With good fortune we begin our study of Maseches Ma'asros, chapter 1, mishnah 1. The opening mishnah lays down a general rule regarding which types of food are subject to the various tithing obligations.
"Kelal amru b'ma'asros" - a general rule was stated concerning ma'asros: three conditions were established for the obligation:
"Kol shehu ochel" - that it be human food. Agricultural produce grown for a purpose other than human consumption - whether it is intended as animal feed or serves some other need, such as dye - is exempt from the tithing obligations.
"V'nishmar" - that the produce grew in a person's private domain and he watched over it while it was growing. Plants that grow on their own in ownerless land produce that is exempt from the obligation of ma'asros.
"V'giduloh min ha'aretz" - that it be a food that grows from the ground and draws its nourishment from it. This condition exempts mushrooms and truffles, since mushrooms are not plants and do not draw their nourishment from the soil itself, but rather feed through fine filaments that absorb from decaying matter. They are therefore entirely exempt from the tithing process.
These are the foods that are subject to ma'asros, and these are what a person is required to tithe.
Two related questions:
Hydroponic vegetables: the poskim are divided as to whether they are subject to ma'aser or not. A comprehensive discussion of the subject appears in Sefer Derech Emunah on Hilchos Terumos (2, 1, s.v. ha'ochel), where Rav Chaim Kanievsky treats the matter at length, and in practice the question of whether there is a rabbinic obligation to tithe hydroponic produce was left unresolved.
Sea vegetables (nori): nori is an alga and not a plant, and therefore it would seem that its berachah is "shehakol," like the law of mushrooms, and similarly it is exempt from ma'asros.
The second rule:
"V'od kelal acher amru" - the Sages stated a further rule:
"Kol shetechilaso ochel v'sofo ochel" - a food that is fit to be eaten already from the beginning of its growth, and is likewise fit to be eaten at the end, meaning that it is edible at every stage.
"Af al pi shehu shomro l'hosif ochel" - even if it has not yet finished growing, and the owner leaves it in place so that it will continue to grow larger, this makes no difference: it is already a food now, and for that reason the obligation of ma'aser applies to it.
"Chayav katan v'gadol" - one is obligated to tithe it equally, whether it is small and undeveloped or fully grown.
By contrast, a food that at its beginning is not fit to be eaten because it is unripe, and only at the end ripens and becomes human food - there is no obligation to tithe it until it is fit to be eaten as human food.
In summary: in this mishnah we learned two rules regarding the obligation of ma'asros. The first rule: the obligation applies to human food that is guarded in its owner's domain and grows from the ground. The second rule: anything whose beginning is food and whose end is food is obligated at every stage, small as well as large, whereas something that is not fit to be eaten at its beginning does not become obligated until it ripens.
In the next three mishnayos we will discuss at what stage the various types of plants, fruits and vegetables ripen sufficiently to be considered fit - the stage known as onas hama'asros.