Discussion
Consider a diploid plant species where the cells in the epidermis (the outermost single cell layer) always divide in the anticlinal orientation. If one such cell within the central zone of the shoot apical meristem (SAM) spontaneously becomes tetraploid at the seedling stage, which one of the following cellular arrangements would be most likely observed if the meristem is examined at the adult stage?
Answer: B
The most likely result is many tetraploid cells in the epidermis.
The shoot apical meristem is layered. Because cells of the epidermal L1 layer divide only anticlinally (with new walls perpendicular to the surface), their daughters stay in the same layer. A cell in the central zone is a stem cell: it keeps dividing throughout the plant's life, and its descendants are displaced outward to form new tissue.
So a central-zone L1 cell that becomes tetraploid founds a clone that remains in the epidermis and grows with every division, giving a sector of many tetraploid epidermal cells by the adult stage. This is the logic of periclinal chimeras used to map meristem layers.
A single tetraploid cell would imply it never divided, which a stem cell does. The clone cannot spread to the whole SAM, because anticlinal division keeps it out of the inner L2 and L3 layers, and an all-diploid meristem would mean the tetraploid lineage was lost.
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