What it does
Flowering plants do something no other living thing does: each pollen tube delivers two male gametes, and both fuse with something. One fuses with the egg to make the zygote (2n); the other fuses with two polar nuclei to make the primary endosperm nucleus (3n). This page walks through it in eleven steps inside one ovule:
- The megaspore mother cell (2n).
- Meiosis gives four megaspores (n).
- Three break down; the functional megaspore remains.
- to 6. Three mitoses without walls: 2, then 4, then 8 free nuclei.
- Walls form: the embryo sac, with 7 cells and 8 nuclei (2 synergids, the egg, 3 antipodals and a central cell with 2 polar nuclei).
- The pollen tube grows down the style and in through the micropyle.
- It bursts into a synergid and releases two male gametes.
- Syngamy and triple fusion: double fertilisation.
- The endosperm and the embryo grow; the ovule becomes a seed.
Each step animates, and the ploidy counter lists every cell present with its ploidy and its chromosome number for the plant you choose. It fits Class 12 Biology: sexual reproduction in flowering plants (megasporogenesis, the female gametophyte, pollination, double fertilisation, endosperm and embryo). For meiosis itself see cell division, and to follow what the zygote’s genes do in the next generation try mendel's pea garden.
How to use it
- Next and Back move one step; the Step list jumps straight to any step; Replay plays the current step again.
- Colours show ploidy: one colour for n, one for 2n, one for 3n, and faded cells are breaking down.
- Plant sets 2n for the chromosome column (pea 14, onion 16, maize 20, rice 24, bread wheat 42). The line under the table works the numbers out: meiosis halves, mitosis keeps, fusion adds.
- The facts under the picture count the cells and nuclei of the embryo sac at each step.
- Check yourself gives five fresh questions: endosperm chromosomes, chromosomes in a named cell, meiotic divisions for a number of seeds, the 7 and 8 count, and which fusion gives the endosperm.
Worked example
Choose maize, 2n = 20, so n = 10. The megaspore mother cell has 20 chromosomes; after meiosis each megaspore has 10. Three mitoses keep that number, so the egg, the synergids, the antipodals and each polar nucleus all have 10. At step 10 the table shows the zygote: 10 + 10 = 20 and the primary endosperm nucleus: 10 + 10 + 10 = 30. The seed coat comes from the mother’s integuments, so it has 20.
How many meiotic divisions make 100 maize grains? Each grain needs one ovule, and each ovule’s embryo sac comes from one meiosis: 100. Each also needs one pollen grain, and one meiosis makes four pollen grains: 100 ÷ 4 = 25. Total 125.
Tips
- “7 cells, 8 nuclei” is the most asked fact: the central cell is one cell with two nuclei.
- Synergids carry the filiform apparatus that guides the pollen tube in.
- In most plants the endosperm starts dividing before the zygote: the embryo needs food first.
FAQ
Is the ovule really straight? Most are bent (anatropous), with the micropyle near the stalk. It is drawn straight here so the steps are easy to follow.
Do all plants have this kind of embryo sac? About 70 % of flowering plants do (the Polygonum type). Others differ in detail.
Is this a botanical reference? No. It is a teaching model of the main steps.
Where the numbers come from
- Megasporogenesis, the Polygonum-type embryo sac, pollen tube growth and double fertilisation are standard plant biology. Drawings and wording are fibub's own; the ovule is drawn straight for clarity. A teaching model, not a botanical reference.
- Diploid chromosome numbers (pea 14, onion 16, maize 20, rice 24, bread wheat 42) are widely known general facts. Checked in the tests against the standard counts (8 nuclei, 7 cells; 2n zygote, 3n endosperm; 125 meioses for 100 seeds).
- Nothing you choose is stored or sent anywhere.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.