Introduction to Plant embryology-2

Dr. PallaviJ.N.L. College Khagaul

Ovule• It is attached to the placenta with a stalk called

funicle.

• The point of attachment of funicle with the body of the ovule is known as hilum which extends above in the form of a ridge called raphe.

• Nucellus consists of parenchymatous cells.

• Nucellus remains covered by one or two coverings called integuments.

• Integuments remain disconnected at one point forming a passage called micropyle.

• Embryo sac consists of three antipodals, two synergids, one egg cell and one secondary nucleus.

• Antipodals are located near the chalaza end and the egg cell and synergids towards the micropylar end.

Types of ovule

(A) Orthotropous:

(Ortho, straight; tropous, turned). When micropyle, chalaza and funicle lie in onestraight line; e.g., Polygonaceae, Urticaceae.

(B) Anatropous:

(Ana, backwards; tropous, turned). Here, the body of the ovule turns backwardsby an angle of 180° and so the micropyle becomes close to the hylum andplacenta; Sympetalae.

(C) Hemitropous:

(Hemi, half; tropous, turned). Here the body of the ovule is placed transverselyor somewhat at right angle to the funicle. Chalaza and micropyle are presenthere in one straight line (Fig. 186); e.g., Ranunculus.

(D) Campylotropous:

(Kampylos, curved). Here the body of the ovule is curved in such a way that thechalaza and the micropyle do not lie in the same straight line; e.g., Leguminosae.

(E) Amphitropous:

Here the curvature of ovule is more pronounced and embryo sac becomeshorseshoe shaped (Fig. 186); e.g., Butomaceae.

(F) Circinotropous:

Here the funicle is very long and the ovule rotates by an angle of 360° in such afashion that it is completely circled around by the funicle. Micropyle facesupward; e.g., Cactaceae.

Megasporogenesis• Development of Megaspore Mother Cell:

• The ovule consists of nucellus and one or two integuments from itsbasal region. Out of the many hypodermal cells of the nucellus onebecomes differentiated gets enlarged in size with dense cytoplasmand conspicuous nucleus, called archesporial cell. The archesporialcell divides transversely and forms an inner primary sporogenous celland an outer primary parietal cell.

• The primary sporogenous cell functions as megaspore mother celland the primary parietal cell undergoes repeated vertical divisionsand forms layers of parietal cells. Sometimes, the archesporial celldoes not divide and directly functions as megaspore mother cell.

• Megasporogenesis (Development of Megaspores):

• The megaspore mother cell is diploid (2n), which undergoes meiosis and forms four haploid (n) megaspores. The first division of megaspore mother cell is transverse, forming two cells. Both the cells again divide transversely and thus four (4) haploid megaspores are formed.

• The megaspores are then arranged in an axial row, called linear tetrad. Out of four megaspores, only one which remains towards the chalazal end behaves as functional megaspore and the other three which remain towards the micropylar end, gradually degenerate. The functional megaspore forms the female gametophyte i.e., the embryo sac.

Megagametogenesis (Formation of female gametophyte i.e., Embryo sac):

• Megaspore (n) is the first cell of the female gametophyte. The functional megasporebecomes enlarged at the expense of the nucellus and thus forms the femalegametophyte i.e., the embryo sac. Initially, the embryo sac is uninucleate and withfurther growth its nucleus divides by three successive divisions and forms eight nuclei.

• Out of eight nuclei, initially four remain towards the micropyle end and the other fourtowards the chalazal end. One nucleus from each pole then moves towards the centreand forms a pair of polar nuclei. These nuclei fuse together and form 2n nucleus, thedefinitive nucleus. It is also known as polar fusion nucleus or secondary nucleus.

• The three nuclei of the micropylar end form the egg apparatus and the rest three at thechalazal end are called antipodal cells. In the egg apparatus, each nucleus is surroundedby viscous mass of cytoplasm without any wall, of which the middle one is the largestand called egg, ovum or oosphere and the rest two (one on each side of the egg) are thesynergids or helping cells. The antipodal cells have viscous mass of cytoplasm, coveredby cellulosic wall.

