosperm supply food to the developing embryo. Such seeds are called endospennic seeds. In plants like corn, wheat, the endosperm tissue is present at the time of seed germination. So these are endosperrnic seeds.
a) Non-endospermic or ex-albuminous seeds: In some cases, the• endosperm is completely utilized by developing embryo. Such seeds are known as non-endospermic seeds. In beans and peas the endosperm tissue is completely digested by the developing embryo and stored in the cotyledons.
The embryo in dicots is octant type. It is composed of an embryonal axis. The cotyledons are attached to this axis. The embryonal axis has two parts:
a) Epicotyl: The part of the axis above the point of attachment of cotyledons is epiCOtyi. It bears a bud called plumule. Plumule develops into shoot. The cotyledon is terminal. It encloses the plumule present at the tip of the embryonal axis.
b) Hypocotyl: The part of the embryonal axis below the point of attachment of cotylbildits is hypocotyl. It possesses a meristem at its tip called radical. The radical is present at the lower end of the embryonal axis. The radical develops into primary root. It is filamentous type in monocots embryo.
A ripened or mature ovule protected by seed coats enclosing a
resting embryo, provided with stored food either in endosperm or cotyledons is called seed. The integument of the ovule forms testa and tegmen. Seed may be endospermic or albuminous when endosperm is present (grain of maize). It may be non-endospermjc or ex-albuminous in which the food in endosperm is digested and stored in cotyledons (pea seed).There are different structures of seed in dicots and moncots.
a) Dicots: Dieots has dicotyledonous seed, (seed with single
cotyledons) e.g. pea seed. In dicot seeds the embryo consists of a ihort axis. The embryonal leaves (cotyledons) are attached to it. The part of the axis above the point of attachment of cotyledons is epicotyl. The part of the axis below the point of attachment of cotyledons in hypocotyl. At the upper end of the epicotyl a minute bud called plumule. It is present between the two cotyledons. It gives rise to embryonal shoot. The lower end of the hypocotyl is radical It give rise to primary root.
b) Monocots: Monocots have monocotyledonous seed (seed with single cotyledons) e.g. grain of maize. In monocot seeds the cotyledon is terminal. The upper part of the embryonal axis bears plumule and the lower part contains radical.
The ripened ovary is called fruit. The seed are enclosed within fruits in angiosperms. It grotects and disperses seeds. The ovary wall undergoes changes to form the fruit wall or pericarp. It pericap may be dry or fleshy. Seed usually undergoes a period of rest. It germinates in suitable conditions and roduces a seedling. This seedling gradually changes into a sporoph rte.
DEVELOPMENT OF ANGIOSPERMS (EMBRYOLOGY)
DEVELOPMENT OF FLOWER
Flower is a modified shoot. It appears as a small bud. It develops much like a foliage bud. Unlike the foliage bud, there is no elongation of the internodes in floral bud. Therefore, the floral leaves are not distributed at intervals on the stem or it branches and its leaves are grouped together at a place. The development sequence of different parts of the flower varies in different types of flowers. The development of flower of Capsella bursa-pastoris (Shepherd’s purse, a member of family Cruciferae) is taken as an example. It has
- The floral primordium is a small axis of meristematic cells. It enlarges above into a round structure called receptacle. It forms a narrow stalk below. This stalk becomes pedicel of the flower later.
- The floral parts arise at the top of the receptacle. These floral parts appear as rounded protuberances ofFig: Development of Flower meritstematic tissue.
4. A circle of four sepals appear first at four points on the receptacle. Sepals form a protective cover for the younger parts.
5. Sepals are followed by stamen primordis Each stamen primordium develops into an anther and fil:
6. The stamen primordia are followed by carpel primordium. It develops as a continuous wall about the apex of the floral axis. It develops into a bicarpellary, 2-celled ovary. It ends in a stigma and a very short style.
