Disciples of Land
Like the Disciples of Hand, classes from the Disciples of Land cannot be selected at the game's start. These are roles one can pick up after a bit of progress in the game.
In essence, they are the "gatherers" of Eorzea. Their actions directly contribute to the Disciples of Hand, in that they gather materials used in the Disciples of Hand's crafts. This is touched on in our Final Fantasy 14: A Realm Reborn crafting guide, but we'll provide some details here as well.
Botanists gather materials from the landscape, including trees, crops, and plants. The materials they gather contribute to Alchemists, Carpenters, and Culinarians. To become a Botanist, complete your class' Level 10 quest and join the Botanist's Guild in Old Gridania.
Fishers gather materials from the water, namely fish (duh). This primarily contributes to Culinarians. To become an Alchemist, complete your class' Level 10 quest and join the Fisherman's Guild in the Limsa Lominsa Lower Decks.
Miners gather materials from the raw land. What they gather goes to the support of Armorers, Blacksmiths, and Goldsmiths. To become a Miner, complete your class' Level 10 quest and join the Miner's Guild in Ul'Dah.