Apple trees could be found in the Orchard biome and were a source of Apples.
There are four types of apple leaves: leaves, Flowering Leaves, leaves with green fruit, and leaves with red fruit. Only the last one, the red fruit, will drop apples when broken. You can also harvest apples by right-clicking the leaves with red fruits.
Apple Trees were apparently removed due to concerns that they made finding Apples too easy.