Some orchids form a modified stem called a back bulb (if it is leafless - see arrow 1) or green bulbs (with leaves - see arrow 2).
They form on lateral or terminal rhizomes during the growing cycle.
Pseudobulbs assist plants to survive during dry periods.
Anatomically, these are not true bulbs, but they are used to propagate certain orchids especially Cymbidium.
Back or green bulbs are removed from the parent plant and placed in a rooting medium to induce a basal offshoot (arrow) with its own root system.