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 
indusium, 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 