DEVELOPMENT OF MICROSPOROPHYLL (STAMEN) A papillate ougrowth of meristematic tissue from the receptacle arises. This outgrowth forms stamen. The apical part of this outgrowth grows more actively. It develops into broader anther. The lower part differentiates into narrow filament. Cross section of embryonic anther consists of a mass of thin-walled cells. These cells are surrounded by an epidermis. But as it matures it becomes four lobed. Now it has a prominent vascular strand in the centre. A microsporangium or pollen sac develops in each lobe.
Development of Microsporangium
A group of hypodermal cells are present at each corner of the young anther. These cells act as sporangial initials. These divide by periclinal walls to form inner primary sporogenous cells or archesporium and outer primary parietal cells. Parietal cells lie immediately below the epidermis.
(a) Formation of sporangial wall
The primary parietal cells undergo periclinal and anticlinal divisions These form three to five concentric layers of sporangial wall and large part of tapetum. The outermost layer of the wall is present beneath the epidermis. It is called endothecium, The endothecium develops fibrous bands of thickenings. So it becomes hygroscopic. These fibrous bands are absent at a point called stomium. The cells remain thin-walled at the stomium. The anthers dehisce at stomium on maturity. The middle layers of the wall collapse at the time of dehiscence. The innermost layer of the wall progressively differentiates into tapetum.
(b) Formation of Microspores (Mierosporogenesis)
The primary sporogenous cells or archesporium undergo several divisions to form microspore (pollen) mother cells. Each microspore mother cell undergoes meiosis. Wall is formed on newly formed cells. Thus a tetrad of haploid microspores or pollen grains is formed. The wall formation in the developing microspore is of two types:
(0 Successive type: In this case, wall is laid down between the daughter nuclei at each nuclear division. It forms isobilateral tetrad.
(ii) Simultaneous type: In this case, walls are formed by furrowing after formation of four nuclei. It forms tetrahedral tetrad. The spore members of each tend separate from one another. They Dispersal of pollen grain The anther becomes mature and the middle layer and tapetum disorganize. The sporangial wall now consists of epidermis and endothecium. The sterile partition between the two pollen sacs disintegrate and two pollen sacs of one side unite together. They form a single compartment. The pollen grains are released through stomi um.
DEVELOPMENT OF MEGASPOROPHYLL (CARPEL) Carpel arises from the tip of the receptacle (thalamus). It is a part of the gynoecium. Carpel consists of a hollow basal swollen part, the ovary. One or more ovules or megasporangia are produced in it. It has a stalk, called style and a stigma. Stigma is the modification of the tissue of at the apex of the style.
Development of Ovule (Megasporangium)
The ovule develops from placenta of the ovary. Meiosis and megaspore formation occurs within ovule. Therefore, ovule represents a megasporangium. Ovule consists of a stalk or funiculus. It bears the nueellus surrounded by one or twc integuments. Following steps occur during formation of ovule:
I) Perkfinal divisions occur in layer of hypodermal cells. It forms ovule primordium. This primordium becomes conical structure with rounded tip.
(ii) The tip of primordium form nucellus. The growth of nucellus is unequal. This various degrees of curvature are formed in main body of the ovule. This results in formation of different types of ovules.
(iii) Integuments arise as rim-like outgrowths from the basal cells of the nucellus. It surrounds nucellus completely except at tip of the ovule. Thus inner integument contains a tubular opening. It is called micropyle.
Formation of Megaspores (Megasporogenesis)
- A hypodermal cell of nucellus becomes larger in size. It has more conspicuous nucleus and denser cytoplasm.
- In some cases, the enlarged cell acts as megaspore mother cell directly. However, in most cases a hypodermal cell first divides by an unequal division to form two cells. The outer cell is called parietal cell and an inner cell is primary sporogenous cell
- Primary sporogenous cell functions directly as megaspore mother cell.
- The parietal cell divides by anticlinal or periclinal divisions. It gives rise wall cells.
- The megaspore mother cell undergoes meiosis. Each nuclear division is accompanied by wall formation. It forms a *linear row of four megaspores.
- The upper three megaspores nearest the micropyle degenerate. The lower surviving megaspore enlarges to form an eight-nucleate embryo sac or female gametophyte.
