If you have small ferns growing on walls or in other dry arid places. then are most likely to belong to the Spleenworts, or so called Asplenium genera. Though sometimes larger ferns will grow as stunted plants on walls, especially Harts Tongue Fern known as Phyllitis scolopendrium. This however is easy to recognize as it is the only hardy fern with plain, simple, undivided leaves. See picture on page one.
Of the Spleenworts there are several but, only two common types. Asplenium trichomanes the Maiden Hair Spleenwort , including its several sub species, (see picture 1). And wall rue or Asplenium ruta-muraria, (see picture 2).
The two Spleenworts are easy to tell apart. As A. trichomanes is a bright green, with distinctive black stems,usually 10cm to 20cm long. A. ruta-muraria however is an overall blueish green and with fronds rarely more than 10cm. long