178 HISTORY OF BRITISH FERNS. 



three, four, or more venules, according to the size of the 

 lobes, and each branch generally bearing a sorus at about 

 midway its length. The sori are thus generally numerous, 

 and rather irregularly disposed ; and it often occurs that 

 they are so numerous as, when fully grown, to become 

 confluent into a mass of fructification covering the whole 

 under surface of the frond. The number of sori produced, 

 and consequently the sparse or crowded disposition of the 

 fructification, is a matter altogether dependent upon the 

 circumstances of growth, and hence exceedingly liable to 

 vary even in the same plant, and within the same year, 

 as heat or cold, drought or moisture, may preponderate. 

 The sori, which are nearly circular, are covered while 

 young as already explained, by a concave or hood -shaped 

 iudusium, which usually becomes torn or split at the point 

 into narrow segments, and the whole soon becomes pushed 

 back or cast off by the growing spore-cases. 



There are many forms or varieties of this species. In 

 the form called angustata, the points of the pinnae and 

 the apex of the frond itself are often considerably narrowed 

 or elongated. One form, which is certainly the rhceticum 

 of Bolton, and nearly allied to angustata, is rather larger, 

 generally, than the typical form, and differs in having its 



