|Watercore in a Lady Apple. Click for close-up.|
You'd think juiciness would be a straightforward trait specific to each apple variety.
Coarse-grained apples, with large cells, hold more juice than fine-graned, smaller-cell breeds. Watermelon vs potato.
Sometimes however things get a little haywire and the tree forces water, tree-sap really, into the apple so fast that some of it ends up between the cells.