Which part of an apple is the true fruit?

The core. Botanically the fruit is the part that develops from the ovary and holds the seeds, which in an apple is the papery casing everyone throws away. The flesh grows from the receptacle, the swollen end of the flower stem, which makes an apple what botanists call an accessory fruit - and puts it in the same structural category as a pear, a strawberry and a rose hip.

Apples do not breed true. Plant a pip from a good apple and the tree will produce something different and usually unpleasant, because apples are extreme outbreeders and every seed is a genetic lottery. Every named variety - every Bramley, every Cox - is therefore a cutting grafted onto rootstock, and the entire commercial crop is a collection of clones descended from a handful of lucky seedlings. The original Bramley tree, from a pip planted around 1809, still stands in Nottinghamshire.

That variability is why cider and Johnny Appleseed go together: the American planter was distributing seeds, not grafts, so what grew was not eating fruit but the sour stuff you press. All of them descend from a wild apple in the mountains of Kazakhstan, where the ancestor species still grows in forests, and where individual wild trees bear fruit that varies from bitter and pea-sized to something you would happily buy.