Structure of Ovule or Megasporangium
- A ridge or stalk-like outgrowth is formed from the placenta of the ovary on which body of ovules is present. Structure of Megasporangium
- Each ovule attached to the placenta by means of a thin stalk called funicle or funiculus/ Funiculum. The point of attachment of the funicle with the ovule is called ilum.
- The main region of the ovule is composed of mass of parenchymatous cells called the nucellus. The nucellus is the main part of the ovule. The nucleus is covered by one or two coats called integuments.
- In most of the ovule, the funicle is attached to the main body of the ovule for some distance (at the lateral side) to form a ridge-like structure known as Raphe.
- Vascular tissues are present inside the funiculus which supply food material from the placenta to the body of the ovule.
- A place from where funicle and integuments arise is called Chalaza.
- The integument is absent just opposite to the chalaza so that a narrow passage (pore) is formed which is called the micropyle.
- Micropyle in bitegmicovule has two parts:
(i) Outer region which is surrounded by the outer integument is called the exostome.
(ii) The inner part of the micropyle is surrounded by an inner integument called the endostome.
- In most of the Angiosperm entire part of the nucleus is utilized by developing the embryo sac but in some of the Angiosperm, some part of the nucleus remains inside the ovules that part of the nucleus present inside the seed in the form of a thin layer known as perisperm. Perisperm is commonly found in Piperaceae(Pipernigrum) and Zingiberaceae Families (Turmeric, Ginger).
- Some filaments are attached with funicle (some times placenta) are known as "Obturators".
- The function of obturators Special Integuments is to guide the passage of the pollen tube towards the micropyle inside the ovary.
Try yourself:Mass of parenchymatous cells on the body of the ovary is also called ______
The mass of parenchymatous cells are also known as the Nucellus. It maybe massive (crassinucellate ovule) or thin (tenuinucellate ovule). Its surrounded by one or more teguments.
(i) Aril: It is the type of third integuments that develops from the funicle at the base of the ovule.
Example: Myristica, Asphodelus and Litchi.
(ii) Arillode: It develops from the tips of the outer integument and grows downwards and surrounds the entire ovule.
Example: Pithecellobium (Inga dulce).
(iii) Sarcotesta: When the outer integument becomes fleshy then it is called Sarcotesta.
Example: Magnoliaceae (Magnolia).
(iv) Operculum: It is a stopper or break like structure that is formed on the micropyle. It is formed due to the elongation of inner integuments.
Example: Lemnaceae family (Lemna).
The flower buds of Eucalyptus erythrocorys have a large, bright red operculum. (v) Caruncle or Strophiole: It is formed due to the proliferation (outgrowth) of outer integuments over the micropyle.
Example: Ricinus communis (Castor).
It is formed by sugary contents so helps in the absorption of water during the germination of seeds and dispersal of seeds by ants called myrmecochory.
(vi) COMA: In some of the plants, unicellular filaments like structures are present on the seed which is formed by cells of the outer surface of the outer integument. Such seeds are known as "Comose seeds".
Example: Calotropis and Gossypium.
Types of Ovule on the Basis of Integuments
- A single integumented ovule is called a unitegmic ovule.
Example: Members of Gamopetalae and Gymnosperm.
- Two integumented ovule is called a bitegmic ovule.
Example: In most of Angiosperms (Polypetalae-Capsella and Monocots).
- The ovule in which integuments are absent is called Ategmic ovule.
Example: Olax, Liriosma, Loranthus & Santalum.
Try yourself:What is ategmic ovule?
Ategmic means the absence of integuments. Integuments are a natural outer, protective layer. Just like skin in animals, integument in plants is thin and layered.
Types of Ovule on the Basis of Nucleus
(i) Tenuinucellate: The nucleus is either less developed or present in the form of a single layer.
Example: Gamopetalae group.
(ii) Crassinucellate: The nucleus is a massive type i.e., it is made up of many layers.
Example: Polypetalae group and Monocots.
The nucleus degenerates in plants of the Composite family and integuments becomes active to form a nucleus like tissue. This is called endothelium or integumentary tapetum. It is a polyploid structure.
The nucleus dissolves in the members of the Podostemaceae family to form a nutritional cavity. This is termed pseudo-embryo-sac.
Types of Ovules on the Basis of Structure
- There are six different types of ovules are found in Angiosperms on the basis of the relationship of the micropyle, chalaza, and hilum with the body of the ovule and orientation on the funiculus.
(i) Atropous or Orthotropous
- The body of the ovule is upright in position. The micropyle, chalaza and hilum lie in one straight line so that this ovule is called a straight or upright ovule.
Examples: Betel, Piper, Polygonum and in Gymnosperms.
- It is the most primitive and most simple type of ovule of Angiosperms. Raphe is absent.
(ii) Anatropous Ovule
- In this type, the body of the ovule completely turned at 1800 angle, due to unilateral growth of funiculus, so it is also called an inverted ovule.
- The chalaza and micropyle lie in a straight line. The hilum and micropyle lie side by side very close to each other. This type of ovule is found in 80% of families of Angiosperms but not in Capsella. In this ovule micropyle facing a downward condition.
- This is the most common type of ovule so that it is considered as a "typical ovule" of Angiosperms.
Examples: Members of Malvaceae, Cucurbitaceae, Solanaceae, Composite family.
- It is also called a resupinate ovule.
(iii) Hemitropous or Hemi-Anatropous Ovule
- In this ovule, the body of the ovule bent on funcile at 900 angle, i.e., the body of ovule present at a right angle to the funiculus. This is an intermediate type between ortho and anatropous ovules.
- This ovule is also called horizontal ovule because the body of the ovule present in a horizontal position on the funiculus. Micropyle and chalaza are present in the same line but micropyle is situated away from the hilum.
Examples: Ranunculus, Primula, Golphimia.
(iv) Campylotropous Ovule
- In this ovule, the body of ovule curved (Curvature is not effective) in this way so micropyle and chalaza do not present in a straight line.
- The nucleus is present in a curved position but the embryo sac remains straight. Micropyle comes close to the hilum.
Examples: Leguminosae, Capparidaceae, Cruciferae family (Capsella).
(v) Amphitropous Ovule
- In this type of ovule, the curvature is more pronounced or effective in the nucellus and due to this effect of the nucellus, the embryo sac becomes horseshoe-shaped.
- Micropyle comes close to the hilum. It is also called as transverse ovule.
Examples: Mirabilis, Lemna, Poppy, Alisma, Butomaceae family.
(vi) Circinotropous Ovule
- In this type of ovule, the body of the ovule inverted and again turned into a straight position due to the growth of the funiculus so that body of the ovule present on the funicle at 3600.
- The entire body of the ovule is surrounded by funiculus. It is also known as a coiled ovule. The micropyle is situated away from the hilum.
Example: Cactaceae family - Opuntia.