Development of endosperm. Endosperm develops after double fertilization in angiosperms. It is triploid and develops from primary endosperm nucleus. In angiosperms three types of endosperm development are observed:
(1) Nuclear endosperm. In nuclear endosperm development nucleus divides by free nuclear division, there is appearance of central vacuole, all the nuclei are pushed towards periphery then starts cytokinesis and results into the formation of cells hut in coconut vacuole persists which is filled with fluid.
(2) Cellular endosperm. In cellular endosperm each and every division is followed by cytokinesis, so endosperm is cellular from the very beginning e.g. Datura, Petunia.

(3) Helobial endosperm. It is intermediate between cellular and nuclear endosperm. The first division is followed by cytokinesis to form two unequal cells, micropylar cell and chalazal cell. Further divisions in each cell are free nuclear divisions. This type of endosperm is found in order Helobiales.

(a) (i) Endosperm development precedes embryo development. The primary endosperm cells divide repeatedly and forms a triploid endosperm tissue. The cells of this tissue are filled with reserve food materials and are used for development. (ii) Endosperm persists in albuminous seeds (castor and coconut). Endosperm are used during seed germination.
(b) Development of embryo. 1. The zygote or oospore divides by a transverse wall into an upper suspensor cell and lower embryonal cell.
2. The suspensor cell which lies towards the micropylar end, divides by transverse divisions to constitute 7-10 cells called suspensor.
3. The upper cell of the suspensor filament towards micropylar end is called haustorial cell, whereas the celllying above the embryo cell is called hypophysis.
4. The haustorial cell enlarges in size and attaches the suspensor to the tip of embryo sac.
5.The embryonal cell divides by second longitudinal division at right angle to the first and then by transverse division to form an octant or eight celled embryo.
6. Out of these eight cells, the lower four cells of octant away from the suspensor give rise to the plumule and the two cotyledons, while the above four cells of octant near the suspensor forms the hypocotyl and stele of radicle.

7. Now this octant divides by a periclinal division to form outer single layered dermatogen from which arises the epidermis layer. The inner cells further divide to form periblem below the dermatogen and the central plerome.
8. The periblem forms the cortex while the plerome gives stele of embryo.
9. The lowermost cell of suspensor, which is lying just above the octant cells is known as hypophysis. The hypophysis divides to give rise the dermatogen and periblem of the radicle.
10. A fully developed embryo of dicotyledons has an embryonal axis differentiated into plumule, two cotyledons and radicle.
11.In the beginning embryo is globular. When two cotyledons differentiate from the sides with faint plumule in the centre, embryo become heart-shaped. Now the embryo undergoes rest and ovule is transformed into seed.
Monocot embryo. Unlike the dicots, where the embryonal mass is formed of eight cells, the anterior cells forming the plumule and the cotyledons, and the posterior forming hypocotyl, the development is much variable in monocots i.e. grass family. In some cases, suspensor does not develop at all. Only one cotyledon appears in monocots as a terminal structure. The plumule always appears laterally from it.

1. The ovule is attached to the placenta by a short stalk called funiculus.
2. The junction between the funiculus and the ovule, is called hilum.
3. The primordium of the ovule grows into a mass of cells, forming the nucellus, the main body of the ovule.
4. It becomes surrounded by two protective envelopes called integuments, which leave a small opening called micropyle, at the tip of the ovule.
5. The basal part of the ovule (opposite to the micropyle) is called chalaza. Cells of the nucellus are rich in reserve food materials.
6. There is a single embryo sac/female gametophyte in the nucellus; it has developed from a megaspore.
7. The embryo sac contains the following cells:
(i) Three antipodal cells at the chalazal end.
(ii) Three cells at the micropylar end, forming egg apparatus, which consists of two synergids and a female gamete.
(iii) Two nuclei in the central cell, called polar nuclei, which fuse to form a diploid secondary nucleus.

Meiosis occurs during sporogenesis.
The structure formed at the end are microspores and megaspores respectively.

