Step 1: Cytoplasmic or extranuclear inheritance refers to traits passed on through organelle DNA rather than nuclear chromosomes.
Step 2: In 1909, Correns crossed variegated and green branches of Mirabilis jalapa and observed that progeny leaf colour always matched the female parent branch regardless of which branch supplied pollen.
Step 3: Since eggs contribute most of the cytoplasm (and the plastids within it) to the zygote while pollen contributes very little cytoplasm, the plastids of the mother plant determined whether leaves were green, white, or variegated.
Step 4: This makes plastids the organelle responsible, giving \[\boxed{\text{Plastids}}\]