Which of the following structures is triploid in angiosperms?

Concept: In angiosperms, double fertilization produces two genetically distinct products with different ploidy levels: the egg cell (n) fuses with one male gamete (n) to form the diploid (2n) zygote, which develops into the embryo; separately, the two polar nuclei of the central cell (n + n) fuse with the second male gamete (n) in triple fusion, forming the triploid (3n) primary endosperm nucleus, which develops into the endosperm.

Application: The radicle, cotyledon, and plumule are all parts of the embryo, which develops entirely through mitotic divisions of the diploid zygote — so each carries the 2n chromosome number. The endosperm, however, develops from the primary endosperm nucleus formed by triple fusion (two polar nuclei + one male gamete, i.e. n + n + n = 3n), making it triploid.

Cross-check — contrast each structure by value:

• Radicle — diploid (2n): the embryonic root, formed by mitotic division of the zygote.

• Cotyledon — diploid (2n): the embryonic seed leaf, likewise arising from the zygote.

• Plumule — diploid (2n): the embryonic shoot tip, likewise arising from the zygote.

• Endosperm — triploid (3n): formed by triple fusion of the two polar nuclei with the second male gamete.

So, among the structures listed, only the endosperm is triploid.