The organic layer is divided into three layers: on the surface is the leaf litter formed by undecomposed vegetable matter; underneath is humus which is the product of decomposed vegetable matter. Between litter and humus is a partially decomposed layer of organic matter ("F: fragmented organic materials")[2] .[3] Some specialists consider this zone to be equivalent to the soil horizon (O) whereas for others, this only includes the humus and the intermediate layer, excluding the litter. Woodland plants that inhabit this zone often have bulbs or rhizomes and include ferns such as bracken, monocots such as bluebells and dog's mercury.