Question

Starting with the zygote, draw the diagrams of the different stages of embryo development in a dicot.

Answer 1

Two celled stage

• In dicots, the zygote divides in a transverse direction to form two unequal cells- the terminal or apical cell and the basal cell.
• By subsequent divisions the basal cell becomes the suspensor cell and the apical cell forms the embryo

Tetrad stage and globular stage

• I shaped embryo- The terminal cell divides by the vertical division forming 4-celled I shaped embryos.
• Octant stage- Now each of the 4 cells divide transversely forming the octant stage (8- celled) of the embryo. The basal cells divide transversely, forming a 6 to 10 -celled suspensor filament.
• Globular embryo stage- Embryonal cells divide longitudinally and form four cells followed by eight then sixteen cells.

Heart-shaped embryo

• The outer eight cells develop into the epidermis of the embryo and inner eight cells develop into the cotyledon, hypocotyl and plumule.
• The suspensor cell at the terminal forms hypophysis- it develops the root apical meristem.
• Two cotyledons develop from the distal part of the embryo and transform into a heart-shaped embryo. The hypophysis develops into the embryonic root and root cap.

Mature embryo

• The cylindrical portion below the level of cotyledons is hypocotyl that terminates at its lower end in the radicle or root tip.
• The root tip is covered with a root cap.
• The portion of the embryonic axis above the level of cotyledons is the epicotyl, which terminates with the plumule or stem tip.