Forms of Ovules
The ovules can be classified into following types on the basis of L.:.;,:.,1%e position of the micropyle, chalaza and funicle:
- Orthotropous or Straight: The ovule is erect. Thus the fuhicle,
chalaza and micropyle all lie in one and the same vertical line. This type of ovule is found in members of family Polygonaceae.
- Anatropous or Inverted: The main body of the ovule bends along the funicle. Thus the micropyle lies close to the hilum and the chalaza lies at the other end. The funicle fuses with the main body of the ovule forming a ridg, the raphe. This is the most common type of ovule. It is found in almost all members of
Sympetalae. It also occurs in other families of both dicots and monocots.
- Amphitropous or Transverse: This is an intermediate type between orthotropous and anatropous. The ovule body is at right angle to its stalk or funicle. The fusion of the integuments with funicle is very slight. Thus the hilum, chalaz_a and micropyle all lie apart from one another. This is very rare type. It is found in Primulaceae, Ranunculaceae and some members of Cruciferae
- Hemitropous: The body of the ovule is straight. But it is twisted transversely at right angles. Thus the chalaza and micropyle are in the same line. They are at right angles to the funicle. Hemitropous ovule is common in Ranunculus.
- Campylotropus or Curved: The body of the ovule is bent upon :tself like a horse-shoe. Thus the micropyle comes to lie near the funicle. It is also rare. It is found in members of family Leguminosae, Caryophyllaceae, Cruciferae, and Poaceae.
- Circinotropous: The nucellus and the axis remain in the same line in the beginning. But rapid growth occurs on one side. Thus the ovule gets inverted. This curvature continues. Thus the ovule turns completely. So once again the micropyle faces upwards. Circinc mpous ovule is found in Plumbago and Opuntia.
DEVELOPMENT OF MALE GANIETOPH’YTE
Microspore is the first cell of male gametophyte. Its germination starts in the microsporangium (pollen sac). The nucleus of the microspore moves toward the wall. It divides into a generative cell and a vegetative cell. The generative cell loses contact with the spore wall. It appears lenticular (lens-shaped). lilies freely in the cytoplasm of the vegetative cell. The germinating microspore at this two celled stage is called pollen grain. Dehiscence occurs in the sporangium wall. Thus the pollen grain is released from the microsporangium. It is carried to the stigma of the carpel during pollination. It is attached on the papillae on the surface of the stigma. The pollen grain absorbs water. The stigma and style provide it nutrition. The intine protrudes out in the form of pollen tube. The vegetative nucleus (tube nucleus) migrates to the pollen tube. The generative nucleus divides to form two male gametes. These nuclei migrate into the pollen tube. The tube nucleus controls the movement of the pollen tube. At this stage of development, the pollen grain is called male gametophyte. The microspore is the first cell of the male gametophyte. The pollen tube elongates and carries the sperm nuciei inside the ovule.
DEVELOPMENT OF EMBRYO SAC OR FEMALE GAMETOPHYTE
Types of Embryo Sac Development There are ten types of embryo sac development. The classification is based on:
I. The number of spores or spore nuclei entering into the formation of embryo sac. Thus embryo sac may be monosporic, bisporic or tetrasporic type.
- The number, arrangement, and chromosome number of the nuclei in the mature embryo sac.
- The total number of nuclear divisions occurring during megasporogenesis and development of female gametophyte.
- Monosporic, Normal or Polygonum Type
It is commonly found in plant. It is commonly called normal type. However, it was first clearly described in Polygonum. Therefore, it is also called as Polygonum type. This embryo sac has four well-defined megaspores. One of which gives rise to the embryo sac. The functional megaspore enlarges. Its nucleus divides. A large vacuole is formed between the nuclei. Thus the daughter nuclei move to the micropylar and chalazal poles of the embryo sac. Each nucleus divides twice. Thus four nuclei are formed at each pole. One nucleus from each pole migrates to the centre of the embryo sac. The two nuclei fuse to form a diploid secondary nucleus. Three nuclei at micropylar end are surrounded by membranes. They form egg apparatus. The central cell enlarged and become egg cell. The other two cells becomes synergid. Thus embryo sac is formed containing 8-nucleoli and later 7-celled during its development.
