• (a.) Increasing in size by growth on all sides, as the lichens.
Am·phig'e·nous adjective (Botany) Increasing in size by growth on all sides, as the lichens. 
amphigenous 1. A reference to certain parasitic fungi growing on both sides of leaves. 2. Increasing in size by growth on all sides; such as, lichens. Lichens are thallophytic plants formed by a mutualistic combination of an alga and a fungus, the algal component being a green or blue-green alga, and the fungal usually an ascomycete.
(of certain parasitic fungi) growing on both sides of leaves.