58 WAYSIDE AND WOODLAND FERNS. 



between. They are situate in the centre of the side branches 

 of the pinnule veins. When numerous, they coalesce as they 

 enlarge, become dark brown, and nearly cover the back of the 

 frond. The spores are ripe in July and August. 



It is a very variable fern. A few of the varieties have been 

 recognized by science, and one or two have been described 

 as distinct species. The Alpine Bladder-fern, separately treated 

 below in deference to the views of some authorities, is one of 

 these. 



The Brittle Bladder-fern is a plant of the Northern type. It 

 is widely distributed between Yorkshire and Orkney, but south of 

 Yorkshire it keeps to the western half of the country, extending 

 as far south as Devonshire. In the Highlands it has been 

 found at an elevation of 4000 feet. It is found also in the 

 Arctic and Northern portions of Europe, the Alpine districts of 

 Middle Europe, Northern Asia, the Himalaya, and North- 

 America. 



Alpine Bladder-fern (Cystopteris alpina). 



Although given a separate reference here, we incline to the 

 view that C. alpina is a variety of C.fragilis sufficiently distinct 

 to be regarded as a sub-species, but with differences too slight 

 to constitute a claim to specific rank. Its principal departure 

 from the type will be found in the finer divisions of the frond, 

 the pinnules being nearly pinnate, and their divisions being 

 further pinnately cut. These final divisions are more distinctly 

 separated than in the type. The stipes is also less brittle. 

 Another distinction is claimed by the " splitters " in the direc- 

 tion taken by the final branches of the veins : in C. fragilis 

 they run in a line with the tip of the teeth ; in C. alpina they 

 terminate at the notch between the teeth. The stature of the 

 plant is less than that of the type. (Plates 53, 63.) 



In any case, its inclusion in the British flora is mainly a 