Stages of Development of Female Gametophyte

• This type of embryo sac development is very common in angiosperms and is known asordinary type or normal type or Polygonum type. This type is also known as monosporictype, because, out of four megaspores, only one remains functional and forms theembryo sac.

Megasporogenesis

• Pollination:Transfer of pollen grains from anther to the stigma (of a pistil) is called pollination.• It is of two main types:• (i) self-pollination and• (ii) Cross-pollination.• (A) Self pollination:• Transfer of pollen from anther of a flower to the stigma of the same flower

(or flower of same plant) is called self pollination. Self-pollination can be two types, autogamy and geitonogamy,

• (1) Autogamy:• It is a type of self-pollination that is found only in bisexual flower. In this

case, the stigma of a flower is pollinated by its own pollen. Autogamy occurs by three methods.

• (2) Geitonogamy:• In this type of pollination, the pollen grains of one flower are transferred to

the stigma of another flower in same plant or genetically similar plant.

• Cross pollination/Allogamy:

• Transfer of pollen grains from the anther of the flower of one plant to the stigma of the flower of other plant by the help of agents is known as cross pollination. Agent may be biotic (non living) or Biotic (living).It is commonly seen in dioecious plant but very rarely in monoeclous. It is highly advantageous than self pollination mainly in formation of new genotypes.

• Types of Cross Pollination:

1. Pollination by Wind or air is known as Anemophilly.

2. Pollination by Water is known as Hydrophilly.

3. Pollination by insect is known as Entomophilly.Ex – Salvia, Cestrum, Ficus

4. Pollination by bird is known as Ornithophilly. Ex – Bombax, Callistemon

5. Pollination by animal is known as Zoophilly.

6. Pollination by Snail is known as Malacophilly. Ex. Lemna, Colocasia, Diptera

7. Pollination by Bat is known as Cheireptrophilly. Ex. – Kigelia, Anthocephalus, Adansonia, Bauhinia

• Fertilisation:

• Fertilisation is angiosperms, is unique. It is called double fertilisation. This is because there are two sperms per pollen tube. One of the sperms fertilises the egg cell to form zygote, while remaining sperm fertilises two polar nuclei, resulting in the formation of a triploid endosperm cell. The phenomenon of fertilization was first reported by Strasburger (1884) in Monotrapa. The male gametes are brought to the egg present in female gametophyte by a pollen tube. This phenomenon is called siphonogamy which was discovered by G. B. Amici in Protulaca plant.

Important events in fertilization are following:

1. Entry of pollen tube into ovule and embryo sac:

(a) After arriving in the ovary, the pollen tube finds its way through style and enters the ovule.

(b) Depending on the place, the retry of pollen tube into the ovule, can be of following three type:

2.Discharge of male gametes (sperms):

(a) The pollen tube contains two sperms (each is a haploid male gamete).

(b) When pollen tube enters the embryo-sac (inside the ovule), it bursts to release its contents i.e.,two sperms along with certain amount of protoplasm .The first male gamete discharge in one of thesynergid.

3.Double fertilization:

(a) Both the male garnet present in pollen tube utilises in fertilisation process of angiosperm isknown as double fertilisation. It is the characteristic feature of angiosperms except FamilyOrchidaceae, Podostemaceae and Trapaceae.

It involves two types of fusion –

a. Syngamy (fusion of egg cell and one male gamete) and

b. Triple fusion (fusion of remaining male garnet and two proper nuclei).

It was first observed by Nawaschin (1898) in Fritilaria and Lilium.

• Development of Embryo (Embryogeny):

• As written earlier also, the highly organized body of a seed plant represents the sporophytic phase of the life-cycle. It begins its existence usually with the fertilized egg, the zygote, which develops into the embryo by characteristic steps showing characteristics of future adult organization of the plant. Embryo, as a whole, assumes a specific form in which an axis and one or more leaf like appendages, the cotyledons, can be recognised.

• Because of its location below the cotyledons, the stem like axis is called hypocotyl. At its lower end (i.e., the root pole), the hypocotyl bears the baby root i.e. (radicle), and at its upper end (i.e., the shoot pole), above the cotyledons, the baby shoot (plumule). The root may be represented by its meristem (apical meristem of the root) or by a primordial root, the radicle. If a primordial shoot is present, it is called epicotyl or plumule.