The sac-like part inside the nucellus that carries the egg is called the embryo sac. Amniotic sac is of three types.
1. Monosporic: An embryo containing only one zygote is called monosporic. About 75% of the plant kingdom is monosporic. In 1879, Strusberger first described the structure of the monosporic embryo of the plant Polygonum divaricatum. Monosporic embryos are of two types.
(i) Oenothera type: Oenothera is a 4-nucleate embryo sac. eg Oenothera lamarckiana.
(ii) Polygonum type: Polygonum is an 8-nucleate embryo sac. For example, Polygonum hydropiper.
2. Bisporic: An embryo containing two spermatozoa is called Bisporic. Bisporic embryos are of two types.
(i) Allium type: Allium is an embryo with 8 nuclei. For example – Allium cepa.
(ii) Endymion type: Endymion is an 8-nucleate embryo sac. For example- Endymion hispanica.
3. Tetrasporic: An embryo containing four gametes is called Tetrasporic. 7 types of tetrasporic embryos.
(i) Adoxa type: Adoxa embryo has 8 nuclei. For example, Adoxa moschatellina.
(ii) Peperomia: Peperomia is a 16-nucleate embryo sac. For example – Peperomia pellucida.
(iii) Penia type: Penia is a 16-nucleate embryo sac.
(iv) Drusa: Drusa embryo contains 16 nuclei. For example – Drusa amatista.
(v) Fritilliaria type: Fritilliaria embryo has 8 nuclei. For example, Fritilliaria imperialis.
(vi) Plumbagella: Plumbagella is an embryo sac with 8 nuclei. For example – Plumbagella micrantha.
(vii) Plumbaga type: Plumbaga is a 16 nucleated embryo sac. For example – Plumbaga zeylanica.