Mishnah 4 in the first chapter of Maseches Ma'asros continues directly from the two previous mishnayos. Here too the Mishnah lists various types of produce and establishes when they reach the season of ma'asros. The difference is that here the Mishnah deals with foods that are fit to be eaten at every stage of their development - they are edible from the moment they begin to sprout, and they do not ripen at any particular point but simply keep growing larger.
The foods listed in the Mishnah:
Among vegetables: "hakishu'im" - cucumbers, "hadelu'im" - gourds or pumpkins, "ha'avatichim" - melons, and "hamelafefonos" - another variety of the cucumber family.
Among fruits: "hatapuchim" - apples, and "ha'esrogim".
All of these foods are fit to be eaten at every stage of their development, and therefore the Mishnah rules: "chayavin gedolim uketanim" - they are obligated in ma'aser whether they are large or small.
"Rabbi Shimon poter es ha'esrogim b'katnan" - in Rabbi Shimon's view, an esrog while it is still small is not fit to be eaten, and therefore it cannot be tithed at that stage. The halachah, however, follows the Tanna Kamma, that the esrog is obligated in ma'aser at every stage.
Bitter and sweet almonds:
Bitter almonds are fit to be eaten while they are small, but as they ripen they become poisonous. The two types of almonds stand in exactly opposite relation to one another, and so the Mishnah rules:
"Hachayav bishkedim hamarim - patur bametukim" - at the first stage, while the bitter almonds are fit to be eaten, the sweet almonds that we eat are still unripe and too undeveloped to have reached the season of ma'asros.
"Hachayav bametukim - patur bamarim" - when the sweet almonds finally become fit to be eaten, at that very same point of development the bitter almonds have become poisonous and unfit to be eaten, and they are not tithed.
In summary: this Mishnah lists foods that are fit to be eaten at every stage of their growth - cucumbers, gourds, melons, the melafefon variety, apples and esrogim - and therefore they are obligated in ma'aser whether large or small, and according to the halachah the esrog is included in this as well. Bitter and sweet almonds, by contrast, become edible at opposite stages, and so the point at which one is obligated is precisely the point at which the other is exempt.