Nearly all ferns support their stems and leaf axes with a sheath of hypodermal sclerenchyma called sterome. The sheath consists of compact fibers with few or no intercellular spaces. Although solving the need for mechanical support, the sterome presents a problem: it blocks air that has diffused through the stomata from diffusing farther into the plant where it is needed for photosynthesis and respiration. In most ferns, the blockage is overcome by means of aerophores, which typically appear as light-colored lines flanking the leaf axes, with one line on either side. Each aerophore bears stomata and overlies a parenchymatous gap in the sterome. The gap contains intercellular spaces that facilitate the diffusion of air inward after it has passed through the stomata. Only rarely do stomata occur off the aerophore on areas of adjacent epidermis overlying the hypodermal sterome. In such locations, stomata would be useless because air diffusing through them would be blocked. In some ferns, aerophores also occur on rhizomes in association with decurrent leaf bases. Developmentally, aerophores arise from the marginal meristem. As such, they mark the boundary between the adaxial and abaxial surfaces of the leaf. Phylogenetically, aerophores constitute a synapomorphy for the clade consisting of the Cyatheales and Polypodiales. Within this clade, aerophores have become variously modified, including their complete loss, reduction to a single row of stomata, co-option as nectaries, and enlargement as peg-like structures at the pinna bases of mucilaginous leaves. Aerophores are absent in the Marattiaceae, which bear a different type of aerating structure here termed a porophore. These are scattered over the leaf axes, appearing as many pale, short longitudinal streaks, with each streak bearing a central pore.