Question

Mutation which occurs in one layer or more layers at the top of apex is known as

• A. Mericlinal chimera
• B. Periclinal chimera
• C. Sectorial chimera
• D. Segmented chimera

Hint:

Chimeras arise from mutations in the apical meristem's cell layers (L-I, L-II, L-III):
Mericlinal: Mutated tissue forms a sector within one or more layers (part of a layer is affected).
Periclinal: An entire layer is mutated, forming a "skin" over other layers (most stable type).
Sectorial: A wedge-shaped section through all layers is mutated (unstable).

Answer 1

The Correct Option is A

A chimera is an organism composed of tissues of two or more genetically different types, often resulting from a somatic mutation in a meristem (growing point). Chimeras are classified based on the arrangement of the mutated and non-mutated tissues in the apical meristem. The apical meristem typically has distinct cell layers (L-I, L-II, L-III).
(1) Mericlinal chimera: In this type, the mutated tissue occupies only a part of one or more cell layers in the apical meristem. This results in a sector of mutated tissue in the plant organs derived from that part of the meristem. It is often unstable and can revert to a non-chimeric state or convert to a periclinal or sectorial chimera.
(2) Periclinal chimera: The mutated tissue occupies an entire cell layer (or layers) in the apical meristem, forming a "skin" of mutated tissue over a core of non-mutated tissue (or vice versa). These are relatively stable.
(3) Sectorial chimera: The mutated tissue occupies a section through all cell layers of the meristem, like a wedge or slice. These are usually unstable.
(4) Segmented chimera: This term is not a standard classification for apical meristem chimeras in the same way as the others. The description "mutation which occurs in one layer or more layers at the top of apex" but implying it's not the entire layer (as in periclinal) or a full sector through all layers (as in sectorial) best fits the definition of a mericlinal chimera, where only a portion of a layer is affected. Mericlinal chimera

Answer 2

Layer-geometry approach: Visualize the apical meristem as stacked cell layers (L-I, L-II, L-III) seen from above. If the mutated tissue forms a full ring occupying an entire layer, that is a periclinal chimera. If it cuts through all layers like a wedge or pie-slice, that is a sectorial chimera. A mericlinal chimera is the intermediate, partial case, the mutation covers only part of one or more layers, not the whole ring and not a full wedge. Since the question specifies mutation in one or more layers at the top of the apex without saying the entire layer is affected, this partial-layer description matches mericlinal chimera exactly.