Etymology Interjectum means to interpose or to interject.

Description Rhizome: creeping, branching, whitish waxy, rather thick, with phylopodia, scales lanceolate, abruptly narrowed above a dilated base, to 11 mm.
Frond: 50 cm high by 8 cm wide, evergreen, new fronds midsummer, monomorphic, blade/stipe ratio: 1:1 to 3:1.
Stipe: jointed at base, straw-colored, narrowly triangular, red-brown scales, to 6 mm, these scales are peltate???, vascular bundles: 3 at the base, unifying upwards into an open v-shape.
Blade: pinnatifid, narrowly oval, truncated base, arching, leathery or herbaceous, mid-green, dull in shade, glabrous.
Pinnae: 18 to 22 pair, alternate, longest pinnae at the middle; margins shallowly serrate; veins free, forking.
Sori: oval when young (and closer to the rachis), more round later, discrete, sunken into the lamina, bulging on the top surface, midway between margin and midrib, on the upper half of the blade, indusium: absent, sporangia: early green, later yellow, then rusty brown; paraphyses absent, maturity: fall.
Dimensionality: lower pinnae, particularly the lowest pair, bend above the plane of the blade (less so than with P. cambricum).
Phenology: fronds emerge in June-July, develop over a four week period.

Culture Habitat: neutral to basic rocks, walls, well-drained sites. Distribution: Portugal to Scotland, east to central Europe. Hardy to -25°C, USDA Zone 5.

Distinctive Characteristics Differs from P. vulgare in possessing the typical hybrid marks: larger in all parts, more vigorous in growth. Additionally, the sori here are oval when young, becoming less so with maturity. P. interjectum is more likely to be found on limestone or mortared walls. P. interjectum fronds emerge in June, four or five weeks after P. vulgare.

Synonyms
Polypodium vulgare var. attenuatum Milde var. prionodes Aschers
Polypodium vulgare L. ssp. prionodes (Aschers.) Rothm.

Compare with other species in the European Group