COMMON HART'S-TONGUE FERN 83 



are properly called a twin sorus. This twin sorus is 

 always produced between two fascicles of veins : that 

 is, the lowermost venule produced by one vein and the 

 uppermost of another below become each a receptacle 

 upon which one of the two contiguous lines of spore- 

 cases is produced. Their indusia are attached on the 

 upper and lower sides of the venules, the other edges 

 overlapping, so that the free margin is outside the fas- 

 cicle of venules to which it belongs. When very 

 young, the separation where they overlap is not ap- 

 parent, but becomes so as they advance toward matu- 

 rity ; till at last they open down the centre, one in- 

 dusium turning up and the other down, the two lines 

 of spore-cases becoming confluent and confused. 



This is the normal condition of Scolopendriwm ; but 

 the genus is one of the most remarkably prolific in 

 varieties, and in remarkable varieties, among all known 

 Ferns, the greater part of these, though not un- 

 frequently altogether monstrous, permanent and re- 

 taining their peculiarities in cultivation. Several forms 

 very distinct in themselves, and distinct also from the 

 parent, have been produced from the spores by artificial 

 treatment, indicating how probably the same process is 

 going on in a state of nature, more slowly perhaps and 

 imperceptibly, but as certainly, giving rise to new 

 forms, some transient, but some to be perpetuated. 



The fructification of Scolopendriu/ni is, of course, as 

 being one of the Polypodiacece, normally dorsal ; but 

 in some of its varieties there is a very curious deviation 

 from the law : the sori are produced on the upper as 

 well as the lower surface, and sometimes abundantly 



G2 



