Question

Arrange the following stages of development of a dicot embryo in the order of their occurrence: (i) Formation of heart shaped embryo
(ii) Formation of typical dicot embryo
(iii) Formation of zygote
(iv) Formation of globular embryo

• A. (i), (ii), (iii), (iv)
• B. (iii), (iv), (i), (ii)
• C. (iii), (i), (ii), (iv)
• D. (iv), (i), (ii), (iii)

Hint:

Embryo development is irreversible and strictly sequential; skipping stages is biologically impossible.

Answer 1

The Correct Option is B

Concept: Embryogenesis in angiosperms is a highly coordinated developmental process that begins after fertilization and results in the formation of a mature embryo capable of developing into a complete plant. In dicot plants, this process follows a fixed morphological sequence involving distinct structural stages. Each stage reflects increasing cellular differentiation, polarity establishment, and organ formation. The development is not random but follows a genetically programmed sequence controlled by differential gene expression, hormone regulation (especially auxins and cytokinins), and cell division patterns.

Step 1: Using fertilization to define the starting point of embryogenesis.
The process begins when the male gamete fuses with the egg cell inside the embryo sac, forming a diploid zygote. This zygote is a single cell but contains the complete genetic blueprint required for the development of a multicellular organism. It marks the transition from gametophytic to sporophytic generation. The zygote undergoes repeated mitotic divisions and establishes polarity (apical-basal axis), which is essential for later differentiation.

Step 2: Using early mitotic divisions to explain globular embryo formation.
After several rounds of cell division, the zygote develops into a globular embryo. This stage is characterized by a spherical structure with rapidly dividing cells. At this stage: - Basic tissue differentiation begins - Primary meristematic regions start to appear - Cell fate begins to be determined However, organ differentiation is still absent, making this a fundamental early embryonic stage.

Step 3: Using cotyledon initiation to explain heart-shaped embryo stage.
In dicot plants, the formation of two cotyledon primordia is a key developmental event. As these cotyledons begin to grow laterally, the embryo takes on a heart-shaped appearance. This stage is biologically significant because: - It marks the first visible organ differentiation - Embryonic polarity becomes clearly established - Shoot and root axis formation becomes more defined This is a transitional stage between undifferentiated and fully differentiated embryo.

Step 4: Using maturation process to explain formation of typical dicot embryo.
The final stage involves extensive differentiation leading to the formation of a fully developed dicot embryo. The embryo now consists of: - Two cotyledons - Plumule (future shoot system) - Radicle (future root system) At this stage, the embryo becomes metabolically prepared for dormancy or germination depending on environmental conditions. Final Conceptual Understanding: Embryogenesis is a stepwise developmental cascade: zygote → globular → heart-shaped → mature dicot embryo. Each stage builds upon the previous one through controlled gene expression and hormonal regulation.