Maasrot, Chapter One, Mishnah Four
Our mishnah begins to discuss when vegetables and other fruits that are eaten in early stages of ripening begin to become liable for tithes.
With regard to when vegetables [are liable to tithes]:
1) Cucumbers, gourds, water-melons, cucumber-melons, apples and etrogs are liable [for tithes], whether gathered in the earlier or later stages of ripening.
2) Rabbi Shimon exempts the etrog in the earlier stages.
3) The condition in which bitter almonds are liable [to tithes] is exempt in the case of sweet almonds, and the condition in which sweet almonds are liable [to tithe] is exempt in the case of bitter almonds.
Section one: All of these vegetables/fruits are eaten at both an early stage in their ripening and a later stage. Hence, even if they are harvested at their earlier stage, they are still liable for tithes.
Section two: Rabbi Shimon holds that etrogs are only eaten at their later stage of ripening and therefore if they are picked at an earlier stage, they are exempt from tithes.
Section three: Bitter almonds are eaten in the early stage of their ripening and sweet almonds are eaten in the later stage of their ripening. Therefore, the opposite rules apply to them. If bitter almonds are harvested at the later stage, they are exempt, because this is not when they are typically eaten, and if sweet almonds are harvested in their early stage, they are exempt because that is not when they are typically eaten.