Megasporophyll Or Carpel Of Angiosperms
Gynoecium or pistil is the female reproductive whorl of flower consisting of one or more carpels or Megasporophll. The carpel is an elongated appendage and is a modified folded leaf. The folding encloses Megasporangia (ovules) in a chamber, the locule. Each carpel consists of three parts
- Stigma, which receives pollen grains.
- Style slender projection of the ovary. The surface of the style may be hairy or smooth.
- The swollen basal portion is termed as Ovary which may have one or many chambers. The ovary contains one or more rounded or oval bodies, the Ovules.
The ovule contains a large oval cell called Embryo sac.
Megasporangium or Ovule Of Angiosperms
The angiosperm ovule has long been considered as a bud because sometimes a shoot develops from it; and because it was borne on placenta which was considered axial; because from it, as a seed, the axis of the new plant develops. It has more commonly been considered of folio nature – a sporophyll, a leaflet, a lobe of a leaf, the tip of a leaf, etc. However, fundamentally, it is recognized as a structure that bears sporogenous tissue in which megaspore develops and the integuments are the protective structures.
The ovules are borne in the ovary on a specialized tissue called the Placenta which is supplied by the poorly developed tissue. Each ovule is attached to the placenta by a short stalk known as Funicle. The funicle maybe long and curved, or short and stout. The long funicle maybe adnate to the ovule body forming a ridge, the Raphe. The place of attachment of the funicle with the main body of the ovule is termed as Hilum. The main body of an ovule consists of a mass of sporogenous tissue called Nucellus which is surrounded, except at the tip, by two coverings or Integuments which arise as outgrowths from the chalazal end. The small opening left at the apex of the integuments is known as Micropyle. The junction of integuments and the nucellus is called Chalaza. The number of integuments is formed first than the outer one. In some plants belonging to gamopetalous families the number of integument is one.
In a mature ovule, towards the micropylar end, a large cell called Embryo-Sac is present embedded in the nucellus. The embryo-sac is actually an enlarged megaspore which contains three small cells towards the chalazal end, these cells are called Antipodal Cells, while a set of three cells known as Egg Apparatus is present at the micropylar end. The two upper cells in this egg apparatus are called Synergids or Help Cells, While, third lower cell is the Egg or Oospore. In the center of the embryo sac a nucleus termed as Secondary Nucleus is present.
Types of Ovule Of Angiosperms
Variation in general form and in position of micropyle distinguish various types of ovules. All forms of the ovules are modifications of a basic type. Various types of ovules found in angiosperms are:
- Orthotropous or Straight
The ovule is orthotropous when it is straight and upright on the placental surface. The micropyle, chalaza and funicle, all are in a straight line, as in members of Polygonaceae.
- Anatropous or Inverted
The anatropous ovule is inverted and is adnate or appressed to the funicle, and the micropyle usually faces the placenta. The ridge formed as a result of the fusion of the funicle and the integument is called Raphe. The anatropous ovule is characterstics of Ranales, Helobiales etc.
- Campylotropous or Curved
The ovule is more or less bean shaped. The micropyle faces the placenta and is attached near the middle of the ovule. These types of ovules are found in the members of Cruciferae, Caryophyllaceae, etc.
- Amphitropous or Transverse
In this type of ovule, the funicle is adnate to the ovule proper for about half of its length and the micropyle faces laterally. Therefore, the ovule is placed at right angles to its funicle.
The nucellus and the axis remain in the same line in the beginning but due to rapid growth on one side, the ovule gets inverted. This curvature continue till the ovule turns completely and once again the micropyle faces upwards, e.g., in Opunta.
(1) Orthotropous, (2) Anatropous, (3) Amphitropous, (4) Campylotropous