main subunit (noun, pl. main subunits) – the main (or primary) branches are the first ones diverging from the central stem (axis) of a fruticose thallus. Some fruticose and all foliose or squamulose thalli have no central stem or axis. Their main lobes (or branches) are therefore the dominant lobes of the main thallus, which are usually all connected to one another, or (sometimes) separately attached to the substrate (i. e., some multi-lobed thalli or imbricate squamules have separately attached primary subunits). Related terms: thallus joint, primary subunit, secondary subunit..