Development of Female Gametophyte:
- The megaspore mother cell undergoes meiosis to form four megaspores.
- Three of them degenerate and the functional megaspore enlarges to form the embryo sac.
- Its haploid nucleus undergoes a mitotic division and the two daughter nuclei move towards the poles.
- Each of them undergoes two successive mitotic divisions.
- Cell wall formation starts in the 8-nucleate stage.
- Three of them at the micropylar end form the egg apparatus, consisting of a female gamete and two synergids.
- Three of them at the chalazal end form the antipodal cells.
- One nucleus from each of the poles moves to the centre; they are called polar nuclei.
- The two polar nuclei fuse in the centre (now the central cell) to form a diploid secondary nucleus.
- Thus, the mature embryo sac is 7-celled and 8-nucleate.

Development of Female Gametophyte:
- The megaspore mother cell undergoes meiosis to form four megaspores.
- Three of them degenerate and the functional megaspore enlarges to form the embryo sac.
- Its haploid nucleus undergoes a mitotic division and the two daughter nuclei move towards the poles.
- Each of them undergoes two successive mitotic divisions.
- Cell wall formation starts in the 8-nucleate stage.
- Three of them at the micropylar end form the egg apparatus, consisting of a female gamete and two synergids.
- Three of them at the chalazal end form the antipodal cells.
- One nucleus from each of the poles moves to the centre; they are called polar nuclei.
- The two polar nuclei fuse in the centre (now the central cell) to form a diploid secondary nucleus.
- Thus, the mature embryo sac is 7-celled and 8-nucleate.

(i) Insect-pollinated flowers are large, colourful and showy; if flowers are small, they are clustered into inflorescene to make them conspicuous.
(ii) The pollen grains and stigmatic surface are sticky.
(iii) They are highly fragrant; flowers pollinated by flies and beetle have foul odours to attract them.
(iv) To sustain animal visits, the flowers offer certain rewards like nectar, edible pollen and safe place for laying eggs.

(a) Syngamy
Syngamy refers to the fusion of a male gamete with the female gamete in the embryo sac to form a diploid zygote.
Triple fusion
Triple fusion refers to the fusion of a male gamete with the secondary nucleus (formed by fusion of two haploid polar nuclei) to form the triploid primary endosperm nucleus.
(b) Embryo development
Th'e zygote undergoes mitotic divisions during embryogeny.
The first division is transverse and results in a basal cell and a terminal cell; the basal cell forms the suspensor while the terminal cell forms the embryo proper.
It first, forms a filamentous proembryo, which later forms in sequence a globular embryo, heart-shaped embryo and then a mature embryo.

(a) Differences: 

(b) Pollen-Pistil Interaction.
The stigma/pistil has the ability to recognise the right type of pollen, i.e. the compatible pollen of the same species and rejects the wrong type of pollen grains.
It is the result of interaction between the chemical components of the pollen and those of stigma. - A compatible pollen grain starts its germination on the stigma.
The intine grows out through one of the germ pores as a protuberance.
The contents of the pollen move into this tube, i.e. the tube nucleus (nucleus of the vegetative cell) and the generative cell (or the two male gametes in those species where pollen is liberated in the three-celled stage).
The pollen tube grows through the tissues of stigma and style and enters the ovule through micropyle.
It enters the embryo sac through the filiform apparatus of one of the synergids to liberate the male gametes.
The events from the deposition of pollen on the stigma till the pollen tube enters the ovule, are collectively referred to as pollen-pistil interaction.

(a) Development of Pollen grain:
   - Microsporocytes undergo mitotic divisions to increase the number of functional microspore mother cells.
   - Each microspore mother cell undergoes meiosis to form a microspore tetrad.
   - All the four microspores remain enclosed in a common wall.
   - They start separating from each other.
   - The nucleus undergoes a mitotic division resulting in the formation of a large vegetative cell and a small generative cell.
   - The spore develops a wall of two layers the outer- exine and inner intine.
   - At this stage, the pollen grains are liberated; but in some cereals the generative cell undergoes mitosis and the pollen grain is 3-celled, at the time of liberation.
(b) Mature Pollen Grain:
   - A mature pollen grain has a protective covering of two layers.
   - The outer is thick, sculptured variously and is made of sporopollenin; this layer is called exine.
   - The inner layer, called intine is thin and is made of cellulose.
   - A mature pollen grain is two celled with a large vegetative cell and a small generative cell floating in the cytoplasm of the vegetative cell.
   - The pollen grains of dicot plants have three germ pores while those of monocots have one.