MOUNTAIN BLADDER-FERN. 59 



matter of historical interest, for though Mr. Backhouse records 

 it from Teesdale, Durham (1872), and Prof. Babington certified 

 the correctness of identity, it was long retained in our lists on 

 the strength of its undoubted occurrence for many years on 

 old walls at Leyton in Essex one of the most unlikely spots 

 in which to find an alpine fern. There can be no doubt that 

 it had been introduced, and had held its own for over fifty 

 years, for the published records for that station extend from 

 1788 to 1840. 



Mountain Bladder-fern (Cystopteris montana). 



This species is very distinct from the Brittle Bladder-fern 

 and its sub-species, presenting a superficial likeness to the 

 Limestone Polypody (Polypodium robertianuni) both in the 

 division of its fronds and the way they are produced at distant 

 intervals along the creeping rootstock. 



The rootstock, which is only sparingly clothed with scales 

 and is almost black in colour, creeps over wet rocks at elevations 

 between 2300 and 3600 feet. The delicate pale-green fronds 

 are produced singly, and vary in their entire length between 10 

 and 15 inches, but nearly two- thirds of these measurements are 

 contributed by the slender stipes, the lower portion of which 

 has a few scattered scales. The leafy portion of the frond is 

 triangular and as broad as long. The pinnae are alternately 

 arranged, but the lowest two are nearly opposite. The pinnules 

 on the lower side of the midrib of these are much more de- 

 veloped than those above, the pinnules are themselves pinnate 

 and their divisions are strongly toothed. The sori are round 

 and small, and on the large lower pinnae there will be found 

 from eighteen to twenty-four of them, arranged on the final 

 branches of the veins. It is in fruit in July and August. 



This must be considered as one of the rarest of ferns, both 

 at home and abroad. In these islands it has only been found 



