Q.Which of the following is a diploid structure ?

The question tests your understanding of embryo development in angiosperms, specifically the sequence of stages from zygote to mature embryo. This is a classic area where the order of events matters critically.

In angiosperm embryology, the zygote undergoes a precise developmental program. The reason statement gives us the starting point: the zygote sits at the micropylar end of the embryo sac (the end where the pollen tube entered) and will indeed develop into the embryo. That much is accurate.

The assertion, however, claims a specific sequence: zygote → heart-shaped embryo → proembryo. This is where we need to recall the actual developmental timeline.

The correct embryonic sequence

1. Zygote formation and first divisions

   After fertilization, the zygote undergoes its first mitotic division to produce two cells: a smaller apical cell (toward the micropyle) and a larger basal cell. The apical cell will form most of the embryo proper, while the basal cell contributes to the suspensor.

2. Proembryo stage

   Through further divisions, a small cluster of undifferentiated cells forms—this is the proembryo. It's an early, globular mass without distinct organ primordia. This stage comes early in development, not late.

3. Globular embryo

   The proembryo develops into a more organized globular embryo, still roughly spherical but with emerging polarity.

4. Heart-shaped embryo

   In dicots, the embryo then transitions to a heart-shaped stage as the two cotyledons begin to bulge out, creating the characteristic bilateral symmetry. This is a later stage, well after the proembryo.

5. Mature embryo

   Further development leads to torpedo and finally mature embryo stages. …