PLANT EMBRYOLOG STUDY
Early development of Capsella bursa–pastoris
Following developmental changes take place in the embryo CopseIla bursa pastoris:
I. First division of Oospore: Its oospore increases in size. It divides transversely in two cells. The cell toward the microphyll end is called suspensor cell. The cells towards other side is called embryonal cell. Embryonal cell forms the major portion of embryo.
- Formation of suspensor and radicle: The suspensor cell, undergoes few transverse divisions. It produces short filament of cells called suspensor. The first cell of suspensor enlarges very much. It becomes basal cell. It pushes the embryo down into the developing endosperm. Suspensor also acts as conductive tissues for the nutrients. The last cell of suspensor adjacent to embryonal cell is called hypophysis. Hypophysis divides further to form radicle.
- Formation of octant: They embryonal cell increases in size. It divides by,three divisiors. Two divisions are vertical and one division is transverse. These divisions form eight groups of cells called octant or pro-erabryo. The four octants towards the chalazal end are the epibasal or anterior octant. The other four octants which are a Ijacent to suspensor are hypobasal or posterior octant.
- Formation of cotyledons and plumule: The epibasal cells further divides to form two cotyledons and plumule. Further
divisions occur in the cotyledonary cells and bibbed mass of cells is formed. These lobes are primary cotyledons. The plumule and epicotyl is produced in the notch between two depressions. Therefore, plumule in dicot is terminal in origin.
•5. Formation of hypocotyl: The hypobasal octants divide to form mass of cells called hypocotyl. Hypocotyl is elongated. It carries radicle at its tip.
- Folding of embryo: The developing embryo increase in size. Therefore, it become curved or folded in different ways. The way of folding of embryo in seed is characteristic feature of each plant.
- Formation of basic layers of meristem: Two successive divisions occur in octants. It produces three layers. The outer layer is called dermatogen, middle is called periblem and central one is called plerome. Dermatogen gives rise to epidermis. Periblem gives rise to cortical portion. Pldome forms the stele in the centre.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 following steps:
- 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 top of the receptacle. Thesefloral parts appear as rourded protuberances of meritstematic tissue.
- A circle of four sepals appear first at four points on the receptacle. Sepals form a protective cover for the younger parts.
- Sepals are followed by stamen primordia. Each stamen primordium develops into an anther and filament.
- 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 arepresent 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
(b) Formation of Microspores (Microsporogenesis)
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:
(i) 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 tetrad separate from one another. They lie freely v, ithin the microsPorangium.
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 stomium.
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. Melosis and megaspore formation occurs within ovule. Therefore, ovule represents a megasporangium. Ovule consists of a stalk or funiculus. It bears the nucellus surrounded by one or two .integuments. Following steps occur during formation of ovule:
(i) Periclinal 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 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 Eirietal 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 relative 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.
- Amnhitropous 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, chalaza 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. flemitropous ovule is common in Ranunculus.
- Campylotropus or Curved: The body of the ovule is bent upon itself 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. Circinotropous ovule is found in Plumbago and Opuntia.DEVELOPMENT OF MALE GAMETOPHYTEMicrospore 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). It lies 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 nuclei inside the ovule.