SECT. vm. CETERACH. 355 



their margins, the outermost of which extend beyond 

 the edges of the segments. They completely cover the 

 sori, and at first hide them entirely from view. On the 

 unexpanded fronds, the scales 

 are white and silvery. 'This 

 plant has an obscure indusium, 

 only perceptible at an early 

 stage of fructification, and sub- 

 sequently as a nearly erect mem- 

 brane, attached to the back of 

 the vein. It is rendered unne- 

 cessary by the arrangement of 

 the scales, which are disposed 

 in regular series along each side 

 of the veins and veinlets, point- 

 ing outwards, and concealing F *- 64 - Ceterach offlcinaram - 

 the sori with their broad bases, which completely over- 

 lap them in their immature condition.' 



It is well known that the seeds of tropical plants 

 have been brought to the western coasts of Scotland 

 and Ireland by the great Atlantic currents ; it might 

 therefore be expected, that the spores of Cryptogams 

 brought to our shores by the same means, and finding a 

 congenial climate, should become naturalized, as appears 

 to have been the case in ancient times, with regard to 

 the Ceterach and Asplenium marinum, from their mi- 

 grations, as traced by Mr. Johnson. c The countries 

 bordering on the basin of the Mediterranean and the 

 islands and eastern shores of the North Atlantic appear 

 to have been the original stations of this remarkable 

 fern. In the British islands its distribution is too partial 

 to admit of its being regarded as strictly indigenous, 

 though probably naturalized here at a period little 

 subsequent to the arrival of Asplenium marinum. It 

 occurs here on limestone-rocks, but more frequently on 

 old walls and ruins, rooted deeply in the decaying mortar, 



A A 2 