- Bisperic or Allium Type
This type of embryo sac is found in Album. It is found in many monocot and dicot families. Two dyad cells are formed during first meiotic ,division during megasporogenesis. One of two dyad cell is aborted. The nucleus of the surviving dyad cell towards the chalazal end divides to form two haploid nuclei. These are called megaspore nuclei. These nuclei move towards opposite ends. These nuclei divide twice to form eight nuclei. One nucleus from each pole migrates to the centre of the embryo sac. Three nuclei at the upper end produce egg apparatus. The nuclei present at lower end form antipodal cells. In this way 8- nucleate bisporic embryo sac develops.
- Tetrasporic Type
In this type of embryo sac wall is not formed after the meiotic nuclear division. All four haploid megapsore nuclei take part in the formation of the embryo sac. The resultant embryo sac may be 8- nuceleate or 16-nucleate. Thus it has two types:
a) Plunmbago Type (8-Nucleate): In this case, the megaspore
ruclei arrange themselves in a cross-like manner. One lies at the micropylar ends and the other lies at the chalazal end. The other two are present at each side of the embryo sac. Each nucleus divides once. Thus pairs of four nuclei are formed. One nucleus from each pair migrates to the centre. They fuse to form tetraploid secondary nucleus. The nucleus at micropylar and form the egg cell. The rest three nuclei degenerate. There are no antipodal cells and synergids.
b) Fritillaria Type (8-Nucleate): This type of embryo sac occurs in a large number of genera. In this case, Three out of four megaspore nuclei are arranged in 3 + 1 fashion. Three nuclei migrate to the chalazal end. The remaining nucleus comes at the micropylar pole. The micropylar nucleus divides to form two haploid nuclei. The three chalazal nuclei fuse. The fusion nucleus divides to form two triploid nuclei. Now the embryo sac contains four nuclei, two haploid micropylar nuclei and two triploid chalazal nuclei. Later each nucleus divides. Thus they produce four haploid nuclei at micropylar end and four triploid nuclei at chalazal end. One nucleus from each pole migrates to the centre. These fuse to forms a tetraploid secondary nucleus. The nuclei at micropylar end form egg apparatus. The nucleus at the chalazal end gives rise to antipodal cells.
c) Penaea Type (16 Nucleate): In this case, 16 nuclei are arranged in quarters. One is present at each end of the embryo-sac and two are present at the sides. Three nuclei of each quarter become cells. The fourth nuclei of each quarter moves towards the center and act as polar nucleus. Therefore, there are four triads and four nolar nuclei. One cell of the micropylar triad is the egg. It is the only functional cell.
d) Drusa Type (16 Nucleate): In this case, one megaspore nucleus moves towards the micropylar. The remaining three megaspore nuclei move towards chalazal end. Each nucleus divides twice. Thus four nuclei are produced at micropylar end and twelve at chalazal end. One nucleus from each migrates towards the centre of the embryo sac. They fuse to form secondary nucleus. The dime nuclei at micropylar end form egg apparatus. The eleven nuclei at chalazal end form antipodal cells.
e) Adoxa Type (8–Nucleate): The four haploid megaspore nuclei present in the cytoplasm undergo a mitotic division. They produce eight nuclei. These nuclei are arranged in typical. manner. Three of them come at the micropylar end. Three comes at the chalazal end. And two come in the centre (fusion nucleus). Thus normal 8.nucleate seven celled embryo sac is formed.
Paperoma tye (16 Nucleate): In this case, each of four megaspores nuclei divides twice. They form 16 nuclei. These are uniformly distributed at the periphery of the embryo sac. Two nuclei at micropylar end form an egg and a Synergid. Eight of them fuse to form secondary nucleus. The remaining three stay at the periphery of the embryo sac.