The pistil, the ovule and the embryo sac · Biology… | StemPy Lab
Course 19.2·Lesson 1 of 3
The pistil, the ovule and the embryo sac
The female part of the flower is the pistil, with a stigma, a style and an ovary. The ovary holds ovules, and each ovule is a megasporangium. One cell of the ovule, the megaspore mother cell, divides by meiosis into four megaspores. Only one survives. Its nucleus divides three times to make the embryo sac, which is seven-celled and eight-nucleate.
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The pistil
The female whorl of a flower is the gynoecium. It is made of one or more pistils, and a pistil is built from one or more carpels. A gynoecium with one pistil is monocarpellary, and a gynoecium with more than one is multicarpellary. When there are several pistils, they may be fused together, which is syncarpous, or free from each other, which is apocarpous.
A note on words. NCERT uses the words pistil and carpel almost as one. Many other books call a syncarpous gynoecium a single compound pistil that is made of fused carpels. The drawing below follows NCERT.
On the left, a pistil drawn lengthways. The stigma sits on top of a thin style, which joins the swollen ovary at the base. The ovary wall surrounds the ovarian cavity, called the locule. A placenta runs up the middle of the cavity, and six small ovules are attached to it, three on each side. On the right, two small circles in cross section. The syncarpous one is a single circle divided into three chambers, with the carpels fused. The apocarpous one is three separate circles, with the carpels free.
Each pistil has three parts.
The stigma is the top. Pollen lands on it, so it is the landing platform of the pistil.
The style is the long, thin part between the stigma and the ovary.
The ovary is the swollen base. Inside it is the ovarian cavity, or locule.
Inside the ovary, a tissue called the placenta lies against the wall. The ovules are attached to the placenta. Some ovaries hold one ovule, as in wheat, rice and mango. Others hold many, as in papaya, watermelon and orchids.
The ovule
An ovule is a megasporangium, which means the sac that makes megaspores. Look at the picture of a typical ovule.
A drawing of a typical ovule. A thick stalk, the funicle, joins the ovule to the placenta at the bottom. The place where they join is the hilum. The body of the ovule has two layers around it, the outer integument and the inner integument, which are open at a small gap near the funicle. That gap is the micropyle. Inside the integuments is a large mass of cells, the nucellus. In the nucellus, nearer the micropyle, lies the pink oval embryo sac. The end of the ovule opposite to the micropyle is the chalaza.
A stalk, the funicle, joins the ovule to the placenta. The place where they join is the hilum.
One or two protective layers, the integuments, surround the ovule. They leave a small opening at one tip. This opening is the micropyle.
The end of the ovule opposite to the micropyle is the chalaza. It is the base of the ovule.
Inside the integuments lies a mass of cells, the nucellus. Its cells are full of reserve food.
Inside the nucellus lies the embryo sac, also called the female gametophyte. Generally an ovule has a single embryo sac, formed from one megaspore.
Megasporogenesis: making the megaspores
Megasporogenesis is the formation of megaspores from a megaspore mother cell, abbreviated MMC.
An ovule forms a single MMC in the micropylar region of the nucellus. It is a large cell, with dense cytoplasm and a prominent nucleus. It is diploid.
The MMC divides by meiosis and forms four haploid megaspores.
In most flowering plants, one megaspore is functional, and the other three degenerate. Usually the functional one is at the chalazal end.
Compare this with the male side. A PMC makes four microspores, and all four become pollen grains. An MMC makes four megaspores, and only one is used. Each ovule needs just one embryo sac.
The embryo sac
The method of forming an embryo sac from a single megaspore is called monosporic development. The functional megaspore grows, and its nucleus divides by mitosis three times in a row.
A flow chart of seven stages, each a drawn cell with the micropylar end at the top. The megaspore mother cell, 2n, divides by meiosis into a stack of four megaspores of n, and three of them, at the micropylar end, are hatched in grey because they degenerate. The one at the bottom, near the chalaza, is the functional megaspore. It divides by mitosis into the two-nucleate stage, with a nucleus at each end and a vacuole between them. Two more mitoses give the four-nucleate stage and the eight-nucleate stage, with four nuclei at each end and no cell walls. When walls form, the embryo sac has seven cells and eight nuclei.
The first division gives 2 nuclei, which move to opposite ends, the poles.
The second division gives 4 nuclei, two at each pole.
The third division gives 8 nuclei, four at each pole.
These divisions are free nuclear. The nuclei divide, but no cell wall forms between them. After the 8-nucleate stage, walls appear. Then the arrangement is as follows.
Three cells at the micropylar end form the egg apparatus: one egg cell and two synergids. The synergids have a thickening at the tip, the filiform apparatus, which guides the pollen tube.
Three cells at the chalazal end are the antipodals.
Two nuclei, one from each pole, move to the centre. They are the polar nuclei. They lie in the large central cell.
A drawing of a mature embryo sac, with the micropylar end at the top and the chalazal end at the bottom. At the top is the egg apparatus: two synergid cells, each with a hatched cap called the filiform apparatus, and the large egg cell between and below them. In the middle of the large central cell lie two polar nuclei. At the bottom are three antipodal cells. The labels give eight nuclei in seven cells.
So a typical embryo sac has eight nuclei but only seven cells. The eight nuclei are the egg, two synergids, two polar nuclei and three antipodals. The seven cells are the egg, two synergids, three antipodals and the central cell, which holds two nuclei.
Every nucleus in the embryo sac is haploid, because the megaspore was haploid and mitosis keeps the number. The central cell, with two haploid nuclei, holds the same number of chromosomes as a diploid cell.
You should find that the nuclei double at each stage until the 8-nucleate stage. The cells stay at one until the walls form.
Try it in Python
The program counts the nuclei and the divisions that lead to the 8-nucleate embryo sac.
Practise with new numbers
Common mistakes
Saying that the embryo sac has eight cells. It has seven cells and eight nuclei.
Saying that all four megaspores survive. Only one does, in most plants.
Counting a diploid central cell. It has two haploid polar nuclei, not one diploid nucleus.
Mixing up the ends. The egg apparatus is at the micropylar end, and the antipodals are at the chalazal end.
Forgetting the free-nuclear stage. No cell walls form until after the 8-nucleate stage.
Remember these
Pistil: stigma, style, ovary. The ovules are borne on the placenta in the locule.
Ovule: funicle, hilum, integuments, micropyle, chalaza, nucellus and embryo sac.
MMC (2n) divides by meiosis into four megaspores (n). One is functional, and three degenerate.
The functional megaspore divides by mitosis three times, free nuclear, and gives 8 nuclei.
The mature embryo sac has 7 cells: egg apparatus (egg and 2 synergids), 3 antipodals and the central cell with 2 polar nuclei.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Name the three parts of a pistil, from the top to the base.
Which end of the ovule has the micropyle, and which end is the chalaza?
How many megaspores does an MMC make, and how many are functional in most plants?
How many nuclei and how many cells are there in a mature embryo sac?
What does the filiform apparatus do?
References
NCERT (2023). Biology, Textbook for Class XII, chapter 1, Sexual Reproduction in Flowering Plants.
Clark M. A. et al. (2018). Biology 2e, OpenStax, chapter 32, Plant Reproduction.
Urry L. A. et al. (2017). Campbell Biology, 11th edition, Pearson, chapter 38.
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