EXPLANATION OF PLATE XVT. 



POLYPODIUM. Plant of P. vulgarc ; piece of the frond ; a magnified spo- 

 rangium with its stalk, and another bursting and discharging spores. 



ON<>CLKA. (1) Pinna of the sterile frond of <>. Strnthioptoris; (2) portion 

 of a fertile frond ; (3) a piece of one pinna cut off to show the manner in 

 which it is rolled up ; and (4) a portion of the hist, magnified, with one 

 side unrolled ; toward the base the sporangia all removed, to show how 

 the fruit-dots are borne each on the middle of a vein. 



PELL7KA. Sterile and fertile plants of P. gracilis, and (1) a portion of the 

 fertile frond enlarged, with a piece of the marginal indusiuin turned back 

 to display the fruit ; the sporangia are all removed from the fruit-bearing 

 tips of the two forks of the lower vein. 



