Monocotyledonous Embryo

L.S. of an embryo of grass

In monocotyledonous plants (e.g., the grass family), the embryo possesses only one cotyledon situated towards one side (laterally) of the embryonal axis. This single cotyledon is called the scutellum.

Two specialised protective sheaths are present, one for the shoot and one for the root, which are unique to monocots:

The development of a monocot embryo (Grasses) follows a highly specific sequence of divisions, heavily characterised by the dominant growth of the single cotyledon:

1. Zygote Formation: After double fertilization, a diploid oospore or zygote (2n) is formed inside the embryo sac.
2. First Division (Asymmetric): The zygote divides transversely to form two unequal cells: a small terminal cell (embryo proper) and a large basal cell (suspensor).
3. Suspensor Formation: The basal cell enlarges and elongates to form the suspensor—a vesicular, haustorium-like structure. Its role is to push the proembryo deep into the endosperm to absorb nutrients and transmit them to the developing embryo.
4. Terminal Cell Divisions: The apical (terminal) cell divides repeatedly, eventually forming the plumule and the single, massive cotyledon (scutellum).
5. Middle Cell Contributions: The middle cell (formed during an early 3-cell stage) undergoes further divisions. It contributes to the formation of the hypocotyl, the radicle, and also adds cells to the suspensor.
6. Scutellar Stage: The scutellum grows exceptionally fast. Because of this rapid terminal growth, the plumule is pushed laterally and comes to lie in a lateral depression on the embryonal axis.
7. Protective Sheaths Differentiate: Specialised sheaths emerge to protect the growing points: the coleoptile develops around the plumule, and the coleorhiza develops around the radicle.
8. Mature Embryo: The final, fully differentiated monocot embryo is complete with the scutellum, laterally displaced plumule (in coleoptile), radicle (in coleorhiza), epicotyl, and epiblast (if present).

Development of Monocot (grass) Embryo

• In monocots (e.g., grasses), the embryo has a single cotyledon called the scutellum, set laterally on the embryonal axis; it is shield-shaped and absorbs nutrients from the endosperm.
• Two protective sheaths unique to monocots are present: the coleoptile encloses the shoot (plumule), and the coleorhiza surrounds the radicle and root cap.
• An epiblast (vestigial second cotyledon) lies opposite the scutellum; the radicle at the lower end forms the primary root, covered by a root cap.
• After double fertilisation, the zygote divides transversely into a small terminal cell (embryo proper) and a large basal cell (suspensor).
• The suspensor pushes the embryo into the endosperm to absorb nutrients; the terminal cell forms the plumule and the single massive scutellum.
• The scutellum grows fast, pushing the plumule sideways into a lateral depression on the axis.
• The coleoptile and coleorhiza then differentiate, giving a mature embryo with scutellum, laterally displaced plumule, radicle, epicotyl, and epiblast (if